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 occupied 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). Th ...
(also called ''capacity'') typically used for measuring
liquids
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
. The
British Imperial
The British Empire was composed of the dominions, colonies, protectorates, mandates, and other territories ruled or administered by the United Kingdom and its predecessor states. It began with the overseas possessions and trading posts esta ...
, the
United States customary, and the United States food labeling fluid ounce are the only 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 traditionally one eighth of a gallon. The British impe ...
, of an imperial
gallon
The gallon is a unit of volume in imperial units and United States customary units. Three different versions are in current use:
*the imperial gallon (imp gal), defined as , which is or was used in the United Kingdom, Ireland, Canada, Austr ...
or exactly 28.4130625 mL.
A US customary fluid ounce is of a
US liquid pint and of a
US liquid gallon
The gallon is a unit of volume in imperial units and United States customary units. Three different versions are in current use:
*the imperial gallon (imp gal), defined as , which is or was used in the United Kingdom, Ireland, Canada, Austr ...
or exactly 29.5735295625 mL, making it about 4.08% 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
The ounce () is any of several different units of mass, weight or volume and is derived almost unchanged from the , an Ancient Roman unit of measurement.
The avoirdupois ounce (exactly ) is avoirdupois pound; this is the United States customa ...
as a unit of
weight
In science and engineering, the weight of an object is the force acting on the object due to gravity.
Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar qua ...
or
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
, 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.
:::
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
Troy weight is a system of units of mass that originated in 15th-century England, and is primarily used in the precious metals industry. The troy weight units are the grain, the pennyweight (24 grains), the troy ounce (20 pennyweights), and th ...
,
avoirdupois ounce
The ounce () is any of several different units of mass, weight or volume and is derived almost unchanged from the , an Ancient Roman unit of measurement.
The avoirdupois ounce (exactly ) is avoirdupois pound; this is the United States custom ...
, and ounces used in international trade, such as
Paris troy
Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...
, 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
The gallon is a unit of volume in imperial units and United States customary units. Three different versions are in current use:
*the imperial gallon (imp gal), defined as , which is or was used in the United Kingdom, Ireland, Canada, Austral ...
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 (4.54 kg) 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 an English unit of volume equal to a quarter 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 equ ...
s, the quart into two pints, the pint into four
gills
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
, 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 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 A wine gallon is a unit of capacity that was used routinely in England as far back as the 14th century, and by statute under Queen Anne since 1707. Britain abandoned the wine gallon in 1826 when it adopted imperial units for measurement. The 1707 w ...
of 231 cubic inches that was used in the United Kingdom prior to 1824. With the adoption of the
international inch
Measuring tape with inches
The inch (symbol: in or ″) is a unit of length in the British imperial and the United States customary systems of measurement. It is equal to yard or of a foot. Derived from the Roman uncia ("twelfth") ...
, the US fluid ounce became gal × 231 in/gal × (2.54 cm/in) = 29.5735295625 mL exactly, or about 4% larger than the imperial unit.
References and notes
{{United States Customary Units
Ounce
The ounce () is any of several different units of mass, weight or volume and is derived almost unchanged from the , an Ancient Roman unit of measurement.
The avoirdupois ounce (exactly ) is avoirdupois pound; this is the United States customa ...
Customary units of measurement in the United States
Imperial units
Alcohol measurement
Cooking weights and measures