diff --git a/clatd.h b/clatd.h
index 0f4809d..ca7369b 100644
--- a/clatd.h
+++ b/clatd.h
@@ -25,6 +25,8 @@
 #define PACKETLEN (MAXMTU+sizeof(struct tun_pi))
 #define CLATD_VERSION "1.3"
 
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+
 // how frequently (in seconds) to poll for an address change while traffic is passing
 #define INTERFACE_POLL_FREQUENCY 30