<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:dbaaae70-b9be-4987-bb7d-c68819934798,g:9ddefddb-841b-462e-b418-e9cbe013a4cf-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:roleRef roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="iclr-20210930.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETS"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Liabilities"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_f613fe1d-ebb2-4636-969b-c7a8ab79d69c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_us-gaap_LiabilitiesCurrent_f613fe1d-ebb2-4636-969b-c7a8ab79d69c" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_39b99df0-b8a1-442e-ba1c-f9c08b19350f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_39b99df0-b8a1-442e-ba1c-f9c08b19350f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_CapitalGovernmentGrantsNoncurrent_be5541c1-3d46-49cf-a0dd-892b80bafa62" xlink:href="iclr-20210930.xsd#iclr_CapitalGovernmentGrantsNoncurrent"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_iclr_CapitalGovernmentGrantsNoncurrent_be5541c1-3d46-49cf-a0dd-892b80bafa62" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedIncomeTaxesNoncurrent_88059798-3bb7-4607-aebb-1bfa689f7fc7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccruedIncomeTaxesNoncurrent"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_us-gaap_AccruedIncomeTaxesNoncurrent_88059798-3bb7-4607-aebb-1bfa689f7fc7" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_853dcf6b-54e4-4878-8c56-1266de892e9f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_853dcf6b-54e4-4878-8c56-1266de892e9f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_a922a2cb-62c7-4853-adb2-0231ac0e7ec3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:calculationArc order="6" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_us-gaap_CommitmentsAndContingencies_a922a2cb-62c7-4853-adb2-0231ac0e7ec3" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_9b11351b-fd79-471e-aa72-106fa420132f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:calculationArc order="7" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_9b11351b-fd79-471e-aa72-106fa420132f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermLineOfCredit_0c3ece46-9f76-47a8-8b84-308b095f4657" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermLineOfCredit"/>
    <link:calculationArc order="8" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Liabilities_c97b6f66-1a51-4d2a-b89e-733a5e353e17" xlink:to="loc_us-gaap_LongTermLineOfCredit_0c3ece46-9f76-47a8-8b84-308b095f4657" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_40a3e098-7506-414c-863c-a763fa4aab6d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_00c47620-4b95-4807-9b6f-eb096132c639" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockValue"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_StockholdersEquity_40a3e098-7506-414c-863c-a763fa4aab6d" xlink:to="loc_us-gaap_CommonStockValue_00c47620-4b95-4807-9b6f-eb096132c639" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_e2caccb1-4c24-47c1-a165-8adc8a0805b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_StockholdersEquity_40a3e098-7506-414c-863c-a763fa4aab6d" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_e2caccb1-4c24-47c1-a165-8adc8a0805b8" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherUndenominatedCapital_ff821cae-4311-4f6d-b0b3-8fdecd5a8285" xlink:href="iclr-20210930.xsd#iclr_OtherUndenominatedCapital"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_StockholdersEquity_40a3e098-7506-414c-863c-a763fa4aab6d" xlink:to="loc_iclr_OtherUndenominatedCapital_ff821cae-4311-4f6d-b0b3-8fdecd5a8285" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_64f136c4-c922-4775-bac3-950587c7cf97" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_StockholdersEquity_40a3e098-7506-414c-863c-a763fa4aab6d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_64f136c4-c922-4775-bac3-950587c7cf97" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_8b9c1375-b84c-4116-b69c-fd92139f59b2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_StockholdersEquity_40a3e098-7506-414c-863c-a763fa4aab6d" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_8b9c1375-b84c-4116-b69c-fd92139f59b2" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_8243eea8-4303-4c08-8f01-2f45366033cb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_eadda2f9-f7f1-49bf-ba01-9cc2c8547374" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LiabilitiesCurrent_8243eea8-4303-4c08-8f01-2f45366033cb" xlink:to="loc_us-gaap_AccountsPayableCurrent_eadda2f9-f7f1-49bf-ba01-9cc2c8547374" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_987f5b5a-0158-4060-83a9-fefa193f0520" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LiabilitiesCurrent_8243eea8-4303-4c08-8f01-2f45366033cb" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_987f5b5a-0158-4060-83a9-fefa193f0520" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesCurrent_00f1e875-2af9-4d7f-9627-00386befc403" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherLiabilitiesCurrent"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LiabilitiesCurrent_8243eea8-4303-4c08-8f01-2f45366033cb" xlink:to="loc_us-gaap_OtherLiabilitiesCurrent_00f1e875-2af9-4d7f-9627-00386befc403" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedIncomeTaxesCurrent_069f0a6c-0742-4b96-8273-8ed9e24d74ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccruedIncomeTaxesCurrent"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LiabilitiesCurrent_8243eea8-4303-4c08-8f01-2f45366033cb" xlink:to="loc_us-gaap_AccruedIncomeTaxesCurrent_069f0a6c-0742-4b96-8273-8ed9e24d74ff" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LinesOfCreditCurrent_af6b0df9-016e-453e-98f1-ad24735421cc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LinesOfCreditCurrent"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LiabilitiesCurrent_8243eea8-4303-4c08-8f01-2f45366033cb" xlink:to="loc_us-gaap_LinesOfCreditCurrent_af6b0df9-016e-453e-98f1-ad24735421cc" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Assets"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_8feeed2c-03de-492f-a5d7-0d0e9768b3c7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsCurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_AssetsCurrent_8feeed2c-03de-492f-a5d7-0d0e9768b3c7" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_0626d1a6-b3bb-436b-8ff3-e3d11cf2776e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_0626d1a6-b3bb-436b-8ff3-e3d11cf2776e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_81e9d069-1cd1-49b9-949a-4647ec5979bf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_Goodwill_81e9d069-1cd1-49b9-949a-4647ec5979bf" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_405928dd-e9f5-4d66-b50c-f2e2994e147a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_405928dd-e9f5-4d66-b50c-f2e2994e147a" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesReceivableNoncurrent_05b85c15-2f4f-427c-96ab-5076df329d21" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxesReceivableNoncurrent"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_IncomeTaxesReceivableNoncurrent_05b85c15-2f4f-427c-96ab-5076df329d21" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_12d704fe-869b-49f5-872e-9e18dd6da867" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxAssetsNet"/>
