diff --git a/utils/src/format.h b/utils/src/format.h
index 3093150b..9bc496b3 100644
--- a/utils/src/format.h
+++ b/utils/src/format.h
@@ -60,16 +60,23 @@ 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.
  */
 #define SCOUTFS_BTREE_MAX_HEIGHT 20
-
-/* btree blocks (beyond the first) need to be at least half full */
-#define SCOUTFS_BTREE_FREE_LIMIT \
-	((SCOUTFS_BLOCK_SIZE - sizeof(struct scoutfs_btree_block)) / 2)
-
 #define SCOUTFS_BTREE_BITS 8
 
 /*
diff --git a/utils/src/mkfs.c b/utils/src/mkfs.c
index 943ffd8d..4a277c3b 100644
--- a/utils/src/mkfs.c
+++ b/utils/src/mkfs.c
@@ -68,13 +68,15 @@ static u64 calc_btree_blocks(u64 nr, u64 max_key, u64 max_val)
 	item_bytes = sizeof(struct scoutfs_btree_item_header) +
 			    sizeof(struct scoutfs_btree_item) +
 			    max_key + sizeof(struct scoutfs_btree_ref);
-	fanout = (SCOUTFS_BLOCK_SIZE - SCOUTFS_BTREE_FREE_LIMIT) / item_bytes;
+	fanout = ((SCOUTFS_BLOCK_SIZE - sizeof(struct scoutfs_btree_block) -
+		   SCOUTFS_BTREE_PARENT_MIN_FREE_BYTES) / 2) / item_bytes;
 
 	/* figure out how many items we have to store */
 	item_bytes = sizeof(struct scoutfs_btree_item_header) +
 			    sizeof(struct scoutfs_btree_item) +
 			    max_key + max_val;
-	block_items = (SCOUTFS_BLOCK_SIZE - SCOUTFS_BTREE_FREE_LIMIT) / item_bytes;
+	block_items = ((SCOUTFS_BLOCK_SIZE -
+		       sizeof(struct scoutfs_btree_block)) / 2) / item_bytes;
 	leaf_blocks = DIV_ROUND_UP(nr, block_items);
 
 	/* then calc total blocks as we grow to have enough blocks for items */