How to calculate the fuel cost of a journey
Working out what a car journey costs needs three numbers: how far you are going, how many miles your car does per gallon, and what fuel costs at the pump. This fuel cost calculator turns those into the cost of the trip, the cost per mile, each passenger’s share and what the same journey adds up to over a year.
The formula
Fuel used in litres = miles ÷ mpg × 4.546. Fuel cost = litres × price per litre. The 4.546 is there because UK mpg is measured in imperial gallons, while fuel is sold by the litre.
A 100-mile trip in a car that does 42mpg uses about 10.82 litres. At £1.62 a litre that is £17.53, or 17.5p a mile. The return trip costs £35.07.
Get the units right
A US gallon is only 3.785 litres, so American mpg figures look lower for the same car. Using a US figure with UK pump prices makes a trip look about a fifth more expensive than it is. Across Europe, economy is quoted in litres per 100km instead — the calculator switches units when you change region.
What 100 miles costs at different mpg
Petrol at £1.62 a litre. The difference between an older large car and an efficient small one is more than double.
| Fuel economy | Litres per 100 miles | Cost per 100 miles | Cost per mile |
|---|---|---|---|
| 30 mpg | 15.15 L | £24.55 | 24.6p |
| 42 mpg | 10.82 L | £17.53 | 17.5p |
| 55 mpg | 8.27 L | £13.39 | 13.4p |
| 65 mpg | 6.99 L | £11.33 | 11.3p |
Every 10p on the price of a litre adds about £1.08 to a 100-mile trip at 42mpg.
Commuting and yearly fuel costs
A daily commute
A 20-mile drive each way, five days a week, is 10,400 miles a year. At 42mpg that burns about 1,126 litres — £1,823.63 at £1.62 a litre. Set trips per week to 10 and the distance to 20 miles one way to see your own version.
A typical year of driving
At 8,000 miles a year and 42mpg, fuel costs £1,402.79. A rise of 10p a litre adds £86.59 to that year. Compare the same mileage in an electric car with the EV charging cost calculator, which uses the same pump price.
Sharing the cost of a trip
Splitting fuel fairly
Fuel cost does not rise noticeably with each passenger, so splitting it evenly is the simplest fair method. Three people sharing a 100-mile trip at 42mpg pay £5.84 each. A 400-mile drive — roughly London to Edinburgh — costs £70.14 in fuel, or £17.54 each for four people.
Mileage allowance is not fuel cost
HMRC’s approved mileage rate is 45p a mile for the first 10,000 business miles in a tax year, then 25p. That covers wear, servicing and insurance as well as fuel, so 100 business miles pays £45 against a fuel cost of about £17.53. Use the calculator for the fuel, not the allowance.
Ways to use less fuel
Speed and driving style
Air resistance rises steeply with speed, so dropping from 70mph to 60mph on the motorway is one of the biggest savings available. Smooth acceleration, reading the road ahead and avoiding long idling all help too.
Weight, drag and tyres
- Take off empty roof boxes and bike racks
- Keep tyres at the pressure in the handbook
- Clear heavy items out of the boot
- Combine short trips — a cold engine uses more fuel
Find your real mpg
Brim the tank, reset the trip meter, and at the next fill divide the miles by the litres added, then multiply by 4.546. Dashboard figures usually read optimistic, and a real figure makes every estimate here more accurate.
Petrol, diesel and carbon
Diesel
Diesel usually costs a little more per litre, but diesel cars often go further on each one. If diesel cost £1.70 a litre, a car doing 55mpg would spend £14.05 on 100 miles — less than the 42mpg petrol car above. Enter both cars’ real figures to compare.
CO₂ per journey
Burning a litre of petrol releases about 2.31kg of CO₂, and a litre of diesel about 2.68kg. So 100 miles at 42mpg in a petrol car is roughly 25kg. For other household running costs, see the electricity cost calculator and our guide to how much it costs to charge an electric car.