    <link:calculationArc order="6" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_12d704fe-869b-49f5-872e-9e18dd6da867" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_e969b7e8-9ca7-42dc-83f1-e5d2dba3d18e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:calculationArc order="7" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_e969b7e8-9ca7-42dc-83f1-e5d2dba3d18e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_3fb91c0e-84ca-445c-8277-9000c92b183d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:calculationArc order="8" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_3fb91c0e-84ca-445c-8277-9000c92b183d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLongTermInvestments_156b1a4d-b4bb-4744-a4b4-f3b6c77ea3a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherLongTermInvestments"/>
    <link:calculationArc order="9" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_OtherLongTermInvestments_156b1a4d-b4bb-4744-a4b4-f3b6c77ea3a8" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_a2cd6b70-4afe-4ea1-927d-027e05b4f81d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityMethodInvestments"/>
    <link:calculationArc order="10" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_Assets_c5b4d803-d511-47b8-b335-cba226e0f68d" xlink:to="loc_us-gaap_EquityMethodInvestments_a2cd6b70-4afe-4ea1-927d-027e05b4f81d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_d386ff0f-e65a-4022-b3b8-0be1673d8e38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_83ebb5ac-f0a4-4a98-8615-c33d9e09d960" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Liabilities"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquity_d386ff0f-e65a-4022-b3b8-0be1673d8e38" xlink:to="loc_us-gaap_Liabilities_83ebb5ac-f0a4-4a98-8615-c33d9e09d960" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_537f2d47-ebc2-4721-ada8-996ccaf768fb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquity_d386ff0f-e65a-4022-b3b8-0be1673d8e38" xlink:to="loc_us-gaap_StockholdersEquity_537f2d47-ebc2-4721-ada8-996ccaf768fb" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsCurrent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_142a48ff-ec0a-47b9-8d0e-411cd8f55548" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_142a48ff-ec0a-47b9-8d0e-411cd8f55548" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_22f032b2-387e-4ae0-bff0-d6f56a33c3ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_22f032b2-387e-4ae0-bff0-d6f56a33c3ef" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_8f5e8c35-512e-4071-8ec0-c71d3b0a486f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_8f5e8c35-512e-4071-8ec0-c71d3b0a486f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_a2cbcafd-925d-4290-9166-e757392aca77" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetGrossCurrent"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:to="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_a2cbcafd-925d-4290-9166-e757392aca77" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NontradeReceivablesCurrent_47b816b2-32e0-4f0b-b985-bf6dd493cfe8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NontradeReceivablesCurrent"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:to="loc_us-gaap_NontradeReceivablesCurrent_47b816b2-32e0-4f0b-b985-bf6dd493cfe8" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_ffd13372-8c04-4e2c-8c8b-ffb1eb07d1b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:calculationArc order="6" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_ffd13372-8c04-4e2c-8c8b-ffb1eb07d1b5" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesReceivable_bc84f7e3-7579-4896-aeee-e776771f2e33" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxesReceivable"/>
    <link:calculationArc order="7" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_AssetsCurrent_26e68c94-4cba-4c8a-945e-3f662fe49af8" xlink:to="loc_us-gaap_IncomeTaxesReceivable_bc84f7e3-7579-4896-aeee-e776771f2e33" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED" xlink:type="simple" xlink:href="iclr-20210930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_2cfd87f0-351d-47e0-9000-61790d17439e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_cd5d8176-a497-4051-9a73-69e69766ce69" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetIncomeLoss_2cfd87f0-351d-47e0-9000-61790d17439e" xlink:to="loc_us-gaap_ProfitLoss_cd5d8176-a497-4051-9a73-69e69766ce69" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest_89e625ff-238a-4cac-a675-2c440421b6f5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetIncomeLoss_2cfd87f0-351d-47e0-9000-61790d17439e" xlink:to="loc_us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest_89e625ff-238a-4cac-a675-2c440421b6f5" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest_74f78923-80b8-4454-b5d0-4b57b11547b0" xlink:href="iclr-20210930.xsd#iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_7f0d878e-06f4-4fe5-9159-d2c3bb27fca0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest_74f78923-80b8-4454-b5d0-4b57b11547b0" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_7f0d878e-06f4-4fe5-9159-d2c3bb27fca0" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_1766fcf7-ffb0-46c4-8397-c41657d095af" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest_74f78923-80b8-4454-b5d0-4b57b11547b0" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_1766fcf7-ffb0-46c4-8397-c41657d095af" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_9ebb63b4-9f21-4d02-bbb0-e7cf328d7452" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_f9c657d1-6720-4c6c-810d-6daaa45c642f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostsAndExpenses"/>
    <link:calculationArc order="1" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_OperatingIncomeLoss_9ebb63b4-9f21-4d02-bbb0-e7cf328d7452" xlink:to="loc_us-gaap_CostsAndExpenses_f9c657d1-6720-4c6c-810d-6daaa45c642f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_b15e09af-1759-43e8-bc15-bda2704bb3b1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_OperatingIncomeLoss_9ebb63b4-9f21-4d02-bbb0-e7cf328d7452" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_b15e09af-1759-43e8-bc15-bda2704bb3b1" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_54d10899-206d-43e9-98f9-b169a4c314c1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_3510ab8d-8898-4a40-84f6-b11472ca403d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_54d10899-206d-43e9-98f9-b169a4c314c1" xlink:to="loc_us-gaap_OperatingIncomeLoss_3510ab8d-8898-4a40-84f6-b11472ca403d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_023f3587-823f-4fbe-a5c1-d49feba6118b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_54d10899-206d-43e9-98f9-b169a4c314c1" xlink:to="loc_us-gaap_InvestmentIncomeInterest_023f3587-823f-4fbe-a5c1-d49feba6118b" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_53eb6e65-33ee-4274-bc9f-ca89c1e2d459" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpenseDebt"/>
