Troy weight

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Troy weight is a system of unitss of mass customarily used for precious metals and gemstones. It derives from the troy system of mass, which dates back to before the time of William the Conqueror. Its name comes from the city of Troyes in France, an important trading city in the Middle Ages.

In troy weight, unlike the more common avoirdupois system, there are 12 ounces in a pound, rather than 16. A troy pound is 5760 grains (approx. 373.24 gramss), rather than 7000 (approx. 453.59 g).

A troy ounce is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains). A grain is exactly 64.79891 milligrams, hence 1 troy ounce is exactly 31.1034768 g, about 10% more than the avoirdupois ounce, which is about 28.3 g. There are also two versions of the fluid ounce, units of volume, of 28.4 ml and 29.6 ml.

See also Conversion of units.