Reading a metric fuel receipt
Almost everywhere outside the United States, fuel is sold by the litre. The receipt says 47.3 litres and a price per litre, and an American driver wants to know what that is in gallons and per gallon. Divide by 3.785411784 and 47.3 litres is 12.50 US gallons. A British driver, who still thinks in imperial mpg even though the pump is in litres, divides by 4.54609 and gets 10.40 imperial gallons.
That second calculation is the one British drivers do without realising, every time they work out their mpg from a fill-up. The pump gives litres, the odometer gives miles, and the gallon they divide by is the imperial one. The manual calculation guide shows the full method with a worked fill-up.
The formulas
US gallons = litres ÷ 3.785411784
imperial gallons = litres ÷ 4.54609
The reciprocals are 0.264172 US gallons per litre and 0.219969 imperial gallons per litre, if you prefer to multiply. Both gallons are exact legal definitions, so the answers here are exact to the precision shown.
Worked examples
A European hatchback lists a 45 litre tank. That is 11.89 US gallons or 9.90 imperial gallons. A large SUV with an 85 litre tank holds 22.45 US gallons or 18.70 imperial. A typical 50 litre fill costs, at 1.75 per litre, 87.50; per US gallon that is 6.62, and per imperial gallon it is 7.96.
Notice the per-gallon price is higher for the imperial gallon, as it must be: you are buying more fuel per gallon. This is why comparing pump prices between the US and UK by converting to gallons without saying which one produces nonsense.
Which gallon do you mean?
If you are converting for an American context, an EPA rating, a US tank size, a US pump price, use the US gallon. If you are converting for a British context, a UK mpg figure, an old British handbook, a UK fuel economy claim, use the imperial gallon. Canada, Australia, Ireland and New Zealand went metric and no longer use either gallon officially, though older documents from those countries mean the imperial one.
Litres and liters are the same unit; the spelling is the only difference, and this page follows the British form.
Mistakes to avoid
- Dividing by 3.785 when the context is British, which overstates the gallons by 20%.
- Using 'gallon' unlabelled in a document that will be read on both sides of the Atlantic.
- Working out mpg from litres without converting them first. Miles divided by litres gives miles per litre, which is a perfectly good unit but not the one anyone quotes.
- Rounding 4.54609 to 4.5. The error is small but it compounds over a year of fill-ups.