Petrol cost calculator
The cost of a UK journey in petrol or diesel, from the miles, your car's mpg and the forecourt price in pence per litre.
How the petrol calculation works
cost = miles ÷ mpg × 4.546 × pence per litre ÷ 100
UK drivers get stuck between two units: cars are rated in miles per imperial gallon, but fuel is priced per litre. The calculator converts gallons into litres (4.546 L each) so the answer matches what you'll pay at the pump. Using a US mpg figure by mistake would overstate your cost by about 20%, because a US gallon is smaller.
Petrol cost per mile at a glance
| mpg (UK) | at 130p/L | at 140p/L | at 150p/L |
|---|---|---|---|
| 30 mpg | 19.7p | 21.2p | 22.7p |
| 35 mpg | 16.9p | 18.2p | 19.5p |
| 40 mpg | 14.8p | 15.9p | 17.0p |
| 45 mpg | 13.1p | 14.1p | 15.2p |
| 50 mpg | 11.8p | 12.7p | 13.6p |
| 55 mpg | 10.7p | 11.6p | 12.4p |
| 60 mpg | 9.8p | 10.6p | 11.4p |
| 65 mpg | 9.1p | 9.8p | 10.5p |
Claiming business mileage? HMRC's approved rate is 45p a mile for the first 10,000 business miles, then 25p. Anything above your real fuel cost per mile is meant to cover wear, servicing and depreciation. The cost per mile calculator shows your full running cost. Driving abroad in km? Use the fuel consumption calculator.
Questions drivers ask
How do I work out the petrol cost of a journey?
Litres used = miles ÷ mpg × 4.546. Cost = litres × price per litre. For 180 miles at 45 mpg with petrol at 138p: 180 ÷ 45 × 4.546 = 18.2 litres, × £1.38 = £25.11.
Why multiply by 4.546?
UK cars are rated in miles per imperial gallon, but UK forecourts sell by the litre. One imperial gallon is 4.546 litres, so this converts gallons used into litres you pay for.
Is diesel cheaper to run than petrol?
Diesel usually costs a few pence more per litre, but diesel cars typically go 20–30% further on a litre. For high-mileage motorway drivers, diesel is usually cheaper per mile; for short town trips the difference shrinks.
How much is petrol per mile?
At 138p a litre, a 45 mpg car spends about 14p per mile on petrol (4.546 × 1.38 ÷ 45). A 35 mpg car spends about 18p.
What mileage allowance does HMRC allow?
HMRC's approved mileage allowance payment for cars and vans is 45p per mile for the first 10,000 business miles in a tax year and 25p after that. It covers all running costs, not just fuel.