The American gallon began as an English wine measure rounded under Queen Anne
Why is a US gallon about a sixth smaller than a British one? The American version descends from the old English wine gallon, fixed at exactly 231 cubic inches in 1706. Britain later scrapped its wine, ale and grain gallons for a single new one based on ten pounds of water, and the two never matched again.
By the late eighteenth century, England had three gallons in everyday use: a corn or Winchester gallon for grain, a wine gallon, and a larger ale gallon of 282 cubic inches. The wine gallon had once been described as a cylinder six inches deep and seven across; under Queen Anne it was pinned to 231 cubic inches, as if pi were simply twenty-two sevenths. The United States adopted that wine gallon as its standard in 1836, and it still equals about 3.785 litres. The imperial gallon, defined until 1976 as the volume of ten pounds of water at 62 °F, is now exactly 4.54609 litres.
The mismatch runs right down the ladder. Both gallons hold four quarts, eight pints or thirty-two gills, but an imperial gill contains five fluid ounces while an American one holds four, and the ounces themselves differ slightly. So an imperial pint, quart or gallon is about 1.2 times its US namesake. America also once had a separate dry gallon for grain, one-eighth of a Winchester bushel, now dropped from statute.
Pinning down a gallon also requires a temperature, because liquids swell when warm. US regulations reference both petroleum and alcoholic drinks to 60 °F. Spirits went metric in America in 1976, yet a 750-millilitre bottle is still sometimes called a fifth, after the old one-fifth-of-a-gallon size.
The unit keeps retreating. The European Union demoted it to a secondary unit, prompting Ireland and Britain to adopt the litre for trade in the 1990s, though British drivers still quote fuel economy in miles per gallon. The United Arab Emirates stopped selling petrol by the gallon in 2010, and Myanmar followed in 2014. One odd case is the Turks and Caicos Islands, where levying the old imperial-gallon duty on the smaller US gallon quietly raised taxes. The word itself traces to Old Norman French, though its ultimate origin is unknown.
Source: Gallon
