diff --git a/include/macros.h b/include/macros.h
index a987cbf..abb6b26 100644
--- a/include/macros.h
+++ b/include/macros.h
@@ -35,3 +35,4 @@
 #define BIT_SIZE(t) (CHAR_BIT * sizeof(t))
 #define BIT(n) (1 << (n))
 #define BITS(_start, _end) ((BIT(_end) - BIT(_start)) + BIT(_end))
+#define BIT_MASK(x, k) ((x) & ((1 << (k)) - 1))