Fluid ounce

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a unit of volume in both the imperial and the US customary systems. It is common to refer to the unit simply as an ounce, especially in cases where no confusion with the unit of mass (also called an ounce) is likely to occur.

Definitions

Imperial fluid ounce

The imperial fluid ounce is of an imperial gallon making it very nearly the volume occupied by one avoirdupois ounce of water.

1 imperial fluid ounce	=	1/160	imperial gallons
=	1/20	imperial pints
=	1/5	imperial gills
=	8	imperial fluid drams
=	28.4130625	millilitres (exactly) (cc)
≈	1.733871455	cubic inches
≈	0.960759940	U.S. fluid ounces

U.S. customary fluid ounce

The U.S. customary fluid ounce is defined to be of a U.S. gallon. This volume of cool, pure water weighs about 1.04 avoirdupois ounces (29.5 g).

1 U.S. fluid ounce	=	1/128	U.S. gallon
=	1/16	U.S. pints
=	1/4	U.S. gills
=	8	U.S. fluid drams
=	1.8046875	cubic inches (exactly)
=	29.5735295625	millilitres (exactly) (cc)
≈	1.040842731	imperial fluid ounces

U.S. food labeling fluid ounce

U.S. 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. This is not meant to concur with the customary US definition exactly but is a common source of confusion.

30 millilitres	≈	1.055852392	imperial fluid ounces
≈	1.014420681	U.S. customary fluid ounces
≈	1.830712323	cubic inches

Other useful conversions

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

1/29.5735295625 × 1000 = 33.8140

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

Similarly, there are exactly 33 1/3 US food-labeling fluid ounces in a liter.

References and notes