|Unit system||Imperial units, US customary units|
|SI units||28.41306 ml|
|US customary units||0.9607599 US fl oz|
|SI units||29.5735295625 ml|
|Imperial units||1.040843 imp fl oz|
A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ?, fl ?, f?, ? ?) is a unit of volume (also called capacity) typically used for measuring liquids. Various definitions have been used throughout history, but only two are still in common use: the British Imperial and the United States customary fluid ounce.
The fluid ounce is distinct from the ounce as a unit of weight or mass, although it is sometimes referred to simply as an "ounce" where context makes the meaning clear, such as ounces in a bottle. An imperial fluid ounce of pure water has a mass of almost exactly one ounce.
The fluid ounce was originally the volume occupied by one ounce of some substance, for example wine (in England) or water (in Scotland). The ounce in question also 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 ounces used in international trade, such as Paris troy, a situation 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.
In 1824, the British Parliament defined the imperial gallon as the volume of ten pounds of water at standard temperature. The gallon was divided into four quarts, the quart into two pints, the pint into four gills, and the gill into five ounces; thus, there were 160 imperial fluid ounces to the gallon.
This made the mass of a fluid ounce of water approximately one avoirdupois ounce (28.35 g), a relationship which remains approximately valid today despite the imperial gallon's definition being slightly revised to be 4.54609 litres (thus making the imperial fluid ounce exactly 28.4130625 ml).
The US fluid ounce is based on the US gallon, which in turn is based on the wine gallon of 231 cubic inches that was used in the United Kingdom prior to 1824. With the adoption of the international inch, the US fluid ounce became 1⁄128 gal × 231 in3 × (2.54 cm/in)3 = 29.5735295625 ml exactly, or about 4% larger than the imperial unit.
For serving sizes on nutrition labels in the US, regulation 21 CFR §101.9(b) requires the use of "common household measures", and 21 CFR §101.9(b)(5)(viii) defines a "common household" fluid ounce as exactly 30 milliliters.