The ounce is the name for a number of different units of mass, and, confusingly, volume. See Imperial system of units and U.S. customary units for more about the frameworks within which these units are defined.
- avoirdupois ounce defined by the avoirdupois system of mass (see pound). There are 16 ounces in the avoirdupois pound. The avoirdupois ounce is approximately 28.3 g.
- troy ounce defined by the troy system of mass. 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 grams), rather than 7000 (approx 453.59 grams). Note: at roughly 31.1 g, the troy ounce is about 10% more than the more common avoirdupois ounce defined by the avoirdupois system of mass.
There are also two versions of the fluid ounce, units of volume, of 28.4 and 29.6 ml.
The symbol for ounce is ℥