How to Work Out the Fuel Cost of a Journey
Updated 7 Nov 2026
The simple formula for petrol or diesel cost per journey in the UK, worked examples at today’s prices, and how to convert mpg and litres per 100 km.
UK pumps sell fuel by the litre, but most drivers know their car’s economy in miles per gallon. The trick is the UK gallon: 4.546 litres.
The formula
Cost = miles ÷ mpg × 4.546 × price per litre
For a 200-mile drive (roughly London to Manchester) in a petrol car doing 50 mpg, at today’s average E10 price of 171.0p, the fuel costs about £31.10. The same trip in a diesel doing 55 mpg, at 198.3p a litre, costs about £32.79.
Converting fuel economy
- Litres per 100 km to mpg (UK): 282.5 ÷ L/100 km. A car using 5.6 L/100 km does about 50 mpg.
- US mpg to UK mpg: multiply by 1.2, because the US gallon is smaller (3.785 litres).
- Official vs. real figures: official WLTP figures are measured in a lab. Real-world economy is often 10–20% worse, more on short trips, in cold weather and at high motorway speeds.
Sharing the cost
Split the fuel by the number of people in the car. A return trip doubles the distance. Parking, tolls, congestion and clean-air zone charges come on top.
Our journey fuel cost calculator does all of this for you: enter where you start and finish, pick your car, and it uses today’s average price in your area.