Minecraft wiki entry for Item_repair, associated text: Two items of the same type and material can be placed anywhere on the crafting grid or grindstone, which results in a single repaired item. The durability of the repaired item equals the sum of the old items' durability plus a 'repair bonus' of 5% of the item's maximum uses, up to a limit of the maximum durability for that item. Repairing gives a slight benefit in conserving inventory space, as it combines two non-stackable objects into one.[1]