Fuel Cost Calculator

Getting a realistic comparison between petrol, diesel and electric

A fuel cost calculator is only as good as the numbers you put into it. The figures published by the Department for Energy Security and Net Zero give a useful starting point: in the week commencing 14 September 2026, UK average pump prices were 168.14 pence per litre for petrol and 190.72 pence per litre for diesel. Those averages move week by week and vary by region and forecourt, so if you know what you paid locally, enter that instead.

For an electric car, the equivalent input is the price per kilowatt hour. Domestic electricity under the Ofgem price cap was 26.11 pence per kWh for 1 July to 30 September 2026, a national average for Direct Debit customers in England, Scotland and Wales. That is a reasonable default if you charge at home on a standard tariff, but see below on why the answer changes so much depending on where you plug in.

What MPG figure to enter, and why official figures flatter

If you already own the car, use your own brim-to-brim calculations over a few tanks rather than the brochure figure. If you are comparing cars you have not bought yet, the official WLTP figure is the only consistent number available, but treat it as a best case. WLTP tests are run in controlled conditions with a warm engine, moderate temperatures, no air conditioning load, no roof bars, no passengers and no motorway speeds. Real journeys include all of those things.

Short trips are the biggest gap. A cold engine uses noticeably more fuel for the first few miles, so a car that returns a healthy figure on a long run can look very different on a school run. Driving style, tyre pressures and load all pull the real figure below the official one, so entering the official MPG will usually give you a cost lower than you will actually pay.

Miles per kWh is the electric equivalent of MPG

For an electric car, efficiency is measured in miles per kWh. A typical mainstream UK electric car achieves roughly 3 to 4 miles per kWh in real-world driving, though this varies with temperature, speed and driving style, and it falls in cold weather. You cannot convert an MPG figure into miles per kWh, because the two measure different things: one is a volume of liquid fuel burned, the other is electrical energy drawn from a battery or charger. They are not interchangeable, which is why the calculator asks for them separately.

If you do not know your car's figure, the 3 to 4 miles per kWh range is a sensible starting assumption, then adjust once you have your own data. Charging losses mean the energy you pay for is slightly more than the energy that reaches the battery.

Where you charge changes the answer more than anything else

Two drivers of the same electric car can face very different running costs. Charging at home on an off-peak overnight tariff is usually cheaper than the price cap rate. Public rapid charging is usually considerably more expensive than charging at home, sometimes by a wide margin. A driver who can charge overnight at home will see a much lower cost per mile than one who relies entirely on motorway rapid chargers.

Run the calculation twice, once at your home rate and once at a realistic public rate, to see the range of outcomes. If most of your mileage is long-distance motorway work, weight the public figure more heavily.

Fuel is only part of what a car costs you

Comparisons based on fuel alone can mislead, because fuel is often the smaller part of the bill over several years. Other costs to weigh up include:

• Vehicle tax, which varies by emissions band and by list price for newer cars
• Insurance, which differs sharply between models and between drivers
• Servicing and repairs, including tyres, which wear faster on heavier vehicles
• Depreciation, usually the single largest cost of ownership
• Emission zone charges, which apply to some cars and not others

Use a car tax calculator to work out the annual tax bill for a specific model, and a depreciation calculator to see what the car is likely to lose in value. If you drive into London, a ULEZ check will tell you whether a charge applies. When you have the figures for two or three candidates, you can compare vehicles side by side and see which is genuinely cheaper to run, rather than which simply uses less fuel.