Fluid ounce
A fluid ounce 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) gallon or 8 fluid drams, about 1.734 cubic inches or 28.413063 ml. This volume of water weighs exactly 1 ounce under certain temperature and pressure conditions.
- The U.S. fluid ounce is (1/128) gallon, about 1.805 cubic inches or 29.573531 ml. This volume of water weighs about 1.04 ounces.
The UK fluid ounce is legally defined as 28.4130625 millilitres precisely in The Units of Measurement Regulations 1995
http://www.hmso.gov.uk/si/si1995/Uksi_19951804_en_2.htm
