Fuel Economy Calculator (MPG and L/100 km)
Your car’s real fuel economy from the distance you drove and the fuel you used.
Good to know. Fill up, reset the trip meter, drive, fill up again. The second fill-up is exactly the fuel you used.
How it works
In plain words
Divide the fuel by the distance and multiply by 100 for litres per 100 km. For miles per gallon, divide the distance by the fuel. A US gallon is 3.785 litres and a UK gallon 4.546.
The formula
L/100 km = litres ÷ km × 100 mpg = miles ÷ gallons US mpg ≈ 235.2 ÷ L/100 km
Worked example
500 km on 35 litres: 35 ÷ 500 × 100 = 7 L/100 km, the same as 14.3 km per litre, 33.6 US mpg or 40.4 UK mpg.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B Exact conversion factors for US customary units.
- Weights and Measures Act 1985, Schedule 1 1 UK gallon = 4.54609 litres.
Sources checked October 3, 2026. 4 automated tests check its answers on every change.
About the fuel economy calculator (MPG and L/100 km)
The fuel economy on the window sticker comes from a standard test; this calculator measures your own. The simplest way is the fill-up method: fill the tank to the brim and reset the trip meter, drive normally, and fill up again. The fuel the second fill-up takes is what you used over the distance on the meter.
Different countries count it differently. Most of the world uses litres per 100 km, where lower is better. The US uses miles per US gallon, and the UK miles per imperial gallon, which is about 20% larger, so the same car shows a higher mpg in Britain than in America.
Measure over a full tank or several, rather than one short trip: city driving, motorway speed, air conditioning, cold weather and a roof box all change the result. If your figure drops suddenly, check the tyre pressure first.
To turn the figure into money, the fuel cost calculator works out a trip, and the cost of owning a car calculator puts fuel next to everything else. The fuel economy converter switches a figure you already have between units.
Questions people ask
How do I calculate my car’s mpg?
Divide the miles driven by the gallons used. 300 miles on 10 US gallons is 30 mpg.
How do I calculate litres per 100 km?
Divide the litres used by the kilometres driven and multiply by 100. 35 litres over 500 km is 7 L/100 km.
Why is UK mpg higher than US mpg for the same car?
A UK (imperial) gallon is 4.546 litres and a US gallon 3.785, so there are more miles in a UK gallon.