Ounce (oz)

An ounce is the small unit of weight in the US customary and British imperial systems. Americans use it for cheese, coffee, meat and postage, while most other countries weigh the same things in grams.

The name comes from the Latin uncia, "a twelfth part," because the Roman pound was divided into twelve unciae (NIST Handbook 44, Appendix B). The pound used for everyday goods, called the avoirdupois pound, is split into sixteenths instead.

Since 1959, the pound has been fixed in kilograms under an agreement of English-speaking countries (NIST). Because the ounce is a set fraction of that pound, it has an exact metric value as well.

In US recipes, an ounce of a dry ingredient means weight, while liquids are usually given in fluid ounces and measured in a cup. The two agree only for water and liquids like it: a fluid ounce of water weighs 1.04 ounces.

For other foods the numbers split apart. A cup holds 8 fluid ounces, but USDA data puts a cup of all-purpose flour at about 4.4 ounces by weight. If a recipe gives flour, sugar or chocolate in ounces, reach for a kitchen scale.

A quick test helps. If the recipe says "fl oz" or the ingredient pours, use a measuring cup; if it says "oz" next to something you scoop or chop, weigh it.

Federal rules make US labels say which ounce they mean. Weight is stated as net weight in ounces and pounds, while liquids are stated in fluid ounces, pints or quarts (21 CFR 101.7). So "net wt 16 oz" on a bag of rice means 1 pound of rice, while "16 fl oz" on a bottle means 1 pint of liquid.

The Nutrition Facts panel rounds for simplicity: for nutrition labeling, one ounce of weight counts as 28 g (21 CFR 101.9). The true figure is 28.35 g, so the difference is tiny.