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Avoirdupois ounce

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Title: Avoirdupois ounce  
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Avoirdupois ounce

This article is about the unit of mass. For the unit of force, see Pound-force. For the unit of volume, see Fluid ounce. For all other uses, see Ounce (disambiguation).

An ounce (abbreviated oz, from the former Italian word onza, now spelled oncia; apothecary symbol: ) is usually the international avoirdupois ounce as used in the United States customary and British imperial systems, which is equal to one sixteenth of a pound or approximately 28 grams. There are also other ounces which are used in a number of different systems and are of different sizes. The most commonly used other ounce is the international troy ounce.


Ounce derives from Latin uncia, a unit that was one twelfth (1/12)[1] 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).[2]

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.

Summary of ounce units
ounce variant equivalent in grams equivalent in grains
International avoirdupois ounce 28.349523125 437.5
International troy ounce 31.1034768 480
Apothecaries' ounce
Maria Theresa ounce 28.0668  
Spanish ounce (onza) 28.75  
French ounce (once) 30.59  
Portuguese ounce (onça) 28.69  
Roman/Italian ounce (oncia) 27.4  
Dutch metric ounce (ons) 100  
Chinese metric ounce (盎司) 50  

International avoirdupois ounce

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 0.4535923716 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.[3]

International troy ounce

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,

  • a fine ounce is a troy ounce of pure gold content in a gold bar, computed as fineness multiplied by gross weight[4]
  • a standard ounce is a troy ounce of 22 carat gold, 91.66% pure (an 11 to 1 proportion of gold to alloy material)
  • in modern day, an ounce of gold (1 troy ounce) is referred as a 99.99% pure gold piece or gold grains (gold shot)

Apothecaries' ounce

The obsolete apothecaries' ounce (abbreviated ) equivalent to the troy ounce, was formerly used by apothecaries (now called pharmacists or chemists).

Maria Theresa ounce

"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.[5][6] Both the weight and the value are the definition of one birr, still in use in present-day Ethiopia and formerly in Eritrea.

Spanish ounce

The Spanish pound (Spanish libra) was 460 g.[7] The Spanish ounce (Spanish onza) was 116 of a pound, i.e. 28.75 g.[8]

Metric ounces

The unit metric ounce is 25 grams[9] and 20 make the metric pound of 500 grams.

Some countries have redefined their ounces in the metric system.[10] 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.[11][12] 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.[13]


Main article: Pound-force

An ounce-force is 1/16 of a pound-force, or 0.2780139 newtons.

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".

Fluid ounce

Main article: Fluid 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.

Other uses

Fabric weight

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.[14][15]

Fabric type Typical weight in ounces
Organza, voile, chiffon 1-3
Most cottons, wools, silks, muslin, linen 4-7
Denim, corduroy, twill, velvet 7-16

Notes and references

External links

  • Dictionary of Units: Ounce
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