diff --git a/Makefile b/Makefile
index e56396a..6483f53 100644
--- a/Makefile
+++ b/Makefile
@@ -1,15 +1,19 @@
 .PHONY: all
 all: gcd/bin/gcd format_bytes/bin/format_bytes format_time/bin/format_time format_time/bin/format_time_ns
 
+.NOTPARALLEL:
 format_time/bin/format_time:
 	@cd format_time && exec make -s
 
+.NOTPARALLEL:
 format_time/bin/format_time_ns:
 	@cd format_time && exec make -s
 
+.NOTPARALLEL:
 format_bytes/bin/format_bytes:
 	@cd format_bytes && exec make -s
 
+.NOTPARALLEL:
 gcd/bin/gcd:
 	@cd gcd && exec make -s