US Gallon vs Imperial Gallon

You are comparing a 14-gallon fuel tank on a US car to a 14-gallon tank on a UK car and wondering which holds more. The UK tank, using the imperial gallon, holds 63.6 liters. The US tank holds 53.0 liters. The difference is a full 10.6 liters, about 20%. This explains the difference between the US gallon vs imperial gallon, where each came from, and how to avoid the 20% error when you cross the Atlantic.

The imperial gallon vs US gallon is a difference of 0.76068 liters, just over three-quarters of a liter, per gallon. That is the gap that adds up to 7.6 liters on a 10-gallon purchase. Here are the exact numbers, from the legal definitions that will not change:

• US gallon: 231 cubic inches, exactly 3.785411784 liters (NIST Handbook 44 Appendix C; NIST SP 811 Appendix B).
• Imperial gallon: 4.54609 liters, exactly (UK Weights and Measures Act 1985 Schedule 1). Its 1824 definition was 277.274 cubic inches (volume of 10 pounds of distilled water at 62°F), but the law now sets it in metric.

One imperial gallon is 1.20095 US gallons. One US gallon is 0.83267 imperial gallons. If you are converting liters, use 0.26417 US gallons per liter or 0.21997 imperial gallons per liter.

The US gallon traces to the English wine gallon of 1707, defined as 231 cubic inches. This was the standard for wine and other liquids in the American colonies, and it stuck after independence. The UK itself used multiple gallons, ale gallon, corn gallon, wine gallon, until 1824, when the British Parliament passed the Weights and Measures Act 1824, creating the imperial gallon as a single standard for all trade.

The 1985 Act sharpened that to 4.54609 liters exactly. The US gallon, meanwhile, was redefined in 1959 when the international inch was set to exactly 2.54 cm.

Understanding what is an imperial gallon in practical terms means knowing where it is still the law. The UK uses the imperial gallon for fuel at the pump and for draught beer (a pint of beer is an imperial pint). Milk and other liquids are sold in metric, but the imperial gallon remains a legal unit for specific purposes under the 1985 Act. Canada uses liters for fuel, not gallons. Australia, New Zealand, and the entire European Union use the metric system exclusively; the imperial gallon is not a legal unit of trade there.

The US uses the US gallon for fuel, milk, and most liquid measures. The US dry gallon, a separate, older unit of 268.8 cubic inches, is legally defined but virtually unused; it appears in agricultural contexts for dry commodities like grains. The dry gallon is out of scope for fuel and cooking, but you may see it in historical data. For every practical purpose here, 'gallon' without a qualifier means the liquid gallon.

The dry gallon is a unit of volume for dry goods, not liquids. It is defined as 1/8 of a US bushel, or 268.8025 cubic inches (about 4.405 liters). It is larger than the US liquid gallon but smaller than the imperial gallon. You will almost never encounter it outside of US agricultural trade for commodities like wheat or corn. NIST Handbook 44 defines it, but for fuel, cooking, or household measuring, the dry gallon is irrelevant. If a spec sheet says 'gallon' and it is for a liquid, it is always the liquid gallon.

The difference extends to every subdivision. The US pint is 473.2 mL; the imperial pint is 568.3 mL, nearly 100 mL larger. The US quart is 946.4 mL; the imperial quart is 1.1365 L. And the fluid ounce, which you might assume is the same, is not. The US fluid ounce is 1/128 of a US gallon (29.57 mL), while the imperial fluid ounce is 1/160 of an imperial gallon (28.41 mL). That means the imperial fluid ounce is about 3.9% smaller than the US fluid ounce, even though the imperial gallon is larger. This is the most common trap in cooking: a UK recipe calling for 8 fluid ounces of milk means 227 mL, not 237 mL. Use the table above for the exact numbers.

The context tells you. If the spec is for a US car, US fuel pump, or US milk jug, it is almost certainly the US gallon. If the spec is from a UK government publication, a UK fuel station, or a UK draught beer measure, it is the imperial gallon. Canada and Australia use liters, so any 'gallon' there is likely a conversion error. For fuel economy, the US EPA publishes MPG using the US gallon (fueleconomy.gov). The UK publishes MPG using the imperial gallon. A car rated 30 MPG in the US is about 36 MPG in the UK, same fuel consumption, different gallons.

When in doubt, look for the exact liter value. If the spec says '1 gal = 3.785 L', it is the US gallon. If it says '1 gal = 4.546 L', it is the imperial gallon. If it says neither, assume the US gallon for US sources and imperial for UK sources. A spec sheet that says 'gallon' without a country or a conversion factor is a failure case. The 20% error is real: a 55 US gallon drum is 208.2 liters, but a '55 gallon' imperial drum would be 250.0 liters, a difference of 41.8 liters, enough to overflow your tank.

1707: Queen Anne's gallon (wine gallon) defined as 231 cubic inches; becomes the basis for the US gallon.
1824: UK Weights and Measures Act 1824 creates the imperial gallon as the volume of 10 pounds of water at 62°F (277.274 in³).
1893: US Mendenhall Order adopts the 231 in³ wine gallon as the US gallon.
1959: International inch redefined as exactly 2.54 cm; US gallon becomes exactly 3.785411784 L.
1963: UK Weights and Measures Act redefines the imperial gallon as 4.546092 L.
1985: UK Weights and Measures Act 1985 Schedule 1 fixes the imperial gallon at 4.54609 L exactly.

This subject suits anyone who moves between US and UK contexts: comparing fuel economy (MPG) across countries, ordering industrial drums (55 US gallons vs 208.2 liters), converting a UK recipe to US cups, or reading a spec sheet that says 'gallon' without a flag. It also suits students who need the exact conversion factors with legal sources.

Skip this if you only work in metric and never encounter gallons. Professional engineers already know the definitions and need a standards document (NIST SP 811 or the UK National Physical Laboratory), not a summary. If you are converting a single recipe and the context is unambiguous, a US recipe calling for 1 US gallon of water, a simple calculator without the US/imperial distinction is fine.That is the one thing that most often goes wrong here.