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
If the car is a company car, the running cost and the tax bill are two separate things. Wagepage's company car tax calculator works out the benefit in kind from the list price and CO2 figure.
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.
That was one of several UK pay changes in April 2026, set out in what changed for UK pay in April 2026.
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.
Carrying passengers is worth 5p a mile
If you drive colleagues on a business journey, your employer can pay you an extra 5p per mile for each passenger, tax-free, on top of the normal mileage rate. It does not have to be reported to HMRC and no National Insurance is due on it.
What that adds up to. A 200-mile round trip with three colleagues in the car: 200 miles at 55p is £110, plus 3 × 200 × 5p, which is another £30. The passengers are worth more than a quarter as much again as the journey itself.
One catch that makes it different from ordinary mileage. There is no Mileage Allowance Relief for passenger payments. With the normal rate, an employer paying you less than 55p means you can claim tax relief on the shortfall. With passenger payments, if your employer does not pay them, there is nothing to claim. It is money that only exists if the employer chooses to hand it over, so it is worth asking rather than assuming.
Motorbikes and bicycles have their own rates
The approved rates are not the same across vehicles, and two of them did not move when the car rate rose to 55p in April 2026:
- Cars and vans: 55p for the first 10,000 business miles, then 25p
- Motorcycles: 24p, flat, with no change at 10,000 miles
- Bicycles: 20p, flat
The motorcycle and bicycle rates have no upper tier, so unlike a car they do not drop after 10,000 miles. Ride enough and a motorbike eventually pays better per mile than a car does past the threshold.
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.
Can I claim extra for carrying passengers?
Your employer can pay 5p per mile for each colleague carried on a business journey, tax-free and without reporting it to HMRC. But there is no Mileage Allowance Relief for passenger payments, so unlike normal mileage you cannot claim the shortfall back if your employer does not pay it.
What is the mileage rate for a motorbike?
24p a mile, flat, with no drop after 10,000 miles. Bicycles are 20p on the same flat basis. Neither moved when the car and van rate rose to 55p in April 2026.