Fuel cost calculator

The fuel cost of a journey is the fuel it burns times the price you pay for it. For a petrol or diesel car, divide the miles by the car's miles per gallon to get gallons, convert to litres at 4.54609 litres to the imperial gallon, and multiply by the pump price per litre. A 200 mile trip at 45 mpg uses 20.2 litres, which at the average unleaded price of 172p a litre (DESNZ weekly road fuel prices) costs £34.75, or £0.17 a mile. For an electric car the sum is simpler: miles divided by miles per kWh gives the energy, and the tariff gives the cost, £15.04 for the same trip at home charging rates. Double the distance for a return journey and divide by the number of people sharing. Fuel duty and VAT are already inside the pump price, so nothing is added on top.

Three numbers decide it: how far you drive, how many miles the car covers on a gallon, and what a litre costs at the pump. A 200 mile drive in a car returning 45 mpg burns 20.2 litres of unleaded. At an average of 172p a litre, that is £34.75, or £0.17 for every mile.

The same run in a diesel at the same economy costs £39.51, at an average of 195.5p a litre. Make it a round trip for three people and the 400 miles cost £69.51 in total, £23.17 each, which is the fair way to split car sharing costs when nobody is making a profit.

British drivers think in miles per gallon, yet every pump prices fuel in pence per litre, so any honest sum has to convert. The UK gallon is the imperial gallon, larger than the American one, which is why US mpg figures for the same car look lower. Continental brochures quote the inverse measure, litres per 100 km, where a smaller number means a thriftier car.

The conversion rests on the legal size of the imperial gallon, and the calculator's own result for it is one of the checked cases further down.

Much of a UK pump price is tax, and the table splits an average litre of each fuel into its parts.

Fuel duty is a fixed sum per litre, currently £0.530 on both unleaded and diesel (HMRC), whatever the oil price does. VAT at 20% is then charged on the price including duty, the same way the VAT calculator adds it to any net price, so part of the tax is tax on tax. At today's prices 47% of an unleaded litre goes to the Treasury.

An electric car's economy is quoted in miles per kWh, and its fuel is priced by the kWh on the electricity bill. The calculator starts from the Ofgem price cap unit rate for 1 October 2026 to 31 December 2026, £0.263 per kWh, which is the most a standard variable tariff can charge on average. At 3.5 miles per kWh, the 200 mile journey above uses 57.1 kWh and costs £15.04, against £34.75 in unleaded.

From 1 October 2026 to 31 March 2027 domestic electricity in Great Britain carries no VAT, so the cap rate holds no tax; Northern Ireland keeps its reduced rate. Off-peak EV tariffs are often far cheaper at night, and public chargers set their own prices, usually above a home tariff, so enter the rate you actually pay.

Official figures come from the WLTP laboratory test, which is closer to real-world driving than the old test but still cannot reflect every driving style, load or journey. Your own number is easy to find. Fill to the brim, reset the trip counter, drive normally, then brim the tank again: the litres that go in are what the miles used.

420 miles on 44 litres works out at 43.4 mpg, or 6.5 litres per 100 km, and the fill-up cost £75.68. Repeat it over two or three tanks and compare with the trip computer, since a single fill-up is easily skewed by where the pump nozzle clicked off.

Here the journey and the price stay the same while the economy steps up from a thirsty car to a frugal one.

Because mpg sits under the line of the sum, each extra mile per gallon saves a little less than the one before. Moving from a thirsty car to an average one saves far more than moving from an efficient car to a very efficient one.

Employees driving their own car for work can be paid Approved Mileage Allowance Payments without tax (HMRC): £0.55 a mile for the first 10,000 business miles in the tax year and £0.25 after that, rates that apply from 6 April 2026. They are meant to cover running costs as a whole, not fuel alone, and unlike salary, where the salary calculator shows the tax taken, they reach you in full.

Company car drivers are reimbursed for business fuel through HMRC's advisory fuel rates, reviewed every quarter and set by fuel and engine size. From 1 September 2026 a petrol company car between 1,401 and 2,000cc is reimbursed at £0.17 a mile. The calculator's cost per mile shows how that compares with your own car's actual fuel cost.

The fuel bill depends on where you buy and how the car is driven as well as on the car, and these factors move it most often.

• Compare forecourts before filling up: the Fuel Finder scheme obliges stations to publish their prices as open data, which comparison apps and maps now show.
• Keep tyre pressure at the level in the handbook; soft tyres raise rolling resistance on every mile.
• Ease off on the motorway, where air resistance grows quickly with speed.
• Take out roof boxes, racks and heavy loads that are not needed for the trip.
• Plan combined errands, since a cold engine uses the most fuel in its first few miles.

The government publishes average prices each week in its weekly road fuel prices, which is where the starting prices here come from. Your local forecourt may be well above or below them, so type in what you actually paid.