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).
, the old Italian word onza
, now spelled oncia
) is a unit
in a number of different systems, including various systems of mass that form part of the imperial
and United States customary
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
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 (or weight, before the distinction between weight and mass
developed). Some of these other ounces are described below.
Summary of ounce units
| ounce variant
|| mass in grams
|| mass in grains |
|International avoirdupois ounce
|International troy ounce
|Apothecaries' ounce |
|Maria Theresa ounce
|Dutch metric ounce
|Chinese metric ounce
Note: The grain values for the Maria Theresa, Dutch and Chinese|
ounces are rounded to the nearest thousandth of a grain.
International avoirdupois ounce
ounce is the most commonly used ounce today. It is defined to be one sixteenth of an avoirdupois pound
. It is therefore equal to 437.5 grains.
In 1958 the United States and countries of the Commonwealth of Nations agreed to define the international avoirdupois pound to be exactly 0.45359237 kilograms. Consequently, since 1958, the international avoirdupois ounce is exactly 28.349523125 grams by definition.
The ounce is commonly used as a unit of mass in the United States. While imperial units have been officially abolished in the United Kingdom, the ounce remains a familiar unit, especially amongst older people.
International troy ounce
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 or silver.
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 gold, 91.66% pure (11 "fine ounces" plus one ounce of alloy material)
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. Both the weight and the value are the definition of one "Birr", still in use in present-day Ethiopia and formerly in Eritrea.
Some countries have redefined their ounces to fit in with the metric system
The Dutch have redefined their ounce (in Dutch, ons) as 100 grams . The Dutch's metric values, such as 1 ons = 100 grams, is inherited, adopted and taught in Indonesia since elementary school. It is also formally written in Indonesian national dictionary (Kamus Besar Bahasa Indonesia) and elementary school's formal manual book.
East Asia has a traditional ounce, known as a tael, of varying value. In China, it has been given a metric value of 50 grams.
Notes and references