diff --git a/cache.h b/cache.h
index 2214d52f61..af4b2c78cf 100644
--- a/cache.h
+++ b/cache.h
@@ -66,8 +66,12 @@ unsigned long git_deflate_bound(git_zstream *, unsigned long);
 #define GIT_SHA1_RAWSZ 20
 #define GIT_SHA1_HEXSZ (2 * GIT_SHA1_RAWSZ)
 
+/* The length in byte and in hex digits of the largest possible hash value. */
+#define GIT_MAX_RAWSZ GIT_SHA1_RAWSZ
+#define GIT_MAX_HEXSZ GIT_SHA1_HEXSZ
+
 struct object_id {
-	unsigned char hash[GIT_SHA1_RAWSZ];
+	unsigned char hash[GIT_MAX_RAWSZ];
 };
 
 #if defined(DT_UNKNOWN) && !defined(NO_D_TYPE_IN_DIRENT)