The ounce (abbreviated oz, from the former Italian word onza, now spelled oncia; apothecary symbol: ℥) 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. The most commonly used ounces today are the international avoirdupois ounce and the international troy ounce.
Ounce derives from Latin uncia, a unit that was one twelfth (1/12) of the Roman pound (libra). Ounce was borrowed twice: first into Old English as ynsan or yndsan from an unattested Vulgar Latin form with ts for c before i (palatalization) and second into Middle English through Anglo-Norman and Middle French (unce, once, ounce).
Inch comes from the same Latin word, but differs because it was borrowed into Old English and underwent i-mutation or umlaut (u → y) and palatalization (k → ch).
Historically, in different parts of the world, at different points in time, and for different applications, the ounce (or its translation) has referred to broadly similar but different standards of mass.
The avoirdupois ounce is the most commonly used ounce today. It is defined to be one sixteenth of an avoirdupois pound. The avoirdupois pound is defined as 7000 grains; one ounce is therefore equal to 437.5 grains.
In the international yard and pound agreement of 1959 the United States and countries of the Commonwealth of Nations agreed to define the international avoirdupois ounce to be exactly kg (28.349523125 g) by definition.
The ounce is commonly used as a unit of mass in the United States.
On January 1, 2000, it ceased to be a legal unit of measure within the United Kingdom for economic, health, safety or administrative purposes.
A troy ounce (abbreviated as t oz) is equal to 480 grains. Consequently, the international troy ounce is equal to exactly 31.1034768 grams. There are 12 troy ounces in the now obsolete troy pound.
Today, the troy ounce is used only to express the mass of precious metals such as gold, platinum, palladium, rhodium or silver. Bullion coins are the most common products produced and marketed in troy ounces, but precious metal bars also exist in gram and kilogram (kg) sizes. (A kilogram bullion bar contains 32.15074657 troy ounces.)
For historical measurement of gold,
The obsolete apothecaries' ounce (abbreviated ) equivalent to the troy ounce, was formerly used by apothecaries (now called pharmacists or chemists).][
"Maria Theresa ounce" was once introduced in Ethiopia and some European countries, which was equal to the weight of one Maria Theresa thaler, or 28.0668 g. Both the weight and the value are the definition of one birr, still in use in present-day Ethiopia and formerly in Eritrea.][
The Spanish pound (Spanish libra) was 460 g. The Spanish ounce (Spanish onza) was of a pound, i.e. 28.75 g.
The unit metric ounce is 25 grams and 20 make the metric pound of 500 grams.
Some countries have redefined their ounces in the metric system. For example, the German apothecaries ounce of 30 grams, is very close to the previously widespread Nuremberg ounce, but the divisions and multiples come out in metric.
In 1820, the Dutch redefined their ounce (in Dutch, ons) as 100 grams. Dutch amendments to the metric system, such as an ons or 100 grams, has been inherited, adopted, and taught in Indonesia beginning in elementary school. It is also listed as standard usage in Indonesia's national dictionary, the Kamus Besar Bahasa Indonesia, and the government's official elementary‐school curriculum.][
East Asia has a traditional ounce, known as a tael, of varying value. The katti of 16 tael, originally lb avoirdupois, is now 600 grams. The corresponding tael or tail is then 37.5 grams. In China, it has been given a metric value of 50 grams.][
An ounce-force is 1/16 of a pound-force, or 0.2780139 newton.
The "ounce" in "ounce-force" is equivalent to an avoirdupois ounce; ounce-force is a measurement of force using avoirdupois ounces. However, it is not necessary to identify it as such or to differentiate it in that way because there is no equivalent measure of force using troy or any other "ounce".
A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a unit of volume equal to about 28 ml in the imperial system or about 30 ml in the US system. The fluid ounce is sometimes referred to simply as an "ounce" in applications where its use is implicit. The imperial fluid ounce is also equivalent to the volume occupied by 1 imperial ounce of water weighed in air at 62 °F.
Ounces are also used to express the "weight", or more accurately the areal density, of a textile fabric in North America, Asia or the UK, as in "16 oz denim". The number refers to the weight in ounces of a given amount of fabric, either a yard of a given width, or a square yard.