MPG to L/100km Converter

The conversion factors used

One UK (imperial) gallon is exactly 4.54609 litres, one US gallon is exactly 3.785411784 litres, and one mile is exactly 1.609344 km. That means 100 km is 100 ÷ 1.609344 ≈ 62.137 miles. A car doing 40 UK mpg uses 62.137 ÷ 40 ≈ 1.553 UK gallons to cover that distance, and at 4.54609 litres to the gallon that's 1.553 × 4.54609  ≈ 7.06, which rounds to 7.1 l/100km - matching the reference table above. Multiplying the constants together once gives a direct shortcut used throughout this page: l/100km = 282.481 ÷ UK mpg (100 × 4.54609  ÷ 1.609344 ≈ 282.481), and l/100km = 235.215 ÷ US mpg (100 × 3.785411784 ÷ 1.609344  ≈ 235.215). Every figure on this page rounds to 1 decimal place on screen.

Why lower is better for l/100km

Mpg and l/100km run in opposite directions, which trips people up the first time they see both on a spec sheet. Mpg counts distance per unit of fuel, so a bigger number means the car goes further on the same gallon - more is better. L/100km counts fuel used over a fixed distance, so a bigger number means the car burns more fuel to cover the same 100 km - less is better. They're not two versions of the same scale; one measures efficiency going up, the other measures consumption going down. A drop from 7.1 to 5.6 l/100km (the 40 to 50 UK mpg rows in the table above) is an improvement, even though the number itself has fallen.

Why UK and US mpg aren't the same number

The UK (imperial) gallon and the US gallon are different sizes - 4.54609 litres against 3.785411784 litres, with the US gallon about a fifth smaller. A car quoted at 30 US mpg converts to 235.215 ÷ 30 ≈ 7.84 l/100km, which converts back to 282.481 ÷ 7.84 ≈ 36.0 UK mpg. So a US 30 mpg car is roughly a UK 36 mpg car - the same fuel economy, but two very different-looking numbers, purely because of gallon size and nothing to do with the engine. Any US spec sheet mpg figure needs converting before it's comparable to a UK one, and l/100km is the easiest common ground since it doesn't depend on either gallon.

Do WLTP and other official figures match real-world driving?

Official fuel economy figures (WLTP in the UK and EU) are measured under standardised lab conditions on a rolling road, not on the road itself. They're useful for comparing one model against another on a level basis, but real-world consumption is typically higher (worse) than the lab figure, and by how much depends on driving style, load, tyre pressure, weather and how much of the journey is motorway versus stop-start town driving. Treat the official mpg or l/100km figure as a baseline for comparing cars, not a promise of what any individual car will actually return.

Related tools

Converting a spec-sheet figure into what a specific journey will actually cost? The journey fuel cost calculator takes an mpg or l/100km figure plus distance and fuel price and gives a cost. Weighing a petrol or diesel car's running cost against an EV rather than converting a fuel-economy figure? The EV vs petrol cost comparator covers that comparison directly.