diff --git a/releasetools/release.sh b/releasetools/release.sh
index 8a7931ccc..cbe1fcbb2 100755
--- a/releasetools/release.sh
+++ b/releasetools/release.sh
@@ -358,7 +358,7 @@ mount $TMPDISKUSR $RELEASEMNTDIR/usr || exit
 
 echo " * Copying files from staging to image"
 synctree -f $RELEASEDIR $RELEASEMNTDIR > /dev/null || true
-expr `df $TMPDISKUSR | tail -1 | awk '{ print $4 }'` - $extrakb >$RELEASEMNTDIR/.usrkb
+expr `df -k $TMPDISKUSR | tail -1 | awk '{ print $4 }'` - $extrakb >$RELEASEMNTDIR/.usrkb
 
 echo " * Unmounting $TMPDISKUSR from $RELEASEMNTDIR/usr"
 umount $TMPDISKUSR || exit