The term ounce is used for both a unit of weight and volume in the US and old British measurement systems, and as a unit of weight in the "troy" system used for precious metals. Today they're defined in terms of equivalent SI units, as follows:
- The avoirdupois (avv-wahr-doo-PWAH) ounce, or common ounce, is a unit of weight in the US system equal to a minuscule amount less than 28.35 gm in the standard SI system. It was also used in the old British system, but ceased being a legal unit of measure in 2000.
- The troy ounce is a unit of weight used only for measuring precious metals. It's equivalent to 31.1 gm.
- The fluid ounce is a unit of volume in both the current US and old British systems. They are similar but not identical in amount: A US fluid ounce is 29.574 ml, while the old British fluid ounce was 28.413 ml.
