About this calculator
How much will that journey cost in petrol or diesel? Enter the distance, your car's fuel economy and the price per litre and get the fuel used, the trip cost and the cost per mile.
Pick UK mpg, US mpg or litres per 100 km to match your dashboard or handbook. UK and US gallons are different sizes, so choosing the right one matters.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
100 miles at 40 UK mpg, £1.45 a litre
- Trip fuel cost
- £16.48
- Fuel used
- 11.365225 litres
- Cost per mile
- £0.16
- Cost per km
- £0.10
- Fuel economy in L/100 km
- 7.062023 L/100 km
100 miles at 40 UK mpg uses 11.365225 litres of fuel and costs £16.48 at £1.45 a litre.
Show the working
- Fuel = miles × 4.546 litres per UK gallon ÷ mpg = 100 × 4.546 ÷ 40 = 11.365225 litres.
- Cost = litres × price per litre = 11.365225 × £1.45 = £16.48.
- UK and US gallons are different sizes, so pick the right mpg type: a UK gallon is 4.546 litres, a US gallon 3.785.
100 km at 6 L/100 km, £1.50 a litre
- Trip fuel cost
- £9.00
- Fuel used
- 6 litres
- Cost per mile
- £0.14
- Cost per km
- £0.09
- Fuel economy in L/100 km
- 6 L/100 km
100 km at 6 L/100 km uses 6 litres of fuel and costs £9.00 at £1.50 a litre.
Show the working
- Fuel = distance in km × L/100 km ÷ 100 = 100 × 6 ÷ 100 = 6 litres.
- Cost = litres × price per litre = 6 × £1.50 = £9.00.
- UK and US gallons are different sizes, so pick the right mpg type: a UK gallon is 4.546 litres, a US gallon 3.785.
250 miles at 30 US mpg, £1.40 a litre
- Trip fuel cost
- £44.16
- Fuel used
- 31.545098 litres
- Cost per mile
- £0.18
- Cost per km
- £0.11
- Fuel economy in L/100 km
- 7.840486 L/100 km
250 miles at 30 US mpg uses 31.545098 litres of fuel and costs £44.16 at £1.40 a litre.
Show the working
- Fuel = miles × 3.785 litres per US gallon ÷ mpg = 250 × 3.785 ÷ 30 = 31.545098 litres.
- Cost = litres × price per litre = 31.545098 × £1.40 = £44.16.
- UK and US gallons are different sizes, so pick the right mpg type: a UK gallon is 4.546 litres, a US gallon 3.785.
How it works
Fuel used = distance ÷ miles per gallon × litres per gallon. A UK gallon is 4.546 litres; a US gallon is 3.785 litres. Trip cost is then litres × price per litre. With litres per 100 km the fuel is simply distance in km × that figure ÷ 100.
UK mpg versus US mpg
The same car is rated about 20% higher in UK mpg than US mpg. 40 UK mpg is roughly 33 US mpg. Always use the figure from the same system your car's handbook uses, or choose litres per 100 km which is unambiguous.
Real-world figures
Real consumption is usually 10 to 25% worse than the official figure, especially at motorway speeds, with a heavy load or in cold weather. Add a margin to be safe for a long trip. Check the price per litre at the pump, not per gallon.
Frequently asked questions
How do I calculate fuel cost for a trip?
Fuel used = distance ÷ mpg × litres per gallon. Multiply by the price per litre.
Is a UK gallon the same as a US gallon?
No. A UK gallon is 4.546 litres and a US gallon is 3.785 litres.
How do I convert mpg to litres per 100 km?
For UK mpg divide 282.48 by the mpg figure. For US mpg divide 235.21. The calculator shows the equivalent for you.
What is my fuel cost per mile?
It is shown in the results: trip cost divided by distance.
Can I use it for an electric car?
Not directly. For electric cars use the electricity cost calculator with the car's kWh use and your price per kWh.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.