In troy weight, unlike the more common avoirdupois system, there are 12 ounces in a pound, rather than 16, and a troy pound is 5760 grains (approx. 373.24 gramss), rather than 7000 (approx. 453.59 grams).
A troy ounce is 480 grains (approx. 31.10 grams), somewhat heavier than an avoirdupois ounce (437.5 grains, approx. 28.35 grams).
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 gramss.
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.