diff --git a/zkapauthorizer.nix b/zkapauthorizer.nix
index 3edef94512311e56a8399cfe9f1c796cf547c839..af01d190358822178fa6c008b5eabef3db67944b 100644
--- a/zkapauthorizer.nix
+++ b/zkapauthorizer.nix
@@ -10,7 +10,7 @@
 let
   hypothesisProfile' = if hypothesisProfile == null then "default" else hypothesisProfile;
   testSuite' = if testSuite == null then "_zkapauthorizer" else testSuite;
-  defaultTrialArgs = [ "--rterrors" ] ++ ( lib.optional ( ! collectCoverage ) "--jobs=4" );
+  defaultTrialArgs = [ "--rterrors" ] ++ ( lib.optional ( ! collectCoverage ) "--jobs=$NIX_BUILD_CORES" );
   trialArgs' = if trialArgs == null then defaultTrialArgs else trialArgs;
   extraTrialArgs = builtins.concatStringsSep " " trialArgs';
 in