    <link:calculationArc order="3" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_54d10899-206d-43e9-98f9-b169a4c314c1" xlink:to="loc_us-gaap_InterestExpenseDebt_53eb6e65-33ee-4274-bc9f-ca89c1e2d459" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_4470fcb1-9864-4cce-9335-c12b745f5d30" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostsAndExpenses"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_a7b6a3e1-a841-4f66-96de-4b7de5f7fb6a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CostsAndExpenses_4470fcb1-9864-4cce-9335-c12b745f5d30" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_a7b6a3e1-a841-4f66-96de-4b7de5f7fb6a" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_d5c9b11b-396d-47ae-95b3-434aa2e5d23b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringCharges"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CostsAndExpenses_4470fcb1-9864-4cce-9335-c12b745f5d30" xlink:to="loc_us-gaap_RestructuringCharges_d5c9b11b-396d-47ae-95b3-434aa2e5d23b" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_d6da5168-c5f3-4d8f-b1fa-ad4eb9de04ab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CostsAndExpenses_4470fcb1-9864-4cce-9335-c12b745f5d30" xlink:to="loc_us-gaap_DepreciationAndAmortization_d6da5168-c5f3-4d8f-b1fa-ad4eb9de04ab" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_fb36f7e5-eff5-4130-aabd-2a5c796ef5e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CostsAndExpenses_4470fcb1-9864-4cce-9335-c12b745f5d30" xlink:to="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_fb36f7e5-eff5-4130-aabd-2a5c796ef5e7" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_63175d63-f039-44c9-8dc4-4208ee2ff09e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CostsAndExpenses_4470fcb1-9864-4cce-9335-c12b745f5d30" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_63175d63-f039-44c9-8dc4-4208ee2ff09e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_97884928-be44-43fb-ac61-91591fb6719b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest_cdfd7ac9-88f9-4b9b-b6e9-6d08397ff7b5" xlink:href="iclr-20210930.xsd#iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ProfitLoss_97884928-be44-43fb-ac61-91591fb6719b" xlink:to="loc_iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest_cdfd7ac9-88f9-4b9b-b6e9-6d08397ff7b5" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_02c1780e-6577-4ea5-a401-43526a7d4387" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ProfitLoss_97884928-be44-43fb-ac61-91591fb6719b" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_02c1780e-6577-4ea5-a401-43526a7d4387" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEStatement" xlink:type="simple" xlink:href="iclr-20210930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEStatement"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEStatement" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentWriteOffOfLossAfterReclassificationAndTax_39dcc904-1505-4e34-837f-f8d99812e045" xlink:href="iclr-20210930.xsd#iclr_OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentWriteOffOfLossAfterReclassificationAndTax"/>
    <link:calculationArc order="1" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:to="loc_iclr_OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentWriteOffOfLossAfterReclassificationAndTax_39dcc904-1505-4e34-837f-f8d99812e045" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding_07fda6c2-ccfb-4a60-8dc8-e0172851375f" xlink:href="iclr-20210930.xsd#iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:to="loc_iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding_07fda6c2-ccfb-4a60-8dc8-e0172851375f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_998b5d6c-1c63-4a9c-b34e-0766e95da7a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_998b5d6c-1c63-4a9c-b34e-0766e95da7a9" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_9ed3f280-6d49-43fc-aed2-2ecab996e529" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_9ed3f280-6d49-43fc-aed2-2ecab996e529" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_7aa1a521-b0cf-4533-b9ea-c7be5c4c2cd5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:to="loc_us-gaap_NetIncomeLoss_7aa1a521-b0cf-4533-b9ea-c7be5c4c2cd5" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentAmortizationAfterTax_4c0a0cd7-a7a2-4f81-ad32-9ed7476a97d7" xlink:href="iclr-20210930.xsd#iclr_OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentAmortizationAfterTax"/>
    <link:calculationArc order="6" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:to="loc_iclr_OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentAmortizationAfterTax_4c0a0cd7-a7a2-4f81-ad32-9ed7476a97d7" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax_d2000f69-25af-472a-8d20-e6867f622b68" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax"/>
    <link:calculationArc order="7" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTax_4623f922-fcc5-4734-9df5-30337929cd39" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax_d2000f69-25af-472a-8d20-e6867f622b68" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED" xlink:type="simple" xlink:href="iclr-20210930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_eccc3e13-0201-4e62-ae39-307f23bed9a1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_ProfitLoss_eccc3e13-0201-4e62-ae39-307f23bed9a1" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized_ae95ee3a-4da7-4ad2-a436-80233d0ff236" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignCurrencyTransactionGainLossUnrealized"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized_ae95ee3a-4da7-4ad2-a436-80233d0ff236" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_2fd4a81f-944e-4cc2-90c1-f9c7feba367d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:calculationArc order="3" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_2fd4a81f-944e-4cc2-90c1-f9c7feba367d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeGainLossOnDerivativeNet_b6a06c76-4e06-43f2-9c77-3d4803b6e4ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeGainLossOnDerivativeNet"/>
