Fuel cost calculator
What a journey costs in fuel, from your car's mpg and the pump price. Splits it between passengers and shows cost per mile.
Your mpg is in imperial gallons, and your fuel is in litres
This is the trap in the middle of every fuel calculation, and it is why two websites can give different answers for the same journey.
A UK car's mpg is miles per imperial gallon, which is 4.54609 litres. But an American gallon is only 3.785 litres. The imperial one is 20.1% bigger.
What the mix-up costs you. 100 miles at 45mpg is 2.22 gallons. Multiply by 4.546 and that is 10.1 litres. Use the American gallon by mistake and you get 8.41 litres, which understates your fuel by 16.7%. At 150p a litre that is the difference between £15.20 and £12.66 for the same drive. This calculator uses the imperial gallon, because the car does.
How it is worked out
Three steps, and you can do it on a receipt:
- Gallons. Miles divided by mpg. 100 ÷ 45 = 2.22 gallons.
- Litres. Gallons times 4.546. 2.22 × 4.546 = 10.1 litres.
- Cost. Litres times the pump price. 10.1 × £1.505 = £15.20.
The pump price is the only figure that moves much, so it is worth putting in the real one rather than a guess. UK petrol has been sitting near 150p a litre through mid-2026, but it varies by a good few pence between towns, and diesel runs higher.
Splitting petrol money
Set the number of people and it divides the total. Four people sharing a 100-mile trip at 45mpg pay £3.80 each, which is usually less than anyone expects and is why "just give me a fiver" is normally generous.
Only fuel is being split here. A longer trip in someone else's car also costs them tyres, servicing and depreciation, which is the reasoning behind the mileage rate below.
Cost per mile, and what you can claim
For business travel in your own car, HMRC's approved mileage rate is 55p a mile for the first 10,000 miles in a tax year, then 25p above that. The rate rose from 45p on 6 April 2026, having sat at 45p since 2011, so a lot of pages still quote the old figure.
The gap between your fuel cost and the approved rate is deliberate. Fuel might be 15p a mile; the other 40p is meant to cover wear, servicing, tyres, insurance and the value the car loses by being driven. Claiming the approved rate is not profit, it is the rest of the cost catching up.
Why your real mpg is lower than the brochure
Official mpg comes from a lab test (WLTP since 2017, NEDC before it), run to a fixed cycle at fixed temperatures with everything switched off. Real driving has hills, traffic, cold starts, roof boxes, air conditioning and your right foot.
Most cars return somewhere around 10% to 20% under the official combined figure, and short cold trips are far worse because an engine below temperature runs rich. Use the trip computer's long-term average if you want the honest number, or work it out by brimming the tank twice and dividing the miles by the litres.
Common questions
How do I work out the fuel cost of a journey?
Divide the miles by your mpg to get gallons, multiply by 4.546 to get litres, then multiply by the pump price. 100 miles at 45mpg is 10.1 litres, which is £15.20 at 150.5p a litre. Or put your numbers in above.
How many litres are in a gallon?
4.54609 litres in a UK imperial gallon, which is what your car's mpg is measured in. An American gallon is 3.785 litres, so it is 20.1% smaller. Using the American figure understates a UK car's fuel by 16.7%.
How much should I charge someone for a lift?
Set the number of people above and it splits the fuel. Four people over 100 miles at 45mpg is £3.80 each. That covers fuel only, so on a longer trip it is fair to add something for the wear on the driver's car.
What is the HMRC mileage rate?
55p a mile for the first 10,000 business miles in a tax year, then 25p above that, for cars and vans. It went up from 45p on 6 April 2026 after 14 years unchanged, so pages quoting 45p are out of date.
Why is the mileage rate higher than my fuel cost?
Because it is not just fuel. The approved rate is meant to cover servicing, tyres, insurance and depreciation as well. If fuel is 15p a mile, the other 40p is the rest of what running the car actually costs.
Why is my mpg worse than the official figure?
Official mpg comes from a lab cycle at a steady temperature. Real driving has traffic, hills, cold starts and air conditioning. Most cars land 10% to 20% below the combined figure, and short cold journeys are much worse than that.
How do I work out my car's real mpg?
Fill the tank to the brim, note the mileage, drive normally, then brim it again. Divide the miles covered by the litres it took, then multiply by 4.546. Or read the trip computer's long-term average, which is close enough for this.