A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a
unit of volume equal to about 28 mL in the imperial system or
about 30 ml in the US system. The fluid ounce is distinct from
the ounce, which measures mass. However, the fluid ounce is
sometimes referred to simply as an "ounce" in applications where
its use is implicit.
In 1824 the British Parliament
defined the imperial gallon as the volume of ten avoirdupois
pounds of water. The gallon was divided into four quarts or
eight pints. Each pint was divided into four gills or twenty
fluid ounces. Thus, the mass of an imperial fluid ounce of water
is one avoirdupois ounce (28.4 g) The US ounce is based on the
earlier definition of one gallon equaling 231 cubic inches.
measurement at first glance does not seem to have any tie to
mass, however it is believed to have somehow come from a
previous measure of a gallon as being 224 cubic inches (just 7
cubic inches less, both numbers being multiples of 7) which was
used because it was the volume of 8 pounds of wine.
Subdivisions of the US gallon are similar to those of the
imperial gallon the difference being that there are only four
ounces to the gill (sixteen to the pint or 128 to the gallon).
Thus the US fluid ounce is about 4% larger than the imperial