    <link:calculationArc order="4" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_DerivativeGainLossOnDerivativeNet_b6a06c76-4e06-43f2-9c77-3d4803b6e4ed" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_ccac99d1-b19a-4631-8a68-a9e36b6a3c4d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensation"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_ShareBasedCompensation_ccac99d1-b19a-4631-8a68-a9e36b6a3c4d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInContractWithCustomerAsset_de87876f-a89a-4b82-9ebb-e41c24e45761" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInContractWithCustomerAsset"/>
    <link:calculationArc order="6" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_IncreaseDecreaseInContractWithCustomerAsset_de87876f-a89a-4b82-9ebb-e41c24e45761" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncashIncomeExpense_e74bf86c-fb3a-44bc-8124-f7a192f31c5f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherNoncashIncomeExpense"/>
    <link:calculationArc order="7" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_OtherNoncashIncomeExpense_e74bf86c-fb3a-44bc-8124-f7a192f31c5f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_4b9166ba-ad89-48fc-8cc9-ef5cb641c2fb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:calculationArc order="8" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_4b9166ba-ad89-48fc-8cc9-ef5cb641c2fb" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet_3008034c-f64c-4429-9048-1e5330a02d57" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet"/>
    <link:calculationArc order="9" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet_3008034c-f64c-4429-9048-1e5330a02d57" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_3200d93a-0e7e-410e-b4cc-b724c3bbaf20" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInDeferredRevenue"/>
    <link:calculationArc order="10" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_3200d93a-0e7e-410e-b4cc-b724c3bbaf20" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_a4a85aed-0100-46c0-97f6-b2d1cd769780" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:calculationArc order="11" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_a4a85aed-0100-46c0-97f6-b2d1cd769780" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_43287dbf-cbe5-4539-87a0-803f1b9c4380" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:calculationArc order="12" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_43287dbf-cbe5-4539-87a0-803f1b9c4380" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_2171c1af-8817-49d5-af14-c750938d61bb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:calculationArc order="13" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_2171c1af-8817-49d5-af14-c750938d61bb" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_95b9e121-9ee6-42ed-81ee-8f5f8bad972e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense"/>
    <link:calculationArc order="14" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_95b9e121-9ee6-42ed-81ee-8f5f8bad972e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxesAndTaxCredits_a09cd4a4-ad3b-46fc-afe8-2e9bb70151c1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxesAndTaxCredits"/>
    <link:calculationArc order="15" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_DeferredIncomeTaxesAndTaxCredits_a09cd4a4-ad3b-46fc-afe8-2e9bb70151c1" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_058ec2d7-4c2d-4fbb-9184-2540991688b4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:calculationArc order="16" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_058ec2d7-4c2d-4fbb-9184-2540991688b4" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_GainLossOnIssuanceOfDebt_30c8d74d-1cf9-451c-ac37-0b2489672c85" xlink:href="iclr-20210930.xsd#iclr_GainLossOnIssuanceOfDebt"/>
    <link:calculationArc order="17" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_18967f7f-c0d2-4949-b2c3-1632bb1beb78" xlink:to="loc_iclr_GainLossOnIssuanceOfDebt_30c8d74d-1cf9-451c-ac37-0b2489672c85" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_d428979b-6ffd-4bfd-a390-02a165b80841" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromLinesOfCredit"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:to="loc_us-gaap_ProceedsFromLinesOfCredit_d428979b-6ffd-4bfd-a390-02a165b80841" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfFinancingCosts_d5712c0e-ae4d-4cdc-949c-41899af6428e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsOfFinancingCosts"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:to="loc_us-gaap_PaymentsOfFinancingCosts_d5712c0e-ae4d-4cdc-949c-41899af6428e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_5d7e09d4-fe50-497a-b63a-4ac968e21315" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromStockOptionsExercised"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_5d7e09d4-fe50-497a-b63a-4ac968e21315" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_673b80c1-a9c9-4867-a979-8dbf92550b64" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock"/>
    <link:calculationArc order="4" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_673b80c1-a9c9-4867-a979-8dbf92550b64" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_9d938af3-9e04-4499-9327-a5ba4b8b1145" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RepaymentsOfLinesOfCredit"/>
    <link:calculationArc order="5" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_9d938af3-9e04-4499-9327-a5ba4b8b1145" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_db547baf-c662-464a-b995-0c9282b17abb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:calculationArc order="6" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_db547baf-c662-464a-b995-0c9282b17abb" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_PaymentsForShareRepurchaseCosts_c88c50b2-38e9-401c-aa54-02f0c3e1abb0" xlink:href="iclr-20210930.xsd#iclr_PaymentsForShareRepurchaseCosts"/>
    <link:calculationArc order="7" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_23c0d8a1-6058-4464-a517-421f85741fd4" xlink:to="loc_iclr_PaymentsForShareRepurchaseCosts_c88c50b2-38e9-401c-aa54-02f0c3e1abb0" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_d6f6369c-93f9-4c42-a4fc-da989ccbc6e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_0e1c5a89-8266-4567-a197-8a4314a4295b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_d6f6369c-93f9-4c42-a4fc-da989ccbc6e3" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_0e1c5a89-8266-4567-a197-8a4314a4295b" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1f9a1f4a-f2c0-4003-afe8-509c84550662" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_d6f6369c-93f9-4c42-a4fc-da989ccbc6e3" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1f9a1f4a-f2c0-4003-afe8-509c84550662" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_3122f2a2-835f-46b2-b87b-d1f96330edfd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_d6f6369c-93f9-4c42-a4fc-da989ccbc6e3" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_3122f2a2-835f-46b2-b87b-d1f96330edfd" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_edfc73b7-9d88-49a4-9bbb-6a127feb95e0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_d6f6369c-93f9-4c42-a4fc-da989ccbc6e3" xlink:to="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_edfc73b7-9d88-49a4-9bbb-6a127feb95e0" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1af2259d-bb96-482d-9c2a-493d4ea6125c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_5502f24e-8f77-4c6b-806d-0bd1ea6dc7ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments"/>
