Journey fuel cost calculator
Work out how much a car journey will cost in petrol or diesel, and how to split it fairly with friends.
The fuel cost formula
Cost = (miles ÷ MPG) × 4.546 × price per litre
The 4.546 converts UK (imperial) gallons into litres, because UK fuel is sold by the litre but economy is quoted in miles per gallon.
Cost per mile at different MPG
At 140p a litre:
| MPG | Pence per mile | 100 miles costs |
|---|---|---|
| 30 | 21.2p | £21.22 |
| 40 | 15.9p | £15.91 |
| 50 | 12.7p | £12.73 |
| 60 | 10.6p | £10.61 |
Tips to use less fuel
- Keep your tyres at the correct pressure.
- Drive smoothly. Hard acceleration and braking use more fuel.
- Cruising at 60mph instead of 70mph can cut motorway fuel use noticeably.
- Remove roof boxes and racks when you're not using them.
Frequently asked questions
How do I calculate fuel cost for a journey?
Divide the miles by your car's MPG to get gallons used. Multiply by 4.546 to convert to litres, then multiply by the price per litre. 200 miles at 45 MPG and 140p a litre is 200 ÷ 45 × 4.546 × £1.40 = £28.29.
Is MPG in UK or US gallons?
UK car MPG figures use the imperial gallon, which is 4.546 litres. A US gallon is smaller (3.785 litres), so US MPG figures look lower for the same car. This calculator uses UK gallons.
What MPG does my car get?
Your car's trip computer usually shows average MPG. The official WLTP figure is a good starting point, but real-world economy is often 10–20% lower, especially on short trips, in winter or at motorway speeds.
What is the HMRC mileage rate?
HMRC's approved mileage allowance for business journeys in your own car is 45p a mile for the first 10,000 miles in a tax year and 25p a mile after that. It's meant to cover fuel plus wear and tear.
Last checked 5 October 2026.