Ounce is a unit of weight or volume used primarily in the United States. As a unit of weight, it is used to measure mass (the amount of matter in an object) or force. In commercial and everyday use, the term weight is understood to mean mass, and the ounce is used as a unit of mass. In science and technology, weight refers to the gravitational force on an object, and so the ounce is also used as a measure of that force.
In the avoirdupois system of weights, there are 16 ounces in a pound. As a unit of force, the avoirdupois ounce equals 0.278 newton in the metric system; as a unit of mass, 28.350 grams in the metric system. This ounce also equals 437.5 grains of mass. The troy weight system, used throughout the world to measure the mass of precious metals, has a heavier ounce. The troy ounce equals 31.103 grams, or 480 grains. In the troy weight system, there are 12 ounces in a pound.
As a measure of liquid volume, one fluid ounce equals 29.574 milliliters. There are 32 fluid ounces in a liquid quart.
