A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of

volume Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...

(also called ''capacity'') typically used for measuring

liquids Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

. The British Imperial, the United States customary, and the United States food labeling fluid ounce are the three that are still in common use, although various definitions have been used throughout history. An imperial fluid ounce is of an imperial

pint The pint (, ; symbol pt, sometimes abbreviated as ''p'') is a unit of volume or capacity in both the imperial and United States customary measurement systems. In both of those systems, it is one-eighth of a gallon. The British imperial pint ...

, of an imperial

gallon The gallon is a unit of volume in British imperial units and United States customary units. The imperial gallon (imp gal) is defined as , and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia ...

, or exactly 28.4130625 mL. A US customary fluid ounce is of a US liquid pint, of a

US gallon The gallon is a unit of volume in British imperial units and United States customary units. The imperial gallon (imp gal) is defined as , and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia, ...

, or exactly 29.5735295625 mL, making it about 4.084% larger than the imperial fluid ounce. A US food labeling fluid ounce is exactly 30 mL.

Comparison to the ounce

The ''fluid'' ounce is distinct from the (international avoirdupois) ounce as a unit of

weight In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition. Some sta ...

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

, although it is sometimes referred to simply as an "ounce" where context makes the meaning clear (e.g., "ounces in a bottle"). A volume of pure water measuring one imperial fluid ounce has a mass of almost exactly one ounce.

Definitions and equivalences

;Imperial fluid ounce : ;US customary fluid ounce :: ;US food labeling fluid ounce 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. This applies to the serving size but not the package size; package sizes use the US customary fluid ounce. :::

History

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 had a 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 imperial gallon was originally defined as the volume occupied by ten avoirdupois pounds () of distilled water weighed in air with brass weights with the barometer standing at at a temperature of . The gallon was divided into four

quart The quart (symbol: qt) is a unit of volume equal to a quarter of a gallon. Three kinds of quarts are currently used: the liquid quart and dry quart of the US customary system and the of the British imperial system. All are roughly equal ...

s, 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 one avoirdupois ounce (), a relationship which remains approximately valid today despite the imperial gallon's definition being slightly revised to be (thus making the imperial fluid ounce exactly ). 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 × × = exactly, or about 4.084% larger than the imperial unit. In the U.K., the use of the fluid ounce as a measurement in trade, public health, and public administration was circumscribed to a few specific uses (the labelling of beer, cider, water, lemonade and fruit juice in returnable containers) in 1995, and abolished entirely in 2000, by ''The Units of Measurement Regulations 1994''.

References and notes

{{United States Customary Units Ounce Customary units of measurement in the United States Imperial units Alcohol measurement Cooking weights and measures