The Gallon: Why the US and UK Use Different Sized Gallons
Source: NIST. Published: 2025-08-15.
Medieval Origins: A Gallon Was a Container
The word gallon comes from the Old French jalon, meaning a liquid measure. Medieval English gallons were not standardised — different commodities were measured in different gallons. There was a wine gallon, an ale gallon, and a corn gallon, each with a different volume, because the taxation and trade conventions governing wine, ale, and grain had evolved independently over centuries. The wine gallon was smaller than the ale gallon; the corn gallon was larger still. Merchants, tax collectors, and buyers all had to know which gallon applied to a given transaction. This proliferation of competing standards was not unusual for the medieval period, but it became increasingly untenable as trade grew more complex and the movement of goods across different jurisdictions became more common.
Queen Anne's Wine Gallon: 231 Cubic Inches
In 1706, during the reign of Queen Anne, the wine gallon was standardised at 231 cubic inches — a value chosen because it was convenient for the customs and trade practices of the time. This definition proved influential: when Britain's North American colonies became the United States, they retained the wine gallon as their standard gallon. The US gallon of 231 cubic inches (approximately 3.785 litres) is a direct descendant of Queen Anne's wine gallon. It is the gallon used in the United States for fuel, milk, and all other liquids, and it remains defined by US law as exactly 231 cubic inches.
The British Imperial Gallon: A Different Approach
In 1824, Britain reformed its weights and measures under the Weights and Measures Act, abolishing the different medieval gallons in favour of a single imperial gallon. Rather than retaining the wine gallon, the Act defined the imperial gallon as the volume of ten pounds of distilled water at a standard temperature and pressure — a definition designed to have a clean relationship to mass. This produced an imperial gallon of approximately 277.42 cubic inches (approximately 4.546 litres), about 20% larger than the US gallon that had descended from the old wine measure. Britain and its then-colonies adopted this new standard; the United States, independent since 1776, did not.
The Consequences: Same Word, Different Volume
The divergence means that a British recipe calling for one pint of milk uses a different quantity than an American recipe calling for one pint — because a British pint is 568 ml while a US pint is 473 ml. A British mile-per-gallon figure for a car is higher than an American one for the same vehicle, because the British gallon is larger. A British fluid ounce differs from a US fluid ounce. This creates persistent confusion for travellers, cooks, and engineers who work across the Atlantic. The only reliable solution is explicit unit identification or conversion to metric — and the Unit Master Volume Converter supports both US and UK gallons precisely to avoid ambiguity.
Canada, Australia, and the Metric Transition
Most countries that were once part of the British Empire transitioned to the metric system in the 1960s and 1970s, replacing both the imperial gallon and other imperial measures with litres and millilitres. Canada metricated in 1971; Australia followed in 1970. The UK itself metricated most commercial measures but retained the imperial pint for draught beer and milk in returnable containers as a cultural concession. The imperial gallon is no longer a legal unit for most trade in the UK. Today the imperial gallon survives primarily in older references, fuel economy discussions comparing British and American cars, and historical contexts — where the 20% difference from the US gallon routinely causes confusion.
Converting Gallons Today
The exact conversion factors are: 1 US gallon = 3.785 411 784 litres (exact, by definition since 1964); 1 UK imperial gallon = 4.546 09 litres (exact, by definition). The two gallons differ by a factor of exactly 1.200 950 095... — approximately 20%. When converting fuel economy between the US and UK — or comparing any other volume stated in gallons — it is essential to know which gallon is being used. The Unit Master Volume Converter offers both US Gallons and UK Gallons as distinct unit options, and users can convert between them directly or via litres. Specifying the gallon type eliminates an ambiguity that has persisted for more than two centuries.
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