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. Various definitions have been used throughout history, but only two are still in common use: the British 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, making it about 4% larger than the imperial fluid ounce.
Imperial fluid ounce
| 1 imperial fluid ounce | = | 1⁄160 | imperial gallon |
| | = | 1⁄40 | imperial quart |
| | = | 1⁄20 | imperial pint |
| | = | 1⁄5 | imperial gill |
| | = | 8 | imperial fluid drams |
| | = | 28.4130625 | millilitres |
| | ≈ | 1.733871455 | cubic inches |
| | ≈ | 0.960759940 | US fluid ounces |
| | ≈ | the volume of 1 avoirdupois ounce of water |