This article is about the unit of mass. For the unit of force, see Pound-force. For the unit of volume, see Fluid ounce. For all other uses, see Ounce (disambiguation). The ounce is a unit of mass with several definitions, the most commonly used of which is equal to approximately 28 grams. The ounce is used in a number of different systems, including those of mass that form part of the United States customary, and imperial, systems. The size of an ounce varies between systems.