fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of volume (also called capacity) typically used for measuring liquids.

The ounce in question also varied depending on the system of fluid measure, such as that used for wine versus ale.

[5] In 1824, the British Parliament defined the imperial gallon as the volume of ten pounds of water at standard temperature.

With the adoption of the international inch, the US fluid ounce became 1⁄128 gal × 231 in3/gal × (2.54 cm/in)3 = 29.5735295625 mL exactly, or about 4.08% larger than the imperial unit.

In the U.K., the use of the fluid ounce as a measurement in trade, public health, and public administration was circumscribed to a few specific uses (the labelling of beer, cider, water, lemonade and fruit juice in returnable containers) in 1995, and abolished entirely in 2000, by The Units of Measurement Regulations 1994.