MPG Calculator
Enter how far you drove and how much fuel it took. We give your mpg in US and UK gallons, and in litres per 100 km.
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The formula
mpg = miles driven / gallons used. L/100 km = litres used / km driven x 100.
The simplest way to measure it
Fill your tank to the top, note the odometer, and drive as usual. At the next fill-up, note how much fuel went in and how far you travelled. Divide the miles by the gallons. For 320 miles on 10 gallons, you got 32 mpg.
Why the units differ
A UK gallon is bigger than a US gallon, about 4.55 litres against 3.79, so the same car gets a higher mpg figure in the UK. The tool shows both, and litres per 100 km, so you can compare with any brochure.
Improve your mpg
Drive smoothly, keep tyres at the right pressure, remove roof racks you do not need and avoid idling for long. Check the fuel economy again after a month to see the effect.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Miles and US gallons; Distance driven: 320; Fuel used: 10 | Miles per US gallon: 32 |
| Miles and UK gallons; Distance driven: 400; Fuel used: 8 | Miles per US gallon: 41.63 |
| Kilometres and litres; Distance driven: 600; Fuel used: 38 | Miles per US gallon: 37.14 |
MPG Calculator: frequently asked questions
Is a bigger or smaller number better?
For mpg, bigger is better. For litres per 100 km, smaller is better.
How accurate is this estimate?
It is as accurate as the numbers you give it. Real-world amounts such as paint, concrete or fuel vary, so allow a margin.
Is what I enter in this tool saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this tool free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026