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The ounce (abbreviated oz; apothecary symbol: ) is a unit of mass used in most British derived customary systems of measurement. It is most pervasive in the retail sale of groceries in the United States, but is also used in many other matters of domestic and international trade between imperial or customary measurement driven countries. Similar customary uses include recipes in cookbooks and sales of bulk dry goods.

Whilst various definitions have been used throughout history, two remain in common use: the avoirdupois ounce equal to approximately 28.3 grams and the troy ounce of about 31.1 grams. The avoirdupois ounce is widely used as part of the United States customary and British imperial systems, but the troy ounce is now only commonly used for the mass of precious metals such as gold, silver, platinum, palladium, rhodium, etc..


Ounce derives from Latin uncia, a unit that was one-twelfth (112)[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] The abbreviation oz came later from the cognate Italian word onza (now spelled oncia).

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] → [tʃ]).


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.

Mass of ounce units
Variant (grams) (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  
English Tower Ounce 29.16 450

Currently in use

International avoirdupois ounce

The international avoirdupois ounce is defined as exactly 28.349523125 g under the international yard and pound agreement of 1959, signed by the United States and countries of the Commonwealth of Nations.

In the avoirdupois system, sixteen ounces make up an avoirdupois pound, and the avoirdupois pound is defined as 7000 grains; one avoirdupois ounce is therefore equal to 437.5 grains.

The ounce is still a standard unit in the United States, but in the United Kingdom it is now only used informally, having ceased to be a legal unit of measure in 2000.[3]

International troy ounce

A troy ounce 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)

Metric ounces

Some countries have redefined their ounces in the metric system.[5] 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.[6][7] 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.[8]


Apothecaries' ounce

The obsolete apothecaries' ounce (abbreviated ℥) equivalent to the troy ounce, was formerly used by apothecaries.

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.[9][10] Both the weight and the value are the definition of one birr, still in use in present-day Ethiopia and formerly in Eritrea.[citation needed]

Spanish ounce

The Spanish pound (Spanish libra) was 460 g.[11] The Spanish ounce (Spanish onza) was 116 of a pound, i.e. 28.75 g.[12]

Tower ounce

The Tower ounce of 450 grains was used in the English mints, the principal one being in the Tower of London. It dates back to the Anglo-Saxon coinage weight standard. It was abolished in favour of the Troy ounce by Henry VIII in 1527.[citation needed]


An ounce-force is 116 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

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a unit of volume equal to about 28.4 ml in the imperial system or about 29.6 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.[citation needed]

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.[13][14]

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

Copper layer thickness of a printed circuit board

The most common unit of measure for the copper thickness on a printed circuit board (PCB) is ounces (oz). It is the resulting thickness when 1 oz of copper is pressed flat and spread evenly over a one-square-foot area. This roughly equals 34.7 µm.[15]

Notes and references

  1. ^ uncia. Charlton T. Lewis and Charles Short. A Latin Dictionary on Perseus Project.
  2. ^ "ounce". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005.  (Subscription or UK public library membership required.)
  3. ^ "The Units of Measurement Regulations 1995 (Article 4)". 2000-09-20. Retrieved 2009-01-26. 
  4. ^ London Bullion Market Association. "Market Basics". 
  5. ^ Wittop Koning, D. A.; Houben, G. M. M. (1980). 2000 jaar gewichten in de nederlanden (in Dutch). Lochem-Poperinge: De Tijdstroom. ISBN 9060879651.  (Dutch)
  6. ^ "Guide to The Hague – Where to turn". Archived from the original on 2008-03-16. Retrieved 2008-01-01. 
  7. ^ nl:Nederlands metriek stelsel
  8. ^ Ons in KBBI
  9. ^ Greenfield, Richard (1965). Ethiopia: a new political history. F. A. Praeger. p. 327. 
  10. ^ Ethiopia observer. 6. 1962. pp. 187–8. 
  11. ^ Diccionario de la Real Academia Española, 23rd edition, libra
  12. ^ Diccionario de la Real Academia Española, 23rd edition, onza
  13. ^ "How to shop the fabric market". Retrieved 2008-12-10. 
  14. ^ "How Many? A Dictionary of Units of Measurement". Retrieved 2008-12-10. 
  15. ^ "Copper Thickness FAQ". Retrieved 2016-11-13. 

External links

  • Dictionary of Units: Ounce
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