Why higher MPG and lower L/100km mean the same thing
Miles per gallon counts distance: how far the car goes on one gallon of fuel. Litres per 100 km counts fuel: how much the car drinks to cover a fixed 100 km. They measure the same efficiency from opposite ends, so the scales run in opposite directions. A car that improves from 30 to 40 mpg has gone from 7.84 down to 5.88 L/100km. Higher is better on one scale and lower is better on the other.
This trips people up constantly, especially when comparing a European brochure with an American one. If you remember one thing from this page, make it this: a small L/100km number is a frugal car, and a big MPG number is a frugal car. They will never both be big for the same vehicle.
Because the two units are reciprocals of each other, the conversion is a division rather than a multiplication. That is also why the gaps are uneven. Going from 10 to 20 mpg saves 11.76 litres per 100 km; going from 40 to 50 mpg saves only 1.18. The MPG or L/100km guide covers why that matters when you are choosing between two cars.
The formula
L/100km = 235.214583 ÷ MPG (US gallon)
The constant is not a rounded convenience. It is 100 multiplied by 3.785411784 (the litres in a US gallon, an exact legal definition) divided by 1.609344 (the kilometres in a mile, also exact). Every number in the converter and every row of the table below is computed from those two definitions rather than from a rounded factor.
If your MPG figure is an imperial one, the constant becomes 282.480936, because the imperial gallon holds 4.54609 litres. Use the US/UK toggle on the converter, or the dedicated MPG (UK) to L/100km page.
A worked example
Take a mid-size sedan rated at 30 mpg combined on a US window sticker. Divide 235.214583 by 30 and you get 7.84 L/100km. The same maths for a 45 mpg hybrid gives 5.23 L/100km, and for a 16 mpg pickup gives 14.70 L/100km.
Run it in reverse to check: 235.214583 divided by 7.84 is 30.0, so the numbers close. That reverse step is the whole of the L/100km to MPG converter.
Where each unit is used
MPG is the official unit on US fuel economy labels (EPA figures, always US gallons) and on UK ones (WLTP figures, always imperial gallons). Canada, Australia, New Zealand, South Africa and every EU country publish L/100km. Japan, India, Brazil and much of South East Asia prefer km/L, which has its own converter.
Litres, or liters in American English, is the same unit either way; only the spelling changes. Both spellings appear on this site because both are searched for, and the maths does not care which you use. The unit itself gets written many ways too: L/100km, l/100, ltr/100km, litres per 100 km, even 'l/km' in a hurry. They all mean litres per hundred kilometres, and this page answers all of them.
Mistakes to avoid
- Using the US constant for a British figure. A UK car claiming 50 mpg is at 5.65 L/100km, not 4.70. The 20% gap between the gallons is the single most common error in fuel economy conversions.
- Multiplying instead of dividing. 30 mpg times 235 is nonsense; 235 divided by 30 is the answer.
- Comparing an official lab figure with a real-world one. Sticker MPG and measured MPG can differ by 10 to 20%, for reasons the real-world MPG guide explains.
- Treating a one-mpg gain as equal everywhere. It is worth far more at 15 mpg than at 50, because of the reciprocal relationship above.