Fluid Ounce

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.

This 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 ounce.