Fluid ounce

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. 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.

Fluid ounce
An example of a 2 fl oz shot glass in British Imperial fluid ounces
General information
Unit systemImperial units, US customary units
Unit ofVolume
Symbolfl oz
Conversions (imperial)
1 imp fl oz in ...... is equal to ...
   SI units   28.41306 ml
   US customary units   0.9607599 US fl oz
Conversions (US)
1 US fl oz in ...... is equal to ...
   SI units   29.57353 ml
   Imperial units   1.040843 imp fl oz

An imperial fluid ounce is 120 of an imperial pint, 1160 of an imperial gallon or approximately 28.41 ml.

A US fluid ounce is 116 of a US fluid pint and 1128 of a US liquid gallon or approximately 29.57 ml, making it about 4% larger than the imperial 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.


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.[1]

In 1824, the British Parliament defined the imperial gallon as the volume of ten pounds of water at standard temperature.[2] 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 29.5735295625 ml exactly, or about 4% larger than the imperial unit.

Definitions and equivalences

Imperial fluid ounce
1 imperial fluid ounce  =  1160imperial gallon
=  140imperial quart
=  120imperial pint
=  110imperial cup
=  15imperial gill
=  8imperial fluid drams
=  28.4130625millilitres[3]
  1.733871455cubic inches
  0.960759940US fluid ounces
  the volume of 1 avoirdupois ounce of water[2]
US customary fluid ounce
1 US fluid ounce =  1128US gallon
=  132US quart
=  116US pint
=  18US cup
=  14US gill
=  2US tablespoons
=  6US teaspoons
=  8US fluid drams
=  1.8046875cubic inches[4]
=  29.5735295625millilitres
  1.040842731imperial fluid ounces
US food labelling 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.[5]

30 millilitres   1.055852392imperial fluid ounces[3]
  1.014420681US customary fluid ounces[4]
  1.830712323cubic inches

References and notes

  1. Connor, R. D.; Simpson, Allen David Cumming; Morrison-Low, A. D.; National Museums of Scotland (2004). Weights and measures in Scotland: a European perspective. NMS. p. 153. ISBN 978-1-901663-88-4. Retrieved 21 September 2012.
  2. The imperial gallon was originally defined as the volume occupied by ten avoirdupois pounds (4.54 kg) of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (102 kPa) at a temperature of 62 °F (16.7 °C).
  3. "The Units of Measurement Regulations 1995 (Schedule)". 20 September 2000. Retrieved 18 April 2006.
  4. One US gallon is defined as 231 cubic inches.
  5. "21 CFR §101.9". Retrieved 2019-05-31.
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