How the journey cost is worked out
Britain buys fuel in litres but measures cars in miles per gallon, so there is a conversion hiding inside every "what will the petrol cost?" question. The calculator divides your distance by your mpg to get gallons, converts to litres using the imperial gallon (exactly 4.54609 litres, not the smaller US one), then multiplies by your pump price.
Tick the round trip box to double the distance for there-and-back journeys, and set the number of people to split the total fairly between passengers. Fuel prices move weekly and vary between forecourts, so always enter the price you will actually pay rather than a remembered one.
Worked examples
The same sums scale up to a useful yearly picture. At 45 mpg and 145p a litre, a mile costs about 14.6p in fuel, so a typical 8,000-mile year is roughly £1,170 at the pump before you touch insurance, tax, servicing or depreciation. Knowing your own pence-per-mile figure makes decisions easier: whether a cheaper supermarket 5 miles away actually saves money, what a lift is really worth, or whether the train fare beats driving once parking is added.
Getting your mpg number right
The single biggest source of error is optimistic mpg. Official WLTP figures come from laboratory tests, and real-world driving typically returns 10% to 20% less, more in town or in winter. Two better sources: your car's long-term trip computer average, or the brim-to-brim method, where you fill the tank to the top, drive normally, refill to the top and divide miles covered by gallons added (litres ÷ 4.54609). If you know litres per 100 km instead, the mpg converter will translate it.
Cutting the cost per mile
Fuel cost is one of the few motoring bills you can influence every single trip. Gentle acceleration, easing off well before queues, and 60 mph rather than 80 on faster roads all cut consumption noticeably, and drag rises steeply with speed. Correct tyre pressures, an empty roof rack and a boot that is not a storage unit help too. Combining short trips matters more than people think, because a cold engine is at its thirstiest in the first couple of miles. Shopping around helps as much as any of it: supermarket forecourts often undercut motorway services by 20p a litre or more, which on a 55-litre tank is over £11.
Figures are estimates for guidance, not financial advice. Real consumption varies with traffic, weather, load and driving style.
Frequently asked questions
What mpg figure should I enter?
Your real-world figure, not the brochure one. Official WLTP figures are set in lab conditions and most drivers get 10% to 20% less. The most accurate source is your car's trip computer averaged over a few weeks, or a brim-to-brim calculation: fill the tank, drive, refill, and divide miles driven by gallons added.
Why does the calculator mix miles per gallon with pence per litre?
Because that is the odd way the UK actually works. Fuel economy is quoted in miles per imperial gallon, but pumps sell fuel in litres. One imperial gallon is exactly 4.54609 litres, so the calculator converts gallons needed into litres before applying your pump price.
Where do I find the current fuel price?
Check the pump, a recent receipt or a price comparison app before you calculate, because prices change weekly and vary a lot between forecourts. Supermarket stations are usually cheapest and motorway services are usually the most expensive, often by more than 20p a litre.
Does this work for electric cars?
Not directly, because EV efficiency is measured in miles per kilowatt hour rather than miles per gallon. Divide your journey miles by your car's miles per kWh to get the energy needed, then price it with the energy running cost calculator using your home tariff or the public charger rate.
How can I cut the cost per mile?
Drive smoothly, stick closer to 60 than 80 on faster roads, keep tyres at the right pressure, remove roof boxes and heavy clutter, and combine short cold-engine trips. Together these routinely improve consumption by 10% or more, which is the same as a 10% cut in pump prices.