How this instrument works
Two countries kept the word "gallon" after their definitions diverged. The United States never adopted Britain's 1824 Imperial reform and instead kept measuring by an older English wine gallon, standardized in US law as exactly 231 cubic inches, or 3.785411784 litres per the US National Institute of Standards and Technology. Britain's own gallon was redefined in the Weights and Measures Act 1985 as exactly 4.54609 litres, a figure that traces back to the original 1824 Imperial gallon. One US gallon therefore equals about 0.832674 UK gallons -- put the other way, the UK measure holds roughly 20% more liquid than the US one for the identical word.
That gap shows up anywhere both regions describe volume with the same unfamiliar-looking number. A car's fuel economy quoted as "40 miles per gallon" from a US source and "40 miles per gallon" from a UK source describe two different amounts of fuel burned, since the larger UK measure means the UK figure represents better real efficiency for an identical stated mpg. Shoppers comparing a US-spec tank capacity to a UK-market vehicle, or a US 55-gallon drum to an Imperial-rated container, run into the same trap: the number alone does not say which measure it means, and the two are never interchangeable without converting.
- Enter the amount you have into US gallons (gal); it opens at 1.
- Read the converted amount from UK / Imperial gallons (gal) beneath it -- it updates as soon as the first field changes.
- The page opens already showing 1 US gallon equal to 0.832674 UK gallons, so you can see the roughly 20% gap before entering your own figure.
- For a fuel tank, drum, or any container rated in US gallons, enter that rated capacity to see what it reads on a UK/Imperial scale, and vice versa.
Worked example -- 1 US gallon to UK gallons
Enter 1 into US gallons (gal), the field's default. UK / Imperial gallons (gal) reads 0.832674 -- one US gallon, 3.785411784 litres by definition, comes to a little over four-fifths of a UK gallon, since the UK measure is defined as the larger 4.54609 litres.
Change the entry to 15, a typical small-car tank size in the US, and UK / Imperial gallons (gal) reads 12.490113 -- the same tank described on a UK forecourt pump or spec sheet would be listed at roughly 12.5 Imperial gallons, not 15, despite holding the identical amount of fuel.
Questions
Why are US and UK gallons different sizes?
The US kept an older English wine gallon, fixed in US law at exactly 231 cubic inches (3.785411784 litres). The UK adopted a larger measure in 1824, later redefined by the Weights and Measures Act 1985 as exactly 4.54609 litres. The two countries simply diverged and neither switched to match the other, so both still call their own unit a "gallon."
How much bigger is a UK gallon than a US gallon?
About 20% bigger. One US gallon equals roughly 0.832674 UK gallons, which means one UK gallon equals about 1.20095 US gallons. A container holding 5 US gallons of liquid holds only about 4.16 UK gallons of the same liquid.
Does this affect fuel economy figures between the US and UK?
Yes. A car rated at "40 mpg" using the UK's larger measure travels farther per unit of fuel than a car rated at "40 mpg" using the smaller US one, because the UK figure divides distance by a bigger volume. Comparing mpg ratings across the two countries without converting first compares two different quantities under one label.
Is the UK gallon the same as the old Imperial gallon?
Yes -- "UK gallon" and "Imperial gallon" name the same unit. It was introduced in Great Britain in 1824, was fixed at exactly 4.54609 litres by the Weights and Measures Act 1985, and that same 4.54609-litre figure remains the UK's legal definition today under the Units of Measurement Regulations 1995.
How many UK gallons are in a standard 55-US-gallon drum?
About 45.80 UK gallons. A 55-US-gallon drum, a common size for shipping liquids in bulk, holds 45.7970801546 UK (Imperial) gallons -- worth checking before assuming a drum rated abroad matches a US-rated one of the same labeled number.
Are US and UK quarts and pints different too?
Yes, by the same underlying ratio, since both are defined as fractions of their own country's gallon: a US quart or pint is smaller than a UK quart or pint by roughly the same 20% gap described here. This page converts only the gallon-to-gallon figure.
References
- NIST Special Publication 811 -- Guide for the Use of the International System of Units
- NIST Handbook 44 -- Current Edition (Weighing and Measuring Devices, incl. unit tables)
- UK Weights and Measures Act 1985, Schedule 1 (original 4.54609-litre gallon definition)
- The Units of Measurement Regulations 1995 (S.I. 1995/1804) -- current legal gallon definition