diff --git a/bignum.c b/bignum.c
index 7548bd1d3..1bd65b605 100644
--- a/bignum.c
+++ b/bignum.c
@@ -182,6 +182,19 @@ void bn_read_uint64(uint64_t in_number, bignum256 *out_number)
 	out_number->val[8] = 0;
 }
 
+// a must be normalized
+int bn_bitcount(const bignum256 *a)
+{
+	int i;
+	for (i = 8; i >= 0; i--) {
+		int tmp = a->val[i];
+		if (tmp != 0) {
+			return i * 30 + (32 - __builtin_clz(tmp));
+		}
+	}
+	return 0;
+}
+
 // sets a bignum to zero.
 void bn_zero(bignum256 *a)
 {
diff --git a/bignum.h b/bignum.h
index 377bf1274..f0877c3e1 100644
--- a/bignum.h
+++ b/bignum.h
@@ -63,6 +63,8 @@ static inline void bn_copy(const bignum256 *a, bignum256 *b) {
 	*b = *a;
 }
 
+int bn_bitcount(const bignum256 *a);
+
 void bn_zero(bignum256 *a);
 
 int bn_is_zero(const bignum256 *a);
diff --git a/tests.c b/tests.c
index 804969bbf..b9b87ffc3 100644
--- a/tests.c
+++ b/tests.c
@@ -275,6 +275,24 @@ START_TEST(test_bignum_is_odd)
 }
 END_TEST
 
+START_TEST(test_bignum_bitcount)
+{
+	bignum256 a;
+
+	bn_zero(&a);
+	ck_assert_int_eq(bn_bitcount(&a), 0);
+
+	bn_read_uint32(0x3fffffff, &a);
+	ck_assert_int_eq(bn_bitcount(&a), 30);
+
+	bn_read_uint32(0xffffffff, &a);
+	ck_assert_int_eq(bn_bitcount(&a), 32);
+
+	bn_read_be(fromhex("ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"), &a);
+	ck_assert_int_eq(bn_bitcount(&a), 256);
+}
+END_TEST
+
 // from https://github.com/bitcoin/bitcoin/blob/master/src/test/data/base58_keys_valid.json
 START_TEST(test_base58)
 {
@@ -2532,6 +2550,7 @@ Suite *test_suite(void)
 	tcase_add_test(tc, test_bignum_copy);
 	tcase_add_test(tc, test_bignum_is_even);
 	tcase_add_test(tc, test_bignum_is_odd);
+	tcase_add_test(tc, test_bignum_bitcount);
 	suite_add_tcase(s, tc);
 
 	tc = tcase_create("base58");