| Fluid ounce | |
|---|---|
 Ababy bottle in both American and British Imperial fluid ounces | |
| General information | |
| Unit system | Imperial units,US customary units |
| Unit of | Volume |
| Symbol | fl oz |
| Conversions (imperial) | |
| 1 imp fl ozin ... | ... is equal to ... |
| SI units | 28.4130625 mL |
| US customary units | 0.96076 US fl oz |
| Conversions (US) | |
| 1 US fl ozin ... | ... is equal to ... |
| SI units | 29.5735295625 mL |
| Imperial units | 1.0408427 imp fl oz |
Afluid ounce (abbreviatedfl oz,fl. oz. oroz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit ofvolume (also calledcapacity) typically used for measuringliquids. TheBritish Imperial, theUnited 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.
Animperial fluid ounce is1⁄20 of an imperialpint,1⁄160 of an imperialgallon, or exactly 28.4130625 mL.
AUS customary fluid ounce is1⁄16 of aUS liquid pint,1⁄128 of aUS gallon, or exactly 29.5735295625 mL, making it about 4.084% larger than the imperial fluid ounce.
AUS food labeling fluid ounce is exactly 30 mL.
Thefluid ounce is distinct from the (internationalavoirdupois)ounce as a unit ofweight ormass, 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.
| 1 imperial fluid ounce | = | 1/160 | imperialgallon |
| = | 1/40 | imperialquart | |
| = | 1/20 | imperialpint | |
| = | 1/10 | imperialcup | |
| = | 1/5 | imperialgill | |
| = | 8 | imperialfluid drams | |
| ≡ | 28.4130625 | millilitres[1] | |
| ≈ | 1.7338715 | cubic inches | |
| ≈ | 0.96076 | US fluid ounces | |
| ≈ | 0.0258014 | US dry quarts | |
| ≈ | 0.0516028 | US dry pints | |
| ≈ | the volume of 1avoirdupoisounce of water[2] | ||
| 1 US fluid ounce | = | 1/128 | US gallon |
| = | 1/32 | US liquid quart | |
| ≡ | 1/16 | US liquid pint | |
| = | 1/8 | US cup | |
| = | 1/4 | US gill | |
| = | 2 | US tablespoons | |
| = | 6 | US teaspoons | |
| = | 8 | US fluid drams | |
| = | 29.5735295625 | millilitres | |
| = | 1.8046875 | cubic inches[3] | |
| ≈ | 1.0408427 | imperial fluid ounces | |
| ≡ | 5775/215042 | US dry quart | |
| ≡ | 5775/107521 | US dry pint | |
| ≈ | the volume of 1.0431756avoirdupoisounces of water | ||
For serving sizes on nutrition labels in the US, regulation 21CFR §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.[4]
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 asParis 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.[5]
In 1824, the British Parliament defined theimperial gallon as the volume of tenpounds of water at standard temperature.[2] The gallon was divided into fourquarts, the quart into two pints, the pint into fourgills, 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 (28.35 g), a relationship which remains approximately valid today despite the imperial gallon's definition being slightly revised to be4.54609 litres (thus making the imperial fluid ounce exactly28.4130625 mL).
The US fluid ounce is based on the US gallon, which in turn is based on thewine gallon of 231 cubic inches that was used in the United Kingdom prior to 1824. With the adoption of theinternational inch, the US fluid ounce became1⁄128 gal ×231 in3/gal ×(2.54 cm/in)3 =29.5735295625 mL 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, byThe Units of Measurement Regulations 1994.[6]