diff --git a/sys/sys/limits.h b/sys/sys/limits.h
--- a/sys/sys/limits.h
+++ b/sys/sys/limits.h
@@ -90,6 +90,26 @@
 #define	QUAD_MIN	(__QUAD_MIN)	/* min value for a quad_t */
 #endif
 
+/*
+ * Programmatically determine the maximum value of
+ * an unsigned and signed type, without integer overflow.
+ *
+ * After determining the byte-size of the type,
+ * e.g. 2 for uint16_t, set the left-most bit. (0x8000)
+ * Set all bits but the highest by subtracting one. (0x7FFF)
+ * Shift all set bits left by one by way of muliply by 2. (0xFFFE)
+ * Finally, set the lowest bit by adding one. (0xFFFF)
+ *
+ * This is a portable way to programmatically determine
+ * the maximum value of an unsigned variable of arbitrary size.
+ */
+#define UTYPE_MAX(x) \
+       ((((1ULL << (sizeof((x)) * 8 - 1)) - 1) * 2) + 1)
+
+#define TYPE_MAX(x) \
+       ((((1ULL << (sizeof((x)) * 8 - 2)) - 1) * 2) + 1)
+
+
 #if __XSI_VISIBLE || __POSIX_VISIBLE >= 200809
 #define	LONG_BIT	__LONG_BIT
 #define	WORD_BIT	__WORD_BIT