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The ounce is a unit
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

 of mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 with several definitions, the most commonly used of which are equal to approximately 28 gram
The gram is a metric system unit of mass....

s. The ounce is used in a number of different systems, including various systems of mass that form part of the imperial
Imperial unit
The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire...

 and United States customary
United States customary units
United States customary units are a system of measurements commonly used in the United States. Many U.S. units are virtually identical to their imperial counterparts, but the U.S. customary system developed from English units used in the British Empire before the system of imperial units was...

 systems. Its size can vary from system to system. The most commonly used ounces today are the international avoirdupois ounce and the international troy ounce.


Ounce derives from Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 uncia, a unit that was one twelfth (1/12) of the Roman
Ancient Roman units of measurement
The ancient Roman units of measurement were built on the Hellenic system with Egyptian, Hebrew, and Mesopotamian influences. The Roman units were comparatively consistent and well documented.-Length:Notes...

 pound (libra). Ounce was borrowed twice: first into Old English as ynsan or yndsan from an unattested Vulgar Latin
Vulgar Latin
Vulgar Latin is any of the nonstandard forms of Latin from which the Romance languages developed. Because of its nonstandard nature, it had no official orthography. All written works used Classical Latin, with very few exceptions...

 form with ts for c before i (palatalization
In linguistics, palatalization , also palatization, may refer to two different processes by which a sound, usually a consonant, comes to be produced with the tongue in a position in the mouth near the palate....

) and second into Middle English
Middle English
Middle English is the stage in the history of the English language during the High and Late Middle Ages, or roughly during the four centuries between the late 11th and the late 15th century....

 through Anglo-Norman
The Anglo-Normans were mainly the descendants of the Normans who ruled England following the Norman conquest by William the Conqueror in 1066. A small number of Normans were already settled in England prior to the conquest...

 and Middle French
Middle French
Middle French is a historical division of the French language that covers the period from 1340 to 1611. It is a period of transition during which:...

 (unce, once, ounce).

An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

comes from the same Latin word, but is different because it was borrowed into Old English and underwent i-mutation or umlaut
Germanic umlaut
In linguistics, umlaut is a process whereby a vowel is pronounced more like a following vowel or semivowel. The term umlaut was originally coined and is used principally in connection with the study of the Germanic languages...

 (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 gram
The gram is a metric system unit of mass....

equivalent in grain
Grain (measure)
A grain is a unit of measurement of mass that is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever...

International avoirdupois ounce 28.3495231 437.5
International troy ounce 31.1034768 480
Apothecaries' ounce
Maria Theresa ounce 28.0668  
Spanish ounce 28.75  
Dutch metric ounce 100  
Chinese metric ounce 50  

International avoirdupois ounce

The avoirdupois
The avoirdupois system is a system of weights based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees by many people in Canada, the United Kingdom, and some other former British colonies despite the official adoption...

 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 1958 the United States and countries of the Commonwealth of Nations
Commonwealth of Nations
The Commonwealth of Nations, normally referred to as the Commonwealth and formerly known as the British Commonwealth, is an intergovernmental organisation of fifty-four independent member states...

 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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 for economic, health, safety or administrative purposes
but remains a familiar unit, especially amongst older people.

International troy ounce

A troy ounce
Troy ounce
The troy ounce is a unit of imperial measure. In the present day it is most commonly used to gauge the weight of precious metals. One troy ounce is nowadays defined as exactly 0.0311034768 kg = 31.1034768 g. There are approximately 32.1507466 troy oz in 1 kg...

 (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
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

, platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

, palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

 or silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

. 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 99.5% (".995") pure gold
  • a standard ounce is a troy ounce of 22 carat
    Carat (purity)
    The karat or carat is a unit of purity for gold alloys.- Measure :Karat purity is measured as 24 times the purity by mass:where...

     gold, 91.66% pure (an 11 to 1 proportion of gold to alloy
    An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

  • 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
Maria Theresa thaler
The Maria Theresa thaler is a silver bullion-coin that has been used in world trade continuously. Maria Theresa Thalers were first minted in 1741, using the then Reichsthaler standard of 9 thalers to the Vienna mark. In 1750 the thaler was debased to 10 thalers to the Vienna Mark...

, 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.

Spanish ounce

The Spanish pound (Spanish
Spanish language
Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...

 libra) was 460 g. The Spanish ounce (Spanish onza) was of a pound, i.e. 28.75 g.

Metric ounces

Some countries have redefined their ounces in the metric system
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...


In 1820, the Dutch redefined their ounce (in Dutch
Dutch language
Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

, ons) as 100 gram
The gram is a metric system unit of mass....

s. Dutch amendments to the metric system, such as an ons of 100 grams, has been inherited, adopted, and taught in Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...

 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
East Asia
East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms...

 has a traditional ounce, known as a tael
Tael can refer to any one of several weight measures of the Far East. Most commonly, it refers to the Chinese tael, a part of the Chinese system of weights and currency....

, of varying value. In China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, it has been given a metric value of 50 grams.


An ounce force is 1/16 of a pound-force, or 1/16 lbf. It is not necessary to identify it as an avoirdupois ounce; there is no troy ounce-force.

Fluid ounce

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a unit of volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

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.

Fabric weight

Ounces are also used to express the "weight", or more accurately 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.

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