<P> An imperial fluid ounce is ​ ⁄ of an imperial pint, ​ ⁄ of an imperial gallon or approximately 28.4 ml . </P> <P> A US fluid ounce is ​ ⁄ of a US fluid pint and ​ ⁄ of a US liquid gallon or approximately 29.57 ml, making it about 4% larger than the imperial fluid ounce . </P> <P> The fluid ounce is distinct from the ounce as a unit of weight or mass, although it is sometimes referred to simply as an "ounce" where context makes the meaning clear, such as ounces in a bottle . </P> <P> 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 . </P>

Does a fluid ounce of water weigh an ounce