Fuel Cost Calculator

Fuel cost is calculated by dividing the journey distance by your vehicle's fuel efficiency, then multiplying by the price per litre. UK fuel efficiency is commonly quoted in miles per gallon (MPG) but fuel is sold in litres, so a conversion is needed: one UK gallon holds 4.546 litres. That conversion factor is why the cost-per-mile formula looks slightly odd. You take the price per litre, multiply it by 4.546 to get the price of a gallon, then divide by your mpg. At £1.72 a litre, the UK average petrol price in the week commencing 21 September 2026, a gallon costs £7.82, so a 40 mpg car burns 19.5p of fuel every mile, a 50 mpg car 15.6p and a 30 mpg car 26.1p. Multiply that figure by the miles you plan to drive and you have the fuel bill for the trip, and nothing else (wear, tax, insurance and depreciation sit outside it).

The HMRC approved mileage allowance for business use is 55p per mile for the first 10,000 miles and 25p per mile thereafter. The 55p figure was raised from 45p on 6 April 2026, the first change to the rate since 2011. Motorcycles attract 24p a mile at any distance, bicycles 20p, and carrying a colleague adds 5p per mile per passenger. These Approved Mileage Allowance Payments are designed to cover fuel, insurance, depreciation and wear, not just fuel, which is why the 55p rate is still nearly three times the 19.5p a mile that a 40 mpg car burns with petrol at £1.72. If your employer pays less than these rates, you can claim tax relief on the difference. The self-employed claim AMAP through Self Assessment.

This calculator lets you enter distance, fuel efficiency and fuel price to estimate the cost of any journey, and shows the equivalent HMRC mileage allowance alongside it for comparison. The gap between the two is what the allowance is meant to absorb beyond fuel: tyres, servicing, tax and the slow loss of the car's value. On a long business trip the difference can be substantial, which is worth knowing before you agree a reimbursement rate with an employer or a client.

Where the pump price goes is less obvious than the formula. In the week commencing 21 September 2026 the UK average was 172.01p a litre for petrol and 195.53p for diesel (DESNZ weekly road fuel prices), with petrol up 10.4p since the end of August. Fuel Duty takes 52.95p a litre, the rate since the 5p cut of March 2022, which the government has extended to 31 December 2026. VAT at 20% is charged on the duty and the fuel together, so it is one sixth of the pump price: 28.67p on a litre of petrol. The two taxes come to 81.62p, about 47% of the price, and the remaining 90.39p pays for the fuel itself, delivery and the retailer's margin. Petrol and diesel pay the same duty, so the 23.5p gap between them comes from the dearer wholesale diesel and the VