The Imperial fluid ounce is 1/160 of an imperial Gallon or 8 Fluid Drams , about 1.734 Cubic Inch es or 28.4130625 Millilitre s.1 This volume of Water weighs very nearly 1 AvoirdupoisOunce (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, 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.2
