diff --git a/build.sh b/build.sh
index 6ef7cb81..21563a34 100755
--- a/build.sh
+++ b/build.sh
@@ -42,7 +42,7 @@ BUILD_ALL_GPU_ARCH=0
 #  FIXME: if INSTALL_PREFIX is not set, check PREFIX, then check
 #         CONDA_PREFIX, but there is no fallback from there!
 INSTALL_PREFIX=${INSTALL_PREFIX:=${PREFIX:=${CONDA_PREFIX}}}
-PARALLEL_LEVEL=${PARALLEL_LEVEL:=""}
+PARALLEL_LEVEL=${PARALLEL_LEVEL:=$(nproc)}
 
 function hasArg {
     (( ${NUMARGS} != 0 )) && (echo " ${ARGS} " | grep -q " $1 ")