diff --git a/hisat2.wdl b/hisat2.wdl
index a86214f91f910f5010923a04a70d12dd4fa525f7..f9a4bc599f0fc43ce759807e4c5f138f02e3aa20 100644
--- a/hisat2.wdl
+++ b/hisat2.wdl
@@ -81,7 +81,7 @@ task Hisat2 {
     }
 
     runtime {
-        memory: "~{estimatedMemoryGb}G"
+        memory: "~{select_first([memoryGb, estimatedMemoryGb])}G"
         cpu: threads
         time_minutes: timeMinutes
         docker: dockerImage