    <link:calculationArc order="1" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1af2259d-bb96-482d-9c2a-493d4ea6125c" xlink:to="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_5502f24e-8f77-4c6b-806d-0bd1ea6dc7ee" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_3b4bdcb9-729e-4b5c-8f52-e199e3cc3361" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1af2259d-bb96-482d-9c2a-493d4ea6125c" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_3b4bdcb9-729e-4b5c-8f52-e199e3cc3361" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_391a2525-6c38-426d-867d-d01da2d8d6f1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:calculationArc order="3" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1af2259d-bb96-482d-9c2a-493d4ea6125c" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_391a2525-6c38-426d-867d-d01da2d8d6f1" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_9ee384c7-9e76-47b4-9125-81754661fd04" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1af2259d-bb96-482d-9c2a-493d4ea6125c" xlink:to="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_9ee384c7-9e76-47b4-9125-81754661fd04" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireLongtermInvestments_56ffc8ea-8ae1-45bb-94b7-935f74955300" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireLongtermInvestments"/>
    <link:calculationArc order="5" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_1af2259d-bb96-482d-9c2a-493d4ea6125c" xlink:to="loc_us-gaap_PaymentsToAcquireLongtermInvestments_56ffc8ea-8ae1-45bb-94b7-935f74955300" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesContractAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesContractAssetsandLiabilitiesDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesContractAssetsandLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ContractsReceivableNet_bdfc4391-34f9-4de0-92b8-0c0383a0f002" xlink:href="iclr-20210930.xsd#iclr_ContractsReceivableNet"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ContractsReceivableGross_d7cc1789-8e39-4924-9914-f58d9eac9888" xlink:href="iclr-20210930.xsd#iclr_ContractsReceivableGross"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_ContractsReceivableNet_bdfc4391-34f9-4de0-92b8-0c0383a0f002" xlink:to="loc_iclr_ContractsReceivableGross_d7cc1789-8e39-4924-9914-f58d9eac9888" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_06757c6d-6daa-4a69-b6f1-0454f13fb729" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_ContractsReceivableNet_bdfc4391-34f9-4de0-92b8-0c0383a0f002" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_06757c6d-6daa-4a69-b6f1-0454f13fb729" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ContractsReceivableGross_10908cea-d7f6-41e7-9c99-2af20b92b1a7" xlink:href="iclr-20210930.xsd#iclr_ContractsReceivableGross"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableGrossCurrent_e73a4ed7-b123-4cc8-8e6a-7350d3893ef8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsReceivableGrossCurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_ContractsReceivableGross_10908cea-d7f6-41e7-9c99-2af20b92b1a7" xlink:to="loc_us-gaap_AccountsReceivableGrossCurrent_e73a4ed7-b123-4cc8-8e6a-7350d3893ef8" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_d4ade8e1-d586-4e62-ae56-2e825475eb95" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetGrossCurrent"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_ContractsReceivableGross_10908cea-d7f6-41e7-9c99-2af20b92b1a7" xlink:to="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_d4ade8e1-d586-4e62-ae56-2e825475eb95" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_AdvancePaymentsNettedAgainstUnbilledContractsReceivable_d4da695b-7843-4d69-8666-fd5c0e9a1c5f" xlink:href="iclr-20210930.xsd#iclr_AdvancePaymentsNettedAgainstUnbilledContractsReceivable"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_d222d570-44ad-40d7-b3a8-d7179825fa0d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetGrossCurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_AdvancePaymentsNettedAgainstUnbilledContractsReceivable_d4da695b-7843-4d69-8666-fd5c0e9a1c5f" xlink:to="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_d222d570-44ad-40d7-b3a8-d7179825fa0d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_730120e9-1837-4d5f-a407-00ada980d537" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_AdvancePaymentsNettedAgainstUnbilledContractsReceivable_d4da695b-7843-4d69-8666-fd5c0e9a1c5f" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_730120e9-1837-4d5f-a407-00ada980d537" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable_30e2a967-2bb4-4335-b91a-c16105697c73" xlink:href="iclr-20210930.xsd#iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent_a714ff77-b564-4c62-aafb-2222860e70c7" xlink:href="iclr-20210930.xsd#iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable_30e2a967-2bb4-4335-b91a-c16105697c73" xlink:to="loc_iclr_IncreaseDecreaseInContractwithCustomerAssetGrossCurrent_a714ff77-b564-4c62-aafb-2222860e70c7" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncreaseDecreaseInContractWithCustomerLiabilityCurrent_dd3b65ae-eb84-456d-a611-c19145b3019a" xlink:href="iclr-20210930.xsd#iclr_IncreaseDecreaseInContractWithCustomerLiabilityCurrent"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_iclr_ChangeinAdvancePaymentsNettedAgainstUnbilledContractsReceivable_30e2a967-2bb4-4335-b91a-c16105697c73" xlink:to="loc_iclr_IncreaseDecreaseInContractWithCustomerLiabilityCurrent_dd3b65ae-eb84-456d-a611-c19145b3019a" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IntangibleassetsDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#IntangibleassetsDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/IntangibleassetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_f2f1e289-f69d-45ef-881e-34901b0d5c2c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_d7f130fa-3c8a-44b6-ab99-76d2b296658f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNet_f2f1e289-f69d-45ef-881e-34901b0d5c2c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_d7f130fa-3c8a-44b6-ab99-76d2b296658f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_5a0de33a-e232-4bd2-9d7c-b09064a3a4bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNet_f2f1e289-f69d-45ef-881e-34901b0d5c2c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_5a0de33a-e232-4bd2-9d7c-b09064a3a4bc" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsNarrativeDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#BusinesscombinationsNarrativeDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_5a19e008-401b-420c-a75b-9a1e9a5b6272" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_Goodwill_5a19e008-401b-420c-a75b-9a1e9a5b6272" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_d29f6aaf-5f66-4ccc-8666-7694b3465666" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_d29f6aaf-5f66-4ccc-8666-7694b3465666" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_5fc5d3bc-3ac1-4f43-8efa-231bee0e57b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_5fc5d3bc-3ac1-4f43-8efa-231bee0e57b8" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_1c4a552c-5324-42c3-960b-4bf6304098f1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_1c4a552c-5324-42c3-960b-4bf6304098f1" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_dd905f5e-49bd-4589-a020-41e3e612e25e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:calculationArc