What is Gallon ?

The US gallon is legally defined as 231 cubic inches, which is exactly 3.785411784 litres. A US liquid gallon of water weighs about 8.34 pounds or 3.78 kilograms at 62 °F (17 °C), making it about 16.6% lighter than the imperial gallon.

The wine, fluid, or liquid gallon has been the standard US gallon since the early 19th century.

The wine gallon, which some sources relate to the volume occupied by eight medieval merchant pounds of wine, was at one time defined as the volume of a cylinder 6 inches deep and 7 inches in diameter, i.e.

The gallon is a unit of measurement for volume and fluid 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 (4 imperial quarts or 8 imperial pints), which is used in the United Kingdom.

6 in × (3
1
/
2
in)2 × π ≈ 230.907 06 cubic inches. It had been redefined during the reign of Queen Anne, in 1706, as 231 cubic inches exactly, which is the result of the earlier definition with π approximated to
22
/
7
. Although the wine gallon had been used for centuries for import duty purposes there was no legal standard of it in the Exchequer and a smaller gallon (224 cu in) was actually in use, so this statute became necessary. It remains the US definition today.

Canada, and some Caribbean nations; the US gallon defined as 231 cubic inches (4 US liquid quarts or 8 US liquid pints) or about 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

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.

In 1824, Britain adopted a close approximation to the ale gallon known as the imperial gallon and abolished all other gallons in favour of it. Inspired by the kilogram-litre relationship, the imperial gallon was based on the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury and at a temperature of 62 °F. In 1963, this definition was refined as the space occupied by 10 pounds of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL (the original "brass" was refined as the density of brass alloys vary depending on metallurgical composition).

This works out at approximately 4.5460903 L (277.41945 in3). The metric definition of exactly 4.54609 cubic decimetres (also 4.54609 L after the litre was redefined in 1964, ≈ 277.419433 in3) was adopted shortly afterwards in Canada, but from 1976 the conventional value of 4.546092 L was used in the United Kingdom until the Canadian convention was adopted in 1985.

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).

Historically, gallons of various sizes were used in many parts of Western Europe. In these localities, it has been replaced as the unit of capacity by the litre.

The imperial fluid ounce is defined as

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.

Gallons used in fuel economy expression in Canada and the United Kingdom are imperial gallons.

Despite its status as a U.S. territory, and unlike American Samoa, the Northern Mariana Islands, Guam, and the U.S. Virgin Islands, Puerto Rico ceased selling gasoline by the US gallon in 1980.

The U.S. liquid gallon is defined as 231 cubic inches and equates to approximately 3.785 litres. One imperial gallon is equivalent to approximately 1.2 U.S. liquid gallons.

The U.S. dry gallon is a measurement historically applied to a volume of grain or other dry commodities.

An imperial gallon is approximately 4.5 litres. The litre is the standard measurement for petrol at service stations across most of the world.

It is expected that oil companies will need four months to complete the shift from measuring in imperial gallons to litres, according to ESMA.