Ounce (oz)
The ounce is a unit of mass used primarily in the United States and United Kingdom. Abbreviated as "oz" from the medieval Italian onza, one avoirdupois ounce equals exactly 28.349523125 grams. From food packaging and postal rates to cooking recipes and precious metals trading, the ounce appears across a wide range of everyday and professional contexts.
Definition
One avoirdupois ounce equals exactly 28.349523125 grams, or one-sixteenth of an avoirdupois pound. It should not be confused with the troy ounce (31.1035 grams), used for precious metals, or the fluid ounce, which measures volume rather than mass. In countries using the metric system, the ounce has no legal standing but is still encountered on imported products.
History
The word "ounce" derives from the Latin uncia, meaning one-twelfth, because the Roman pound (libra) was divided into twelve unciae. When medieval Europe adopted the avoirdupois system for general trade, the pound was redefined as 16 ounces rather than 12, but the name stuck. The troy ounce, still used for precious metals, preserved the older 12-per-pound ratio. In 1959, the International Yard and Pound Agreement standardised the avoirdupois ounce at exactly 28.349523125 grams by defining the pound as 0.45359237 kilograms and dividing by 16.
Common Uses
In the United States, food nutrition labels list serving sizes in ounces, postal services price letters by the ounce, and recipes frequently call for ingredients measured in ounces. Bartenders pour drinks in fluid ounces. In the UK, ounces appear in older recipes and informal conversation, though metric labelling is legally required. Globally, the troy ounce remains the standard unit for quoting gold, silver, and platinum prices on commodity exchanges.
Did You Know? Facts About Ounce
- A standard slice of bread weighs about one ounce (28 grams).
- Gold is measured in troy ounces, which are heavier than regular ounces - one troy ounce equals 31.1 grams versus 28.35 grams.
- The phrase "every ounce of strength" dates back to at least the 14th century in English literature.
- A AAA battery weighs about 0.4 ounces (11.5 grams).
- The US Postal Service charges by the ounce for first-class letters, making it one of the few government services still using imperial units.