order="5" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_dd905f5e-49bd-4589-a020-41e3e612e25e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_8540b28d-0b65-40d7-8103-8ea6994a39b9" xlink:href="iclr-20210930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability"/>
    <link:calculationArc order="6" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_8540b28d-0b65-40d7-8103-8ea6994a39b9" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_70f63edf-aaa9-4f9a-b446-fcfd84f75d90" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:calculationArc order="7" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_70f63edf-aaa9-4f9a-b446-fcfd84f75d90" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_1a03a77b-7e7e-40b6-9cea-0ff3f9f52eb6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:calculationArc order="8" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_1a03a77b-7e7e-40b6-9cea-0ff3f9f52eb6" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_1fdf9442-a5e2-4b73-af05-b4ae1c33eb74" xlink:href="iclr-20210930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability"/>
    <link:calculationArc order="9" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_1fdf9442-a5e2-4b73-af05-b4ae1c33eb74" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_39af36d3-f80c-4351-842f-ff7685f39876" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:calculationArc order="10" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_39af36d3-f80c-4351-842f-ff7685f39876" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_ecbfd3cc-f872-41e9-8bf0-cf9959e04913" xlink:href="iclr-20210930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent"/>
    <link:calculationArc order="11" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent_ecbfd3cc-f872-41e9-8bf0-cf9959e04913" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_7a115fda-8f14-4b21-8008-dc16b88176d6" xlink:href="iclr-20210930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:calculationArc order="12" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_7a115fda-8f14-4b21-8008-dc16b88176d6" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_76187535-2626-42f2-b89d-f9742ab8d655" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:calculationArc order="13" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_3a885c6d-22c5-4324-be45-a931347ae4b6" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_76187535-2626-42f2-b89d-f9742ab8d655" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#BusinesscombinationsFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_369ec2df-bb1e-4d55-ae71-d0d47bf93a50" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_SharesIssuedSharesShareBasedPaymentArrangementAcquisitions_36c3532c-e84a-4009-bfe1-2ba2ec6bb33e" xlink:href="iclr-20210930.xsd#iclr_SharesIssuedSharesShareBasedPaymentArrangementAcquisitions"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferred1_369ec2df-bb1e-4d55-ae71-d0d47bf93a50" xlink:to="loc_iclr_SharesIssuedSharesShareBasedPaymentArrangementAcquisitions_36c3532c-e84a-4009-bfe1-2ba2ec6bb33e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_bf0412bf-9790-46c6-a7a0-305415a3cb12" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferred1_369ec2df-bb1e-4d55-ae71-d0d47bf93a50" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_bf0412bf-9790-46c6-a7a0-305415a3cb12" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_add26dda-01dd-4aff-aaac-592f7657c14c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferred1_369ec2df-bb1e-4d55-ae71-d0d47bf93a50" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_add26dda-01dd-4aff-aaac-592f7657c14c" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_bce78b47-112d-4bde-93a5-199b5bc92261" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferred1_369ec2df-bb1e-4d55-ae71-d0d47bf93a50" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_bce78b47-112d-4bde-93a5-199b5bc92261" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_0cdb0047-3b94-42a8-a366-60e01fed85e9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue"/>
    <link:calculationArc order="1" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_0cdb0047-3b94-42a8-a366-60e01fed85e9" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_962ca5e2-7f41-4c96-9ea9-e79dbe47c1a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_962ca5e2-7f41-4c96-9ea9-e79dbe47c1a8" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c1b157d6-d6ce-4efc-803d-58a95dcd1be8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_Goodwill_c1b157d6-d6ce-4efc-803d-58a95dcd1be8" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_a690e3a8-e241-458c-8ed3-25ae9019829a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:calculationArc order="4" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_a690e3a8-e241-458c-8ed3-25ae9019829a" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_fbdf469e-0253-42ae-b914-1324c4a48667" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_fbdf469e-0253-42ae-b914-1324c4a48667" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_f8c244ca-4468-47f5-ac0d-35c4c31a1182" xlink:href="iclr-20210930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:calculationArc order="6" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_f8c244ca-4468-47f5-ac0d-35c4c31a1182" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_cc39bc3d-0771-4eff-82e8-6f3a6231395f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:calculationArc order="7" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_cc39bc3d-0771-4eff-82e8-6f3a6231395f" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_f006b465-1d5d-41a3-a6ba-715c87f09a42" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:calculationArc order="8" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_f006b465-1d5d-41a3-a6ba-715c87f09a42" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_40fc55e9-2c52-4b45-8c49-50414bd5d723" xlink:href="iclr-20210930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability"/>
