Ounce (oz)

The ounce is a unit of mass equal to one sixteenth of an avoirdupois pound, or exactly 28.349523125 grams. Its symbol is oz, from the Italian onza. It is the everyday small weight of the United States and remains in common use in the United Kingdom alongside metric units.

There are several ounces, and confusing them is easy. The avoirdupois ounce described here is the one used for food and general goods. The troy ounce, used for gold, silver and platinum, is about 31.103 grams, some ten per cent heavier, which is why a troy pound of twelve ounces weighs less than an avoirdupois pound of sixteen. The fluid ounce is not a mass at all but a volume, and the imperial and US fluid ounces differ from each other by about four per cent.

American food packaging is built on the ounce. Net weights appear in ounces below sixteen and in pounds and ounces above, and recipe quantities for cheese, chocolate, pasta and meat are given the same way. Postal rates for letters step in ounces, and boxing, wrestling and other weight-class sports state limits in pounds and ounces.

The word comes from the Latin uncia, meaning a twelfth part, which is also the ancestor of the inch. That original twelfth survives in the troy ounce but not in the avoirdupois one, whose sixteenth-of-a-pound relationship came later from the wool trade of medieval Europe. Two units with the same name and different arithmetic is the direct result.

Precious metals are the other ounce's territory. Gold, silver, platinum and palladium are priced per troy ounce of 31.1035 grams, about ten per cent heavier than the avoirdupois ounce used for groceries, and every quoted bullion price in the world refers to it. A one-ounce gold coin is therefore heavier than an ounce of butter, and a bar described as a kilogram contains 32.15 troy ounces rather than 35.27 ordinary ones. The troy system also keeps its own subdivisions, with 20 pennyweights to the ounce, which is why a jeweller's scale and a kitchen scale are calibrated differently even when both are marked in ounces.

One ounce equals 28.349523125 grams exactly, one sixteenth of a pound, or 437.5 grains.

Ounce conversion table