Fuel cost calculator
Work out the petrol or diesel cost of a journey from the distance, your car’s mpg or litres per 100 km and the price a litre, and split it between passengers.
- Fuel for the return trip
- £17.38
- 100 miles at 45 mpg, 172.01p a litre
- Fuel used
- 10.1 litres
- 100 miles ÷ 45 mpg × 4.546 litres a gallon
- Cost a mile
- 17.4p
- £17.38 ÷ 100 miles
- The same economy as
- 6.3 litres per 100 km
- UK gallon: 4.546 litres
UK mpg uses the imperial gallon of 4.546 litres. A US gallon is smaller, so a US mpg figure is about a sixth lower for the same car. The UK average pump price on 21 September 2026 was 172.01p a litre for petrol and 195.53p for diesel (DESNZ weekly road fuel prices). Real-world economy is usually worse than the official figure, especially on short trips and in cold weather, so use your trip computer’s average if you have one. It doesn’t include parking, tolls or wear on the car.
What goes in
Enter the distance in miles or kilometres, whether it’s one way or there and back, your car’s fuel economy and the price a litre. Most UK cars show economy in miles per gallon; European figures use litres per 100 km, and you can use either. Add the number of people sharing to see each person’s share.
What the result means
A 50-mile trip each way is 100 miles. At 45 mpg, that uses 10.1 litres, and at 172.01p a litre it costs £17.38, about 17.4p a mile. Split three ways, it’s £5.79 each.
The fuel used is the distance ÷ mpg × 4.546 litres in a gallon. In litres per 100 km, it’s the distance in km × the figure ÷ 100.
The figures behind it
UK mpg uses the imperial gallon, which is 4.54609 litres by law. A US gallon is smaller, so a US mpg figure for the same car is about a sixth lower.
The default price is the government’s UK average for unleaded petrol on 21 September 2026, 172.01p a litre. Diesel was 195.53p. Pump prices change weekly and vary by area, so use the price you pay.
Your real economy is usually worse than the official figure, especially on short trips, in traffic and in cold weather. Your trip computer’s average is the better number to use. The cost covers fuel only, not parking, tolls or wear on the car.
Sources
- Weekly road fuel prices, Department for Energy Security and Net Zero
- Weights and Measures Act 1985, Schedule 1, legislation.gov.uk