diff --git a/Include/pymath.h b/Include/pymath.h
index f869724..63ca972 100644
--- a/Include/pymath.h
+++ b/Include/pymath.h
@@ -227,4 +227,12 @@
  * behavior. */
 #define _Py_InIntegralTypeRange(type, v) (_Py_IntegralTypeMin(type) <= v && v <= _Py_IntegralTypeMax(type))
 
+/* Return the smallest integer k such that n < 2**k, or 0 if n == 0.
+ * Equivalent to floor(log2(x))+1.  Also equivalent to: bitwidth_of_type -
+ * count_leading_zero_bits(x)
+ */
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(unsigned int) _Py_bit_length(unsigned long d);
+#endif
+
 #endif /* Py_PYMATH_H */