Troy ounce

The troy ounce is the traditional unit of mass for precious metals. It derives from the troy system of mass[?], which dates back to before the time of William the Conqueror. Its name is thought to derive from the town of Troyes in France. 1 troy ounce is defined as exactly 480 grains, where 1 grain is exactly 64.79891 milligrams, hence 1 troy ounce is exactly 31.1034768 grams.

The symbol for ounce is ℥

Note: The troy ounce is about 10% more than the more common ounce defined by the avoirdupois system of mass, which is 28.3 g. There are also two versions of the fluid ounce[?], units of volume, of 28.4 and 29.6 ml.

See also Conversion of units.