Fuel cost calculator for any units
Work out what a journey costs in fuel. Distance can be in miles or kilometres, economy in UK mpg, US mpg, litres per 100 km or kilometres per litre, and the pump price per litre or per gallon — the conversions use exact factors, so mixing units does not introduce errors. You also get the cost per mile and per kilometre, and the option to split it between passengers.
How to use the Fuel Cost Calculator
- Enter the distance and choose miles or kilometres.
- Enter your fuel economy and the unit it is quoted in.
- Enter the pump price and whether it is per litre or per gallon.
- Tick the return box for a round trip, then press Calculate fuel cost.
The conversions, exactly
Unit mix-ups are the main source of error in fuel sums, so the exact factors matter: one imperial gallon is 4.54609 litres, one US gallon is 3.785411784 litres, and one mile is 1.609344 kilometres. A UK gallon is about 20% bigger than a US one, which is why a car rated at 45 mpg in Britain is rated 37.5 mpg in America for identical consumption.
| Convert | Formula |
|---|---|
| UK mpg → L/100km | 282.481 ÷ mpg |
| US mpg → L/100km | 235.215 ÷ mpg |
| km/L → L/100km | 100 ÷ (km/L) |
| UK mpg → US mpg | mpg × 0.832674 |
Worked example
250 miles at 45 mpg (UK) with petrol at £1.45 a litre. Gallons used = 250 ÷ 45 = 5.5556 imperial gallons, which is 5.5556 × 4.54609 = 25.26 litres. At £1.45 a litre the fuel costs £36.62 — 14.6p a mile, or 9.1p a kilometre. The same car is 6.28 L/100km, 37.5 US mpg or 15.93 km per litre.
Why real consumption is worse than the brochure
Manufacturer figures come from a standardised laboratory cycle, and real journeys are not standardised. Short trips on a cold engine, urban stop-start, a roof box, under-inflated tyres, a full load, air conditioning, heavy right feet and motorway speeds above about 60 mph all cut economy measurably. Air resistance rises with the square of speed, so the power needed rises with the cube — cruising at 80 mph instead of 65 mph can cost a fifth of your range. If you want accuracy, take the figure from your own brim-to-brim measurement: fill the tank, reset the trip, fill again, then divide the distance by the litres put in.
Cost per mile and comparing options
The cost-per-mile figure is the one to use when deciding between driving and the train, or when invoicing mileage. Remember that fuel is not the full cost of running a car: tyres, servicing, depreciation and insurance all scale with distance to some degree, which is why approved mileage rates are considerably higher than the fuel-only cost. An electric car is worth calculating the same way — enter the economy in miles per kWh as km/L and the price per kWh, and the arithmetic still holds.
Frequently asked questions
Why is UK mpg different from US mpg?
The gallons differ. An imperial gallon is 4.54609 litres and a US gallon is 3.785411784 litres, so the same car scores about 20% higher in UK mpg than in US mpg.
How do I convert mpg to litres per 100 km?
Divide 282.481 by UK mpg, or 235.215 by US mpg. At 45 UK mpg that is 6.28 L/100km.
Should I use the manufacturer’s mpg figure?
It is a starting point, but lab cycles flatter real driving. A brim-to-brim measurement from your own journeys gives a far better number.
Does this work for an electric car?
Yes, with a substitution: enter miles or km per kWh in place of the economy figure and the price per kWh in place of the price per litre.
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Last updated 2026-09-23.