Fuel cost is three numbers multiplied together: how far you go, how much fuel that distance consumes, and what a litre costs at the pump. The awkward part is that British drivers quote economy in miles per imperial gallon while fuel is sold in litres, so a conversion has to happen somewhere. This calculator does it explicitly and shows the litres as well as the money.
One imperial gallon is 4.54609 litres and one mile is 1.609344 kilometres, which is why miles per imperial gallon converts to litres per 100 km by dividing 282.481 into it. That constant is simply 100 x 4.54609 / 1.609344. If your figure is American, the equivalent constant is 235.215, because a US gallon is only 3.785411784 litres — roughly a fifth smaller.
A 120 mile trip at 42 mpg imperial works out at 6.726 litres per 100 km, so it burns 12.99 litres. At £1.42 per litre that is £18.44, or 15.4 pence per mile. The pump price is an editable input because it varies by week, brand and motorway service area, and it is the only figure in the sum you cannot control.
The formula
distance- Journey length in miles or kilometres
economy- Fuel consumption as mpg imperial, mpg US or L/100 km
4.54609- Litres in one imperial gallon
price- Pump price per litre, editable
How to use this calculator
- Enter the journey distance and pick miles or kilometres.
- Enter your economy figure and say which unit it is in.
- Set the pump price per litre to what you are actually paying.
- The gauge shows the one-way cost; litres, pence per mile and the annual figure sit below.
Worked examples
120 miles at 42 mpg
120 miles is 193.12 km. At 42 mpg imperial the car uses 6.726 litres per 100 km, so the trip burns 12.99 litres. At £1.42 per litre that is £18.44 one way and £36.89 there and back.
The same trip in a thirstier car
Drop to 28 mpg and consumption rises to 10.089 L/100 km, so the same 120 miles takes 19.48 litres and costs £27.67. That is £9.22 more for an identical journey.
How to read your score
Frequently asked questions
Is my economy figure imperial or US mpg?
If you bought the car in Britain or Ireland it is almost certainly imperial. A US gallon is 3.785411784 litres against 4.54609 for the imperial gallon, so the same car reads about 20% lower in US mpg. A car quoted at 50 mpg imperial is roughly 42 mpg US.
Should I trust the trip computer?
Treat it as optimistic. On-board consumption meters typically read 3 to 10 per cent better than a brim-to-brim measurement, partly because they estimate injector flow rather than measure fuel. Measure over two or three tanks and use that figure here.
Why is the pump price an input rather than a fixed number?
Because it changes constantly and varies by tens of pence between a supermarket and a motorway services. The £1.42 default is a plausible UK figure at the time of writing, not a live price, so overwrite it with the number on your receipt.
Does driving style change the answer much?
More than most people expect. Aerodynamic drag rises with the square of speed, so 80 mph can use 20 to 25 per cent more fuel than 65 mph over the same distance. Roof boxes and underinflated tyres each cost a few per cent as well.
Does this work for a diesel or an LPG conversion?
Yes — the maths only cares about litres and price per litre. Enter the LPG price and the economy you achieve on gas, which is normally 15 to 25 per cent worse in L/100 km than on petrol.
Fuel cost reference
| Economy | 50 miles | 100 miles | 200 miles | 400 miles | Pence/mile |
|---|---|---|---|---|---|
| 25 mpg | £12.91 | £25.82 | £51.64 | £103.29 | 25.8p |
| 30 mpg | £10.76 | £21.52 | £43.04 | £86.07 | 21.5p |
| 35 mpg | £9.22 | £18.44 | £36.89 | £73.78 | 18.4p |
| 40 mpg | £8.07 | £16.14 | £32.28 | £64.55 | 16.1p |
| 45 mpg | £7.17 | £14.35 | £28.69 | £57.38 | 14.3p |
| 50 mpg | £6.46 | £12.91 | £25.82 | £51.64 | 12.9p |
| 60 mpg | £5.38 | £10.76 | £21.52 | £43.04 | 10.8p |
| 70 mpg | £4.61 | £9.22 | £18.44 | £36.89 | 9.2p |
Computed with 1 imperial gallon = 4.54609 litres and 1 mile = 1.609344 km. Change the price input to match your local pump.