Fluid ounce

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A fluid ounce (also known as a fluidounce) is a unit of volume in both the Imperial system of units and the U.S. customary units system. However, the two measures are not quite the same:

• The Imperial fluid ounce is 1/160 of an imperial gallon or 1/20 of an imperial pint or 8 fluid drams, about 1.734 cubic inches or 28.4130625 millilitres. This volume of water weighs very nearly 1 avoirdupois ounce (it is the volume occupied by one ounce at 62 °F, weighed in air with brass weights).

• The U.S. fluid ounce is defined to be 1/128 of a U.S. gallon or 1/16 of a U.S. pint, exactly 1.8046875 cubic inches or 29.5735295625 milliliters. This volume of cool, pure water weighs about 1.04 avoirdupois ounces (29.5 g).
• * U.S. regulation 21CFR101.9(b)(5)(viii) also defines a fluid ounce of exactly 30 mL, for use in nutrition labeling.

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