How the fuel cost calculator works
Car fuel economy in the UK is quoted in miles per imperial gallon, but fuel is sold by the litre. The calculator works out how many gallons the journey needs, converts that to litres (one UK gallon is 4.54609 litres), then multiplies by the price per litre.
Fuel cost = litres used × price per litre
Tick "return trip" to double the distance, and enter the number of people in the car to split the cost fairly. The price defaults to the UK average for unleaded petrol of 174.6p a litre, taken from the government's weekly road fuel price statistics for 19 October 2026. Diesel averaged 199.3p. Prices vary a lot by area and forecourt, so put in what you actually pay.
Worked example: 100 miles at 45 mpg
- Gallons needed: 100 ÷ 45 = 2.222
- Litres: 2.222 × 4.54609 = 10.1 litres
- Cost: 10.1 litres × £1.746 = £17.64, or 17.6p a mile
Make it a return trip with four people sharing and it's £35.28 in total, or £8.82 each.
Cost per mile at different fuel economies
At 174.6p a litre for petrol and 199.3p for diesel:
| Fuel economy | Petrol per mile | Diesel per mile | Petrol per 100 miles |
|---|---|---|---|
| 30 mpg | 26.5p | 30.2p | £26.46 |
| 40 mpg | 19.8p | 22.7p | £19.84 |
| 50 mpg | 15.9p | 18.1p | £15.87 |
| 60 mpg | 13.2p | 15.1p | £13.23 |
What MPG should I use?
Use your own figure if you can. Most cars show average mpg on the dashboard, though these readouts are often a little optimistic. For a more accurate number, brim the tank, reset the trip counter, and the next time you fill up divide the miles driven by the litres you put in, then multiply by 4.54609. Our MPG calculator does this for you.
The official WLTP figure in your car's brochure is a good starting point, but real-world economy is usually lower. Motorway driving at 70 mph, short trips with a cold engine, roof boxes, heavy loads and air conditioning all use more fuel. Long steady runs at 50 to 60 mph often beat the official figure.
Splitting fuel costs fairly
For lift shares and trips with friends, splitting the fuel cost equally is the usual approach. If you're giving someone a lift for money, keep it to a share of running costs: charging passengers more than the cost of the trip could count as carrying passengers for hire, which your insurance may not cover.
Claiming business mileage? For 2026/27, HMRC's approved mileage allowance for cars rose to 55p a mile for the first 10,000 business miles in the tax year (backdated to 6 April 2026), with 25p a mile after that. Those rates are meant to cover all running costs, not just fuel.
Tips to cut fuel costs
- Check pump prices before you set off. Supermarket forecourts are often several pence a litre cheaper than motorway services.
- Keep tyres at the right pressure and remove roof bars when you don't need them.
- Drive smoothly: hard acceleration and braking can use far more fuel than steady driving.
- Thinking about switching? Compare the cost of charging with the EV charging cost calculator, or work out monthly payments with the car finance calculator.
Frequently asked questions
How much does it cost to drive 100 miles?
At 45 mpg and 174.6p a litre, 100 miles costs £17.64 in petrol. At 35 mpg it's £22.68, and a diesel at 50 mpg paying 199.3p a litre costs £18.12.
How many litres are in a UK gallon?
One imperial gallon is 4.54609 litres. A US gallon is smaller at 3.785 litres, so US mpg figures look lower for the same car.
How do I work out fuel cost per mile?
Multiply the price per litre by 4.54609, then divide by your mpg. At 174.6p a litre and 45 mpg, that's 793.7p ÷ 45 = 17.6p a mile.
How much fuel will I use on a 200-mile trip?
At 45 mpg you'll use 200 ÷ 45 × 4.54609 = 20.2 litres. At 174.6p a litre that's £35.28.
Should I split petrol money with passengers?
Splitting the fuel cost equally is common and usually fine. Avoid charging more than the cost of the journey, as that can count as carrying passengers for hire and may not be covered by your insurance.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.