Free calculator
MPG calculator
Fill the tank, note the miles until the next fill, and this works out the mpg your car actually returns, plus your cost per mile.
Prices prefilled from the official Fuel Finder data, updated 28 August 2026. Change any box and the answer updates as you type.
How to measure your real mpg
Fill the tank right up and zero the trip counter. Drive as normal. Next time you fill up, fill it right up again and note two numbers, the miles on the trip counter and the litres the pump put in. Enter them here and you have your car's true economy for that tank, which is far more useful than the official figure, because it reflects your roads, your driving and your car as it actually is.
One tank can be skewed by a single long motorway run or a week of short cold starts, so for a reliable figure repeat it over three or four fills and take the average.
Why it is worth knowing
Once you know your real mpg, every other fuel sum becomes accurate. Our fuel cost calculator and commute cost calculator both work best with the number you measure here rather than a guess. And if the figure is lower than you hoped, our guide on how to use less fuel has the habits that lift it.
Common questions
How do I calculate mpg?
Divide the miles driven by the gallons used. Since UK pumps show litres, divide the litres by 4.546 first to get gallons. Fill the tank fully both times and use the trip counter for the miles.
Why is my mpg lower than the official figure?
Official figures come from a standard test that is gentler than real roads. Cold starts, short trips, hills, speed, load and tyre pressure all pull real world economy below the brochure number, so a gap of ten to twenty percent is normal.
What is a good mpg?
It depends on the car. A small petrol car commonly returns 45 to 55 mpg in mixed driving, a family diesel 50 to 60, and a large SUV or performance car 25 to 35. Compare your figure with your own car's official number rather than another model.