diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index d5f23a37f..7167e96f1 100755
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -407,6 +407,7 @@ int main(int argc, char ** argv)
 
                 env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmp;
                 env["NIX_STORE"] = store->storeDir;
+                env["NIX_BUILD_CORES"] = settings.buildCores;
 
                 auto passAsFile = tokenizeString<StringSet>(get(drv.env, "passAsFile", ""));