Fluid ounce
Encyclopedia
A fluid ounce is a unit of volume
equal to about 28.4 m
L
in the imperial system
or about 29.6 mL in the US system
. The fluid ounce is distinct from the ounce
, which measures mass
. However, the fluid ounce is sometimes referred to simply as an "ounce" in applications where the volumetric "fluid ounce" is implied by context.
In 1824 the British Parliament defined the imperial gallon as the volume of ten avoirdupois
pounds of water. The gallon was divided into four quarts or eight pints. Each pint was divided into four gills or twenty fluid ounces, that is, five ounces for each gill. Thus, the mass of an imperial fluid ounce of water is one avoirdupois ounce (28.4 g)
The US ounce is based on the earlier definition of one gallon equaling 231 cubic inches. This measurement at first glance does not seem to have any tie to mass, however it is believed to have come from a previous measure of a gallon as being 224 cubic inches (just 7 cubic inches less, both numbers being multiples of 7) which was used because it was the exact volume of 8 pounds of wine. Thus one pint of a wine gallon is equal to the volume of one pound of wine, and there are 16 fluid ounces in a pint, just as there are 16 ounces in a pound. Subdivisions of the US gallon are similar to those of the imperial gallon the difference being that there are only four ounces to the gill (sixteen to the pint or 128 to the gallon). Thus the US fluid ounce is about 4% larger than the imperial ounce.
US customary fluid ounce
US food labelling fluid ounce

 30 millilitres ≈
align=right1.055852392imperial fluid ounces

≈
align=right1.014420681US customary fluid ounces

≈
align=right1.830712323cubic inches
}
Other useful conversions
Given the definitions above, the number of US fluid ounces in one litre is:
There are thus about 34 customary US fluid ounces in one litre.
Similarly, there are exactly 33 US foodlabelling fluid ounces in a litre.
Volume
Volume is the quantity of threedimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
equal to about 28.4 m
Milli
Milli is a prefix in the metric system denoting a factor of one thousandth . Adopted in 1795, the prefix comes from the Latin mille, meaning one thousand ....
L
Litre
pic200pxrightthumbOne litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
in the imperial system
Imperial unit
The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire...
or about 29.6 mL in the US system
United States customary units
United States customary units are a system of measurements commonly used in the United States. Many U.S. units are virtually identical to their imperial counterparts, but the U.S. customary system developed from English units used in the British Empire before the system of imperial units was...
. The fluid ounce is distinct from the ounce
Ounce
The ounce is a unit of mass with several definitions, the most commonly used of which are equal to approximately 28 grams. The ounce is used in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems...
, which measures mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
. However, the fluid ounce is sometimes referred to simply as an "ounce" in applications where the volumetric "fluid ounce" is implied by context.
In 1824 the British Parliament defined the imperial gallon as the volume of ten avoirdupois
Avoirdupois
The avoirdupois system is a system of weights based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees by many people in Canada, the United Kingdom, and some other former British colonies despite the official adoption...
pounds of water. The gallon was divided into four quarts or eight pints. Each pint was divided into four gills or twenty fluid ounces, that is, five ounces for each gill. Thus, the mass of an imperial fluid ounce of water is one avoirdupois ounce (28.4 g)
The US ounce is based on the earlier definition of one gallon equaling 231 cubic inches. This measurement at first glance does not seem to have any tie to mass, however it is believed to have come from a previous measure of a gallon as being 224 cubic inches (just 7 cubic inches less, both numbers being multiples of 7) which was used because it was the exact volume of 8 pounds of wine. Thus one pint of a wine gallon is equal to the volume of one pound of wine, and there are 16 fluid ounces in a pint, just as there are 16 ounces in a pound. Subdivisions of the US gallon are similar to those of the imperial gallon the difference being that there are only four ounces to the gill (sixteen to the pint or 128 to the gallon). Thus the US fluid ounce is about 4% larger than the imperial ounce.
Definitions and equivalences
Imperial fluid ounce1 imperial fluid ounce  =  imperial gallon Gallon The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semiofficially within Canada, the United States liquid gallon and the lesser used United States dry... 

=  imperial pint Pint The pint is a unit of volume or capacity that was once used across much of Europe with values varying from state to state from less than half a litre to over one litre. Within continental Europe, the pint was replaced with the metric system during the nineteenth century... 

=  imperial gill Gill (unit) The gill is a unit of measurement for volume equal to a quarter of a pint. It is no longer in common use, except in regard to the volume of alcoholic spirits measures but it is also kept alive by the occasional reference, such as in the cumulative song, "The Barley Mow".Imperial gillUnited States... 

=  8  imperial fluid drams  
≈  28.4130625  millilitres (cm^{3} Cubic centimetre A cubic centimetre is a commonly used unit of volume extending the derived SIunit cubic metre, and corresponds to the volume of a cube measuring 1 cm × 1 cm × 1 cm... ) 

≈  1.733871455  cubic inches  
=  1  avoirdupois Avoirdupois The avoirdupois system is a system of weights based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees by many people in Canada, the United Kingdom, and some other former British colonies despite the official adoption... ounce Ounce The ounce is a unit of mass with several definitions, the most commonly used of which are equal to approximately 28 grams. The ounce is used in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems... of water at 62 °F (16.7 °C) 

≈  0.960759940  US fluid ounce 
US customary fluid ounce
1 US fluid ounce  =  US gallon  
=  US quart  
=  US pint  
=  US cup  
=  US gill  
=  2  tablespoons  
=  6  teaspoons  
=  8  US fluid drams  
≡  1.8046875  cubic inches  
≡  29.5735295625  millilitres (or the same number of cubic centimetres (cm³))  
≈  1.040842731  imperial fluid ounces  
≈  1.041  avoirdupois ounce (29.5 g) of water at 62 °F (16.7 °C) 
US food labelling fluid ounce
 US regulation 21 CFRCode of Federal RegulationsThe Code of Federal Regulations is the codification of the general and permanent rules and regulations published in the Federal Register by the executive departments and agencies of the Federal Government of the United States.The CFR is published by the Office of the Federal Register, an agency...
101.9(b)(5)(viii) also defines a fluid ounce as exactly 30 millilitres, but this is for use in nutrition labeling only. {

 30 millilitres ≈
align=right1.055852392imperial fluid ounces

≈
align=right1.014420681US customary fluid ounces

≈
align=right1.830712323cubic inches
}
Other useful conversions
Given the definitions above, the number of US fluid ounces in one litre is:
 × 1000 = 33.8140
There are thus about 34 customary US fluid ounces in one litre.
Similarly, there are exactly 33 US foodlabelling fluid ounces in a litre.