What is a Troy Ounce?

The ounce is used as a measure of either volume (called fluid ounce) or weight (called ounce weight). In the US, a fluid ounce (29.573 529 562 5 milliliters) is slightly larger than the Imperial fluid ounce (approx. 28.4 milliliters) established by the British Empire. Furthermore, ounce weight can refer to the troy ounce (approx. 31.1 grams or 480 grains -- a medieval measure based on cereal grains) used for precious metals and abbreviated as t oz. The troy ounce is slightly heavier than the avoirdupois ounce (approx. 28.3 grams). Accordingly, an ounce of gold by weight is slightly more than an ounce of steel. When not specified as fluid or weight, the ounce is generally assumed to be ounce weight.