Fluid ounce

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a unit of volume equal to about 28.4 mL 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.[1] 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 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 avoirdupois ounce 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 avoirdupois ounce (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

  1. ^ Rowlett, Russ (September 13, 2001). "Gallon". How Many? A Dictionary of Units of Measurement. University of North Carolina at Chapel Hill. http://www.unc.edu/~rowlett/units/dictG.html#gallon. Retrieved 2010-10-31. 
  2. ^ a b "The Units of Measurement Regulations 1995 (Schedule)". 2000-09-20. http://www.opsi.gov.uk/si/si1995/Uksi_19951804_en_2.htm. Retrieved 2006-04-18. 
  3. ^ The imperial gallon was originally defined as the volume occupied by ten avoirdupois pounds of water at 62 °F (16.7 °C), weighed in air with brass weights.
  4. ^ a b One US gallon is defined as 231 cubic inches.
  5. ^ "Food and Drug Administration, HHS" (PDF). http://a257.g.akamaitech.net/7/257/2422/01apr20051500/edocket.access.gpo.gov/cfr_2005/aprqtr/pdf/21cfr101.9.pdf. Retrieved 2006-04-18.