Gallon
The gallon (/ˈɡælən/) is a unit of measurement for liquid capacity in both the US customary units and the British imperial systems of measurement. Three significantly different sizes are in current use: the imperial gallon defined as 4.54609 litres, which is used in the United Kingdom, Canada, and some Caribbean nations; the US gallon defined as 231 cubic inches (3.785 l), which is used in the US and some Latin American and Caribbean countries; and the least-used US dry gallon defined as 1⁄8 US bushel (4.405 l).
While there is no official symbol for the gallon (as there are for SI units), gal is in common use.
The gallon currently has one definition in the imperial system, and two definitions (liquid and dry) in the US customary system. Historically, there were many definitions and redefinitions.
There were more than a few systems of liquid measurements in the pre-1884 United Kingdom.
The imperial (UK) gallon, now defined as exactly 4.54609 litres (about 277.42 cubic inches), is used in some Commonwealth countries and was originally based on the volume of 10 pounds (approximately 4.54 kg) of water at 62 °F (17 °C). The imperial fluid ounce is defined as 1⁄160 of an imperial gallon; there are four quarts in a gallon, two pints in a quart, and 20 Imperial fluid ounces in an imperial pint.
...
Wikipedia