Imperial fluid ounce

Fluid ounce
This shot glass holds two US fluid ounces.
Unit information
Unit system	Imperial units, US customary units
Unit of	Volume
Symbol	fl oz
Unit conversions (imperial)
1 imp fl oz in ...	... is equal to ...
SI units	28.41306 ml
US customary units	0.9607599 US fl oz
Unit conversions (US)
1 US fl oz in ...	... is equal to ...
SI units	29.57353 ml
Imperial units	1.040843 imp fl oz

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of volume (also called capacity) typically used for measuring liquids, gases, or particulate solids (such as shrimp). Today it is either an Imperial or a U.S. fluid ounce, two different amounts, but both are slightly less than 30 millilitres (ml).

Whilst various definitions have been used throughout history, two are in common use: the imperial and the United States customary fluid ounce.

An imperial fluid ounce is  ¹⁄₂₀ of an imperial pint,  ¹⁄₁₆₀ of an imperial gallon or approximately 28.4 ml.

A US fluid ounce is  ¹⁄₁₆ of a US fluid pint and  ¹⁄₁₂₈ of a US liquid gallon or approximately 29.57 ml.

The fluid ounce is distinct from the ounce avoirdupois and the Troy ounce, which are units of weight or mass, although they do have a historical relationship, and it is sometimes referred to simply as an "ounce" where context makes the meaning clear, such as ounces in a bottle.

The fluid ounce was originally the volume occupied by one ounce of some substance, such as wine (in England) or water (in Scotland). The ounce in question varied depending on the system of fluid measure, such as that used for wine versus ale. Various ounces were used over the centuries, including the Tower ounce, troy ounce, avoirdupois ounce, and various ounces used in international trade, such as Paris troy. The situation is further complicated by the medieval practice of "allowances", whereby a unit of measure was not necessarily equal to the sum of its parts. For example, the 364-pound woolsack (165 kg) had a 14-pound allowance (6.4 kg) for the weight of the sack and other packaging materials.