A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of volume (also called capacity) typically used for measuring liquids. It is equivalent to approximately 30 millilitres (ml). Whilst various definitions have been used throughout history, two remain in common use: the imperial and the United States customary fluid ounce. An imperial fluid ounce is 1⁄20 of an imperial pint, 1⁄160 of an imperial gallon or approximately 28.4 ml. A US fluid ounce is 1⁄16 of a US fluid pint and 1⁄128 of a US liquid gallon or approximately 29.57 ml. The fluid ounce is distinct from the ounce, which is a unit of weight or mass, although they do have a historical relationship, and it is sometimes referred to simply as an "ounce" where context makes the meaning clear, such as ounces in a bottle.
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History
The fluid ounce was originally the volume occupied by one ounce of some substance, such as wine (in England) or water (in Scotland). The ounce in question varied depending on the system of fluid measure, such as that used for wine versus ale. Various ounces were used over the centuries, including the Tower ounce, troy ounce, avoirdupois ounce, and various ounces used in international trade, such as Paris troy. The situation is further complicated by the medieval practice of "allowances", whereby a unit of measure was not necessarily equal to the sum of its parts. For example, the 364-pound woolsack (165 kg) had a 14-pound allowance (6.4 kg) for the weight of the sack and other packaging materials.
In 1824, the British Parliament defined the imperial gallon as the volume of ten pounds of water at standard temperature. The gallon was divided into four quarts, the quart into two pints, the pint into four gills, and the gill into five ounces. Thus, there were 160 imperial fluid ounces to the gallon making the mass of a fluid ounce of water approximately one avoirdupois ounce (28.4 g). This relationship is still approximately valid even though the imperial gallon's definition was later revised to be 4.54609 litres, making the imperial fluid ounce exactly 28.4130625 ml.
The US fluid ounce is based on the US gallon, which is based on the wine gallon of 231 cubic inches that was used in England prior to 1824. With the adoption of the international inch, the US fluid ounce became 29.5735295625 ml exactly, or about 4% larger than the imperial unit.