Two gallons, one litre
The litre is defined the same way everywhere: one cubic decimetre. The gallon is not. The United States uses a gallon of 231 cubic inches, which is 3.785411784 litres exactly. The United Kingdom, Ireland (historically), Canada (historically) and a handful of Caribbean countries use or used the imperial gallon of 4.54609 litres exactly. The imperial gallon is 20.095% bigger.
That gap is the origin of nearly every fuel economy confusion on this site. Convert a tank capacity, a fuel purchase or an mpg figure with the wrong gallon and the answer is off by a fifth. This page and its reverse handle the volume step on its own; the US vs UK gallon guide explains the history.
The formulas
litres = US gallons × 3.785411784
litres = imperial gallons × 4.54609
Both factors are legal definitions rather than measurements. The US figure follows from the inch being defined as 25.4 mm: 231 cubic inches times 16.387064 cubic centimetres per cubic inch is 3,785.411784 cm³. The imperial figure was set directly in litres by the UK Weights and Measures Act 1985.
Worked examples: fuel tanks and purchases
An American sedan with a 14.5 gallon tank holds 54.9 litres. A pickup with a 26 gallon tank holds 98.4 litres. A British hatchback described in an old handbook as having a 10 gallon tank holds 45.5 litres, because that handbook meant imperial gallons.
Buying fuel in Europe with an American mindset: 40 litres at the pump is 10.57 US gallons. If the price is 1.80 per litre, that is 6.81 per US gallon, which is the number to compare with a US pump price. The litres to gallons page does that direction.
Where each gallon is used today
The US gallon is used at every pump in the United States and in a few countries that follow US practice, including Liberia, Belize, Guatemala and parts of the Caribbean. The imperial gallon is no longer used at pumps in the UK, which went to litres between 1985 and 1995, but it survives in British mpg figures and in the fuel economy conventions of a few Commonwealth countries. Canada abandoned it in the 1970s.
Litres or liters, the spelling follows the country; the volume is identical.
Mistakes to avoid
- Assuming 'gallon' means the same thing in a British and an American document.
- Rounding to 3.8 or 4.5. Each is under 1% out, which adds up over a fleet or a year.
- Converting a US gallon price to a per-litre price with the imperial factor, which makes fuel look 20% cheaper than it is.
- Confusing the US liquid gallon with the US dry gallon (4.405 L), a unit almost nobody outside agriculture has met.