MPG calculator
This calculator works out the real miles per gallon from the distance driven and the fuel used, in litres or imperial gallons, then converts that into a cost per mile from the pump price entered. For example, 300 miles on 30 litres gives 45.5 MPG, or 14.0p a mile at 140p a litre. No statutory rate applies to this calculation.
What does the MPG number actually tell you?
Miles per gallon is how far the car goes on one imperial gallon of fuel, and a higher number is better. On its own it is hard to act on, so this tool also prices the fuel used and divides it by the miles, giving pence per mile.
That single figure makes it possible to compare a petrol car with an electric one, or to check whether a work mileage rate actually covers the fuel used. Measuring over a whole tank rather than a quick top-up gives a more reliable reading, since a small quantity of fuel exaggerates any error.
What does the worked example show?
Suppose you drove 300 miles and refilled with 30 litres. That is 6.60 imperial gallons, so 300 divided by 6.60 gives 45.5 MPG. With fuel at 140p a litre, the 30 litres cost £42.00, which across 300 miles is 14.0p a mile. If the same trip had taken 35 litres, the MPG would fall and the cost per mile would climb.
Common questions
How do I measure my MPG?
Fill the tank, reset the trip meter, drive normally, then fill up again and note the litres it took and the miles on the trip. Enter both here. Doing it over a full tank rather than a small top-up gives a truer figure.
Why is my MPG lower than the official figure?
Official WLTP figures come from a controlled test. Real driving, with cold starts, short trips, motorway speeds and a loaded car, usually lands 10 to 20 per cent lower. A gap of that size is normal, not a fault.
Litres or gallons, which should I enter?
Enter litres if you filled up in the UK, because pumps dispense litres. Choose gallons only if you genuinely measured in imperial gallons. The tool converts at 4.54609 litres per imperial gallon either way.
How does MPG become cost per mile?
The fuel used is priced at the pump rate you enter, then divided by the miles covered. That gives pence per mile, which is far easier to compare against an electric car or a work mileage rate than MPG alone.
Is this imperial or US MPG?
Imperial, as used in the UK. One imperial gallon is 4.54609 litres, about 20 per cent larger than a US gallon, so UK MPG figures read higher than US ones for the same car.
What does the MPG number actually tell you?
Miles per gallon shows how far a car travels on one imperial gallon of fuel, and a higher figure means better economy. On its own it is hard to act on, so this tool also prices the fuel used and divides it by the miles driven, giving pence per mile. That single figure makes it possible to compare a petrol car against an electric one, or to check whether a work mileage rate actually covers what fuel costs.
What does the worked example show?
Driving 300 miles on 30 litres works out at 6.60 imperial gallons, giving 45.5 MPG. At 140p a litre, those 30 litres cost an estimated £42.00, which across 300 miles comes to 14.0p a mile. A lower fuel quantity for the same distance would raise the MPG figure and lower the cost per mile.
Sources & Rate Verification
Rates verified against official statutory guidance on 6 September 2026.