diff --git a/include/haproxy/htx-t.h b/include/haproxy/htx-t.h
index 687e403c7..2ea6bc8b2 100644
--- a/include/haproxy/htx-t.h
+++ b/include/haproxy/htx-t.h
@@ -164,6 +164,12 @@ static forceinline char *hsl_show_flags(char *buf, size_t len, const char *delim
 #undef _
 }
 
+/* Overhead induced by HTX on buffers during transfers. In addition to the size
+ * of the HTX structure itself, and meta data for one block, another block is
+ * accounted to favored zero-copy xfer.
+ */
+#define HTX_BUF_OVERHEAD     (sizeof(struct htx) + 2 * sizeof(struct htx_blk))
+
 /* HTX flags.
  * Please also update the htx_show_flags() function below in case of changes.
  */
diff --git a/include/haproxy/htx.h b/include/haproxy/htx.h
index 59e885a67..c991c81e5 100644
--- a/include/haproxy/htx.h
+++ b/include/haproxy/htx.h
@@ -673,10 +673,10 @@ static inline size_t buf_room_for_htx_data(const struct buffer *buf)
 	size_t room;
 
 	room = b_room(buf);
-	if (room <= sizeof(struct htx) + 2 * sizeof(struct htx_blk))
+	if (room <= HTX_BUF_OVERHEAD)
 		room = 0;
 	else
-		room -= sizeof(struct htx) + 2 * sizeof(struct htx_blk);
+		room -= HTX_BUF_OVERHEAD;
 
 	return room;
 }