diff --git a/kmod/src/btree.c b/kmod/src/btree.c
index a64e6337..32421239 100644
--- a/kmod/src/btree.c
+++ b/kmod/src/btree.c
@@ -169,13 +169,6 @@ static inline unsigned int all_len_bytes(unsigned key_len, unsigned val_len)
 		len_bytes(key_len, val_len);
 }
 
-/* number of bytes needed to insert potentially max size child item */
-static inline unsigned int parent_min_free_bytes(void)
-{
-	return all_len_bytes(SCOUTFS_BTREE_MAX_KEY_LEN,
-			     sizeof(struct scoutfs_btree_ref));
-}
-
 /*
  * The minimum number of bytes we allow in a block.  During descent to
  * modify if we see a block with fewer used bytes then we'll try to
@@ -196,7 +189,7 @@ static inline unsigned int parent_min_free_bytes(void)
 static inline unsigned int min_used_bytes(void)
 {
 	return (SCOUTFS_BLOCK_SIZE - sizeof(struct scoutfs_btree_block) -
-		parent_min_free_bytes()) / 2;
+		SCOUTFS_BTREE_PARENT_MIN_FREE_BYTES) / 2;
 }
 
 /* total block bytes used by an existing item */
@@ -986,7 +979,7 @@ static int try_split(struct super_block *sb, struct scoutfs_btree_root *root,
 	int ret;
 
 	if (right->level)
-		all_bytes = parent_min_free_bytes();
+		all_bytes = SCOUTFS_BTREE_PARENT_MIN_FREE_BYTES;
 	else
 		all_bytes = all_len_bytes(key_len, val_len);
 
diff --git a/kmod/src/format.h b/kmod/src/format.h
index 89b95ff1..9bc496b3 100644
--- a/kmod/src/format.h
+++ b/kmod/src/format.h
@@ -60,6 +60,18 @@ struct scoutfs_block_header {
 /* level 0 segments can have two full keys in the value :/ */
 #define SCOUTFS_BTREE_MAX_VAL_LEN 768
 
+/*
+ * The min number of free bytes we must leave in a parent as we descend
+ * to modify.  This leaves enough free bytes to insert a possibly maximal
+ * sized key as a seperator for a child block.  Fewer bytes then this
+ * and split/merge might try to insert a max child item in the parent
+ * that wouldn't fit.
+ */
+#define SCOUTFS_BTREE_PARENT_MIN_FREE_BYTES				\
+	(sizeof(struct scoutfs_btree_item_header) +			\
+	 sizeof(struct scoutfs_btree_item) + SCOUTFS_BTREE_MAX_KEY_LEN +\
+	 sizeof(struct scoutfs_btree_ref))
+
 /*
  * A 4EB test image measured a worst case height of 17.  This is plenty
  * generous.