One of the most common conversion errors comes from a single word that means two different things: the gallon. The US gallon and the imperial (UK) gallon differ by about 20%, which is more than enough to ruin a recipe, a fuel-economy comparison, or a chemistry calculation.
The numbers
- 1 US gallon = 3.785411784 litres (exact)
- 1 imperial (UK) gallon = 4.54609 litres (exact)
So an imperial gallon is roughly 1.2 times larger than a US gallon. A “gallon” of petrol in London is meaningfully more fuel than a “gallon” in Los Angeles.
Why they differ
The US gallon is based on the old English wine gallon, while the imperial gallon was redefined by the British in 1824 based on the volume of ten pounds of water. The two systems diverged and never reconverged.
The difference cascades to every sub-unit: US and imperial quarts, pints, and fluid ounces are all different sizes too. A US pint is 16 US fluid ounces; a UK pint is 20 imperial fluid ounces.
How to convert safely
Always know which gallon you have before converting:
- Convert US gallons to litres or litres to US gallons.
- For the imperial gallon, use the UK gallon unit in the volume converter.
A note on fuel economy
Miles per gallon (mpg) suffers from the same ambiguity. UK mpg figures look better than US mpg figures for the same car simply because the UK gallon is bigger. When comparing vehicles across regions, convert both to litres per 100 km using the fuel economy converter for a fair comparison.