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Troy ounce


The troy ounce (oz t) is a unit of imperial measure, now commonly used to measure the mass (weight, in common parlance) of precious metals. One troy ounce is defined as exactly 31.1034768 g (approximately 1.0971 oz.), which may be used to denote the value of a precious metal. One troy ounce of gold is denoted with the ISO 4217 currency code XAU, while one troy ounce of silver is denoted as XAG.

The troy ounce is about 10% heavier (ratio 192/175) than the avoirdupois ounce, used for everyday purposes in the United States.

The name "troy" is first attested in 1390. Though it is often connected to a fair at the city of Troyes, France, this story may have been invented in the 18th century.

The troy ounce is part of the troy weights system, many aspects of which were indirectly derived from the Roman monetary system. The Romans used bronze bars of varying weights as currency. An aes grave ("heavy bronze") weighed 1 pound. One twelfth of an aes grave was called an uncia, or in English an "ounce". Later the avoirdupois ounce was standardised as 116 of a pound, but the troy ounce, which is 112 of a troy pound (a troy pound is lighter than an avoirdupois pound), is still used for measuring precious metals. At 480 grains, the troy ounce is about 10% heavier than the avoirdupois ounce, which is defined to equal 437 12 grains and is 28.349523125 g (exact).


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