What is Gallon ?

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 gallon was removed from the list of legally defined primary units of measure catalogued in the EU directive 80/181/EEC for trading and official purposes, with effect from 31 December 1994.

Under the directive the gallon could still be used – but only as a supplementary or secondary unit.

One of the effects of this directive was that the United Kingdom amended its own legislation to replace the gallon with the litre as a primary unit of measure in trade and in the conduct of public business, effective from 30 September 1995.

Ireland also passed legislation in response to the EU directive, with the effective date being 31 December 1993.

Though the gallon has ceased to be the legally defined primary unit, it can still be legally used in both the UK and Ireland as a supplementary unit.

The United Arab Emirates started selling gasoline by the litre in 2010, along with Guyana, and Panama in 2013.

The two former had used the Imperial gallon and the latter the US gallon until they switched. Myanmar (Burma) switched from Imperial gallon to litre sales before 2014.

The Imperial gallon continues to be used as a unit of measure in Anguilla, Antigua and Barbuda, the Bahamas, the British Virgin Islands, the Cayman Islands, Dominica, Grenada, Montserrat, St. Kitts & Nevis,St. Lucia, and St. Vincent & the Grenadines

Other than the United States, the US gallon is used in Liberia, Belize, Colombia, The Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Nicaragua, and Peru, but only for the sale of gasoline. All other products are sold in litres and its multiples and submultiples

Antigua and Barbuda planned to switch over to using litres by 2015, but as of 2018 the switch-over had not been effected.

In the Turks & Caicos Islands, both the U.S. gallon and Imperial gallon are used, due to an increase in tax duties disguised by levying the same duty on the 3.79 L U.S. gallon as was previously levied on the 4.55 L Imperial gallon

Both the US liquid and imperial gallon are divided into four quarts (quarter gallons), which in turn are divided into two pints.

These pints are divided into two cups (though the imperial cup is rarely used now), which in turn are divided into two gills (gills are also rarely used).

Thus a gallon is equal to four quarts, eight pints, sixteen cups or thirty-two gills. The imperial gill is further divided into five fluid ounces, whereas the US gill is divided into four fluid ounces.

Thus an imperial fluid ounce is
1
/
20
of an imperial pint or
1
/
160
of an imperial gallon, while a US fluid ounce is
1
/
16
of a US pint or
1
/
128
of a US gallon.

The imperial gallon, quart, pint, cup and gill are approximately 20% larger than their US counterparts and are therefore not interchangeable.

The imperial fluid ounce, on the other hand, is only 4% smaller than the US fluid ounce and therefore they are often used interchangeably.

The term derives most immediately from galun, galon in Old Northern French, but the usage was common in several languages, for example jale in Old French and gęllet (bowl) in Old English. This suggests a common origin in Romance Latin, but the ultimate source of the word is unknown.

The gallon originated as the base of systems for measuring wine, and beer in England. The sizes of gallon used in these two systems were different from each other: the first was based on the wine gallon (equal in size to the US gallon), and the second on either the ale gallon or the larger imperial gallon.

The corn or dry gallon was used in the United States until recently for grain and other dry commodities. It is one-eighth of the (Winchester) bushel, originally a cylindrical measure of 18
1
/
2
inches in diameter and 8 inches in depth.

Unit of liquid volume. (1) UK (Imperial) gallon equals 277.42 cubic inches, 0.16 cubic feet, 4.55 liters, or 1.2 US gallons. One UK gallon of water weighs 10 pounds. (1) US gallon equals 231 cubic inches, 0.13 cubic feet, 3.78 liters, or 0.83 UK gallon. One US gallon of water weighs 8.33 pounds.

That made the dry gallon (9
1
/
4
)2 × π cubic inches ≈ 268.80252 in3. The bushel, which like dry quart and pint still sees some use, was later defined to be 2150.42 cubic inches exactly, making its gallon exactly 268.8025 in3 (4.40488377086 L).

In previous centuries, there had been a corn gallon of around 271 to 272 cubic inches.