Index: sys/sys/bitstring.h
===================================================================
--- sys/sys/bitstring.h
+++ sys/sys/bitstring.h
@@ -75,6 +75,13 @@
 #define	_BITSTR_MASK (~0UL)
 #define	_BITSTR_BITS (sizeof(bitstr_t) * 8)
 
+
+#ifdef roundup2
+#define        _bit_roundup2 roundup2
+#else
+#define        _bit_roundup2(x, y)        (((x)+((y)-1))&(~((y)-1))) /* if y is powers of two */
+#endif
+
 /* bitstr_t in bit string containing the bit. */
 static inline int
 _bit_idx(int _bit)
@@ -104,9 +111,8 @@
 }
 
 /*----------------------------- Public Interface -----------------------------*/
-/* Number of bytes consumed by a bit string of nbits bits */
-#define	bitstr_size(_nbits) \
-	(((_nbits) + _BITSTR_BITS - 1) / 8)
+/* Number of bytes allocated for a bit string of nbits bits */
+#define	bitstr_size(_nbits) (_bit_roundup2(_nbits, _BITSTR_BITS) / 8)
 
 /* Allocate a bit string initialized with no bits set. */
 #ifdef _KERNEL
@@ -123,7 +129,7 @@
 }
 #endif
 
-/* Allocate a bit string on the stack with no bits set. */
+/* Allocate a bit string on the stack */
 #define	bit_decl(name, nbits) \
 	((name)[bitstr_size(nbits) / sizeof(bitstr_t)])
 
Index: tests/sys/sys/bitstring_test.c
===================================================================
--- tests/sys/sys/bitstring_test.c
+++ tests/sys/sys/bitstring_test.c
@@ -102,6 +102,17 @@
 	bit_nclear(test_struct.bitstr, 0, 8);
 }
 
+ATF_TC_WITHOUT_HEAD(bitstr_size);
+ATF_TC_BODY(bitstr_size, tc)
+{
+	size_t sob = sizeof(bitstr_t);
+
+	ATF_CHECK_EQ(0, bitstr_size(0));
+	ATF_CHECK_EQ(sob, bitstr_size(1));
+	ATF_CHECK_EQ(sob, bitstr_size(sob * 8));
+	ATF_CHECK_EQ(2 * sob, bitstr_size(sob * 8 + 1));
+}
+
 BITSTRING_TC_DEFINE(bit_set)
 /* bitstr_t *bitstr, int nbits, const char *memloc */
 {
@@ -407,6 +418,7 @@
 {
 
 	ATF_TP_ADD_TC(tp, bitstr_in_struct);
+	ATF_TP_ADD_TC(tp, bitstr_size);
 	BITSTRING_TC_ADD(tp, bit_set);
 	BITSTRING_TC_ADD(tp, bit_clear);
 	BITSTRING_TC_ADD(tp, bit_ffs);