Fuel cost calculator
Work out what a journey costs in fuel, or what a year of driving adds up to. Prefilled with today's live UK prices — petrol £1.70, diesel £1.94 — so you can start with a realistic figure and change it to whatever your local forecourt is charging.
Prices shown are the median across every reporting UK forecourt, not the mean — a small number of remote and motorway sites sit far above the rest and would otherwise pull the figure up by around 2p.
How the calculation works
Fuel economy in the UK is quoted in miles per gallon, but fuel is sold in litres, so every calculation has a conversion in the middle of it:
litres = (miles ÷ mpg) × 4.54609cost = litres × price per litreThe 4.54609 is the imperial gallon in litres. The US gallon is 3.785 litres, which is why American mpg figures look worse than British ones for the same car — a common source of confusion when comparing figures online.
Common questions
How do you work out the cost of a journey?
Distance divided by miles per gallon gives the gallons used. Multiplied by 4.54609 that becomes litres, since a UK gallon is 4.54609 litres. Multiplied by the price per litre, that is the cost. The calculator on this page does exactly that and shows the litres so you can check the arithmetic.
What mpg should I use?
Use your car's real-world figure rather than the manufacturer's official one, which is measured under laboratory conditions and is usually optimistic. Most modern trip computers show a long-term average. If you do not know it, 45 mpg is a fair starting point for a small modern petrol car, 55 for a modern diesel, and 35 for a large petrol SUV.
Does driving style change the cost much?
Considerably. Motorway speed is the biggest single factor: aerodynamic drag rises with the square of speed, so 80 mph typically uses around 25% more fuel than 60 mph over the same distance. Under-inflated tyres, roof boxes and heavy loads each add a few percent.
Is it worth driving further for cheaper fuel?
Only sometimes, and the calculator makes it checkable. A 10-mile detour in a 45 mpg car uses about a litre of fuel — roughly £1.60. If your tank takes 50 litres, you need the cheaper forecourt to be about 3p a litre lower just to break even, before your time is counted. Big savings come from choosing well among forecourts you already pass, not from long detours.
Now find the cheapest fuel near you
The calculator tells you what a journey costs. Your postcode tells you where to fill up before it.