Ounce (unit of mass and weight)

Common variants and conversions

Modern usage most often refers to two distinct mass ounces and one volume ounce:

• Avoirdupois ounce: the standard ounce used for everyday goods and common commerce. One avoirdupois ounce equals approximately 28.35 grams; by international agreement it is exactly 28.349523125 grams. There are 16 avoirdupois ounces in one avoirdupois pound.
• Troy ounce: historically used for precious metals (gold, silver, platinum) and gemstones. A troy ounce is about 31.10 grams; its internationally recognized value is 31.1034768 grams. Twelve troy ounces make a troy pound.
• Fluid ounce (a volume measure): distinct from the mass ounces above. The US fluid ounce equals about 29.57 milliliters, while the imperial (UK) fluid ounce is about 28.41 milliliters. Because a "fluid ounce" measures volume, its relationship to an ounce of mass depends on the density of the substance measured.

History and development

The ounce traces back to the Roman uncia, which was one‑twelfth of a libra (pound). Over centuries the basic idea of subdividing a pound persisted across Europe, but local standards of the pound and its subdivisions diverged. Through medieval and early modern trade, systems such as avoirdupois and troy evolved for different trades—general goods and bullion, respectively—leading to the multiple definitions in use today.

Uses, examples, and distinctions

In commerce, the avoirdupois ounce appears on packaged foods, postal rates, and consumer scales in countries using customary measures. The troy ounce remains the conventional unit for trading and pricing precious metals and is widely used by mints and bullion dealers. The fluid ounce is common in recipes, beverage labelling, and pharmacology—note that recipes and labelling must distinguish between fluid ounces and ounces of weight.

Further notes and references

When reading measurements it helps to confirm which ounce is meant: a label or specification should indicate whether it is avoirdupois, troy, or fluid. For background on the concept as a mass unit, on distinctions between mass and weight, or on its place in imperial and United States customary systems generally see resources about historical measures and modern conversions in customary systems.

Because of its long history and multiple meanings, the ounce illustrates how local practice and trade needs can produce several coexisting definitions of a common unit name.