diff --git a/kmod/src/format.h b/kmod/src/format.h
index 2124d970..71027e90 100644
--- a/kmod/src/format.h
+++ b/kmod/src/format.h
@@ -213,6 +213,7 @@ struct scoutfs_key {
 #define SCOUTFS_SYMLINK_KEY		8
 #define SCOUTFS_EXTENT_KEY		9
 #define SCOUTFS_ORPHAN_KEY		10
+#define SCOUTFS_MAX_UNUSED_KEY		255
 
 #define SCOUTFS_MAX_ITEM_LEN 512
 
@@ -441,4 +442,7 @@ struct scoutfs_link_backref {
 	__le64 offset;
 } __packed;
 
+#define SCOUTFS_MAX_KEY_SIZE \
+	offsetof(struct scoutfs_dirent_key, name[SCOUTFS_NAME_LEN])
+
 #endif