    <link:calculationArc order="9" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability_40fc55e9-2c52-4b45-8c49-50414bd5d723" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_ad8b040a-ad33-4bf9-8aec-e22d33a02e61" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther"/>
    <link:calculationArc order="10" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_ad8b040a-ad33-4bf9-8aec-e22d33a02e61" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_1920099e-bdba-4115-a050-a2dbf9b0952c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets"/>
    <link:calculationArc order="11" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_1920099e-bdba-4115-a050-a2dbf9b0952c" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_3b82f339-1cd7-4119-af95-ec707437d178" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:calculationArc order="12" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_3b82f339-1cd7-4119-af95-ec707437d178" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_c643b5e0-9e50-4f67-aa74-5cbccf22e07d" xlink:href="iclr-20210930.xsd#iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability"/>
    <link:calculationArc order="13" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability_c643b5e0-9e50-4f67-aa74-5cbccf22e07d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_4945a94e-b13e-4fe5-bf65-a6f57eb11fcb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets"/>
    <link:calculationArc order="14" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_4945a94e-b13e-4fe5-bf65-a6f57eb11fcb" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_634cfe43-ae1c-4ec3-8590-abe881cb8297" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:calculationArc order="15" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_634cfe43-ae1c-4ec3-8590-abe881cb8297" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_2eeaf5ce-3a4d-4dee-a06e-45fb22a7bd21" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:calculationArc order="16" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_71f6d9d2-3f5f-493d-8225-3ec73759e354" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_2eeaf5ce-3a4d-4dee-a06e-45fb22a7bd21" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesLeaseCostsDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#OperatingleasesLeaseCostsDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/OperatingleasesLeaseCostsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_38071895-d5ee-4939-a1eb-978c4eb1ca51" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseCost"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseCost_4d4bb06a-d3e4-4027-b22e-143548c38b59" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseCost"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LeaseCost_38071895-d5ee-4939-a1eb-978c4eb1ca51" xlink:to="loc_us-gaap_OperatingLeaseCost_4d4bb06a-d3e4-4027-b22e-143548c38b59" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubleaseIncome_84e5df75-35c5-41dc-88b8-564ea4a6a356" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubleaseIncome"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LeaseCost_38071895-d5ee-4939-a1eb-978c4eb1ca51" xlink:to="loc_us-gaap_SubleaseIncome_84e5df75-35c5-41dc-88b8-564ea4a6a356" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesOperatingLeaseMaturityDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#OperatingleasesOperatingLeaseMaturityDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/OperatingleasesOperatingLeaseMaturityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_bc5abcc2-da7a-4bf9-bb6f-4f23ab6a3c7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo_6ef0015b-147e-4a19-a27a-9e425a9ba3b7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_bc5abcc2-da7a-4bf9-bb6f-4f23ab6a3c7a" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo_6ef0015b-147e-4a19-a27a-9e425a9ba3b7" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour_3eb2f800-2c7c-411f-902b-11efd3b3fce4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_bc5abcc2-da7a-4bf9-bb6f-4f23ab6a3c7a" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour_3eb2f800-2c7c-411f-902b-11efd3b3fce4" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFive_76e2a306-d73a-498b-8f84-35dfd23beb6a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFive"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_bc5abcc2-da7a-4bf9-bb6f-4f23ab6a3c7a" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFive_76e2a306-d73a-498b-8f84-35dfd23beb6a" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths_a8e5c4ec-a0f8-4335-9e5d-b2c257932d1e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_bc5abcc2-da7a-4bf9-bb6f-4f23ab6a3c7a" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths_a8e5c4ec-a0f8-4335-9e5d-b2c257932d1e" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree_d5df0a66-5388-4cc6-a2ea-848e76596995" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_bc5abcc2-da7a-4bf9-bb6f-4f23ab6a3c7a" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree_d5df0a66-5388-4cc6-a2ea-848e76596995" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive_8adde56d-97bd-4dbe-ac5b-c6487c0df613" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive"/>
    <link:calculationArc order="6" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_bc5abcc2-da7a-4bf9-bb6f-4f23ab6a3c7a" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive_8adde56d-97bd-4dbe-ac5b-c6487c0df613" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/OperatingleasesOperatingLeaseMaturityDetails_1" xlink:type="simple" xlink:href="iclr-20210930.xsd#OperatingleasesOperatingLeaseMaturityDetails_1"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/OperatingleasesOperatingLeaseMaturityDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_33ab7b70-6ae9-4694-920b-e3bceae5ff8f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount_543eaeab-ef0d-41e9-82a0-45e366a1b565" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_33ab7b70-6ae9-4694-920b-e3bceae5ff8f" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount_543eaeab-ef0d-41e9-82a0-45e366a1b565" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_bfd83612-fda7-4ac4-88e7-1b276260e139" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_33ab7b70-6ae9-4694-920b-e3bceae5ff8f" xlink:to="loc_us-gaap_OperatingLeaseLiability_bfd83612-fda7-4ac4-88e7-1b276260e139" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#NoncurrentbankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_81672e9b-8756-4a6f-b753-8af6fe240d3f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_44a0b3b0-ae36-4d16-b74e-73ad5da2ca0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_DebtInstrumentCarryingAmount_81672e9b-8756-4a6f-b753-8af6fe240d3f" xlink:to="loc_us-gaap_LongTermDebtCurrent_44a0b3b0-ae36-4d16-b74e-73ad5da2ca0a" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_f5458d72-493b-402f-becd-b63d8f2ef724" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_DebtInstrumentCarryingAmount_81672e9b-8756-4a6f-b753-8af6fe240d3f" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_f5458d72-493b-402f-becd-b63d8f2ef724" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_ef601f92-cd9f-44b4-ad01-4406f94a14ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebt"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_d3f3bd85-9f0b-4fda-9bc5-88acb8fde647" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LongTermDebt_ef601f92-cd9f-44b4-ad01-4406f94a14ed" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_d3f3bd85-9f0b-4fda-9bc5-88acb8fde647" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_e05e1ccc-1fe4-43c3-8b59-1d3e0ae82428" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_LongTermDebt_ef601f92-cd9f-44b4-ad01-4406f94a14ed" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_e05e1ccc-1fe4-43c3-8b59-1d3e0ae82428" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesScheduleofContractualMaturitiesofDebtDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#NoncurrentbankcreditlinesandloanfacilitiesScheduleofContractualMaturitiesofDebtDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/NoncurrentbankcreditlinesandloanfacilitiesScheduleofContractualMaturitiesofDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_ca361fb6-f6a9-4b95-a7f2-227fa32d26aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_ea65130d-90fc-42bc-b0be-f5a662c0e045" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_DebtInstrumentCarryingAmount_ca361fb6-f6a9-4b95-a7f2-227fa32d26aa" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_ea65130d-90fc-42bc-b0be-f5a662c0e045" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_e65479cd-4053-491d-9b1b-20345baae912" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_DebtInstrumentCarryingAmount_ca361fb6-f6a9-4b95-a7f2-227fa32d26aa" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_e65479cd-4053-491d-9b1b-20345baae912" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_LongTermDebtMaturityYearFourAndThereafter_38a093ce-6fa1-4fc4-8820-17ce9c471997" xlink:href="iclr-20210930.xsd#iclr_LongTermDebtMaturityYearFourAndThereafter"/>
    <link:calculationArc order="3" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_DebtInstrumentCarryingAmount_ca361fb6-f6a9-4b95-a7f2-227fa32d26aa" xlink:to="loc_iclr_LongTermDebtMaturityYearFourAndThereafter_38a093ce-6fa1-4fc4-8820-17ce9c471997" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_fe151179-bead-4b52-88df-378bff85e9b2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear"/>
    <link:calculationArc order="4" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_DebtInstrumentCarryingAmount_ca361fb6-f6a9-4b95-a7f2-227fa32d26aa" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_fe151179-bead-4b52-88df-378bff85e9b2" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_e5465a31-3a42-422f-a128-1a6bc9205cd8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree"/>
    <link:calculationArc order="5" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_DebtInstrumentCarryingAmount_ca361fb6-f6a9-4b95-a7f2-227fa32d26aa" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_e5465a31-3a42-422f-a128-1a6bc9205cd8" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/IncometaxesIncomeTaxesRecognizedDuringPeriodDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#IncometaxesIncomeTaxesRecognizedDuringPeriodDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/IncometaxesIncomeTaxesRecognizedDuringPeriodDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_aa4976b3-9464-4bc8-94fd-dd1b1bef3a90" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherTaxExpenseBenefit_67dcf2cf-b4ba-4b06-904f-adf919ea0c62" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherTaxExpenseBenefit"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefit_aa4976b3-9464-4bc8-94fd-dd1b1bef3a90" xlink:to="loc_us-gaap_OtherTaxExpenseBenefit_67dcf2cf-b4ba-4b06-904f-adf919ea0c62" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_iclr_IncomeTaxExpenseBenefitExcludingOtherTaxExpenseBenefit_960cf13c-8162-4d8d-9cfb-861c666cba43" xlink:href="iclr-20210930.xsd#iclr_IncomeTaxExpenseBenefitExcludingOtherTaxExpenseBenefit"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefit_aa4976b3-9464-4bc8-94fd-dd1b1bef3a90" xlink:to="loc_iclr_IncomeTaxExpenseBenefitExcludingOtherTaxExpenseBenefit_960cf13c-8162-4d8d-9cfb-861c666cba43" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#NetincomeperordinaryshareReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_843954f4-c66f-4c09-a3c4-f1dfcb9a3b1a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_22b3de06-2c20-4865-9d97-43dce7764b33" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_843954f4-c66f-4c09-a3c4-f1dfcb9a3b1a" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_22b3de06-2c20-4865-9d97-43dce7764b33" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_fa71a94e-532e-447d-8201-f4f91531accd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment"/>
    <link:calculationArc order="2" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_843954f4-c66f-4c09-a3c4-f1dfcb9a3b1a" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_fa71a94e-532e-447d-8201-f4f91531accd" xlink:type="arc"/>
  </link:calculationLink>
  <link:roleRef roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNetIncomeAttributabletotheGroupandNetIncomeAttributableTotheGroupIncludingNCIRedemptionAmountDetails" xlink:type="simple" xlink:href="iclr-20210930.xsd#NetincomeperordinaryshareReconciliationofNetIncomeAttributabletotheGroupandNetIncomeAttributableTotheGroupIncludingNCIRedemptionAmountDetails"/>
  <link:calculationLink xlink:role="http://www.iconplc.com/role/NetincomeperordinaryshareReconciliationofNetIncomeAttributabletotheGroupandNetIncomeAttributableTotheGroupIncludingNCIRedemptionAmountDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_0f6f052f-e285-475d-b11f-d073977064ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_682ba934-874d-4592-aa7f-529a27e5ae3d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:calculationArc order="1" weight="1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_0f6f052f-e285-475d-b11f-d073977064ce" xlink:to="loc_us-gaap_NetIncomeLoss_682ba934-874d-4592-aa7f-529a27e5ae3d" xlink:type="arc"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityAccretionToRedemptionValueAdjustment_559dd948-d0ed-4de7-8c2d-48e24903b3b0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityAccretionToRedemptionValueAdjustment"/>
    <link:calculationArc order="2" weight="-1.0" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_0f6f052f-e285-475d-b11f-d073977064ce" xlink:to="loc_us-gaap_TemporaryEquityAccretionToRedemptionValueAdjustment_559dd948-d0ed-4de7-8c2d-48e24903b3b0" xlink:type="arc"/>
  </link:calculationLink>
</link:linkbase>
