Troy weight is a system of units 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.

A troy ounce, the only currently used unit of the system, is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains). A grain is exactly 64.798 91 mg, hence one troy ounce is exactly 31.103 476 8 g, about 10 per cent more than the avoirdupois ounce, which is exactly 28.349 523 125 g. The troy ounce is the only ounce used in the pricing of precious metals, such as gold and silver, and this is the only remaining use of the troy ounce. In troy weight, there are 12 ounces in a pound, rather than 16 in the more-common avoirdupois system.

A troy pound is 5760 grains (about 373.24 g), rather than 7000 (about 453.59 g).

The use of troy weight for precious metals provides the technical curiosity that an ounce of gold has more mass than an ounce of feathers, but a pound of gold has less mass than a pound of feathers.

See also

• Carat
• Conversion of units.