Fluid ounce

Look up fluid ounce in Wiktionary, the free dictionary.

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of volume equal to ¹⁄₂₀ of a pint or ¹⁄₁₆₀ of a gallon in the imperial system or ¹⁄₁₆ of a pint or ¹⁄₁₂₈ of a gallon in the US system. The imperial gallon was originally defined as the volume occupied by 10 pounds avoirdupois of water at 62 degree Fahrenheit, but is currently defined as 4.54609 litres, making the imperial fluid ounce 28.4130625 mL. The US gallon is defined as 231 cubic inches, making the US fluid ounce 29.5735295625 mL, or about 4% larger than the imperial ounce. 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.

History [edit]

The fluid ounce was originally the volume occupied by one ounce of some substance such as wine (in England) or water (in Scotland). The ounce in question 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, ounce avoirdupois, and various ounces used in international trade such as Paris troy. The situation is 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 had a 14-pound allowance 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 avoirdupois pounds of water. The gallon was divided into four quarts and eight pints. Each pint was divided into four gills and 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 fluid ounce is based on the US gallon, which is based on the wine gallon, used in England prior to 1824.

Definitions and equivalences [edit]

Imperial fluid ounce

1 imperial fluid ounce	=	1⁄160	imperial gallon
	=	1⁄20	imperial pint
	=	1⁄5	imperial gill
	=	8	imperial fluid drams
	≈	28.4130625	millilitres[2] (cm3)
	≈	1.733871455	cubic inches
	=	1	ounce avoirdupois of water at 62 °F (16.7 °C)[3]
	≈	0.960759940	US fluid ounce

US customary fluid ounce

1 US fluid ounce	=	1⁄128	US gallon
	=	1⁄32	US quart
	=	1⁄16	US pint
	=	1⁄8	US cup
	=	1⁄4	US gill
	=	2	tablespoons
	=	6	teaspoons
	=	8	US fluid drams
	≡	1.8046875	cubic inches[4]
	≡	29.5735295625	millilitres (or the same number of cubic centimetres (cm3))
	≈	1.040842731	imperial fluid ounces
	≈	1.041	ounce avoirdupois (29.5 g) of water at 62 °F (16.7 °C)

US food labelling fluid ounce

US regulation 21 CFR 101.9(b)(5)(viii) also defines a fluid ounce as exactly 30 millilitres, but this is for use in nutrition labeling only.^[5]

30 millilitres	≈	1.055852392	imperial fluid ounces[2]
	≈	1.014420681	US customary fluid ounces[4]
	≈	1.830712323	cubic inches

Other useful conversions

Given the definitions above, the number of US fluid ounces in one litre is:

^{1 fl oz}⁄_{29.57353 mL} × 1000 ^mL⁄_L = 33.8140 ^{fl oz}⁄_L

There are thus about 34 customary US fluid ounces in one litre.

Similarly, there are exactly 33 ¹⁄₃ US food-labelling fluid ounces in a litre.

References and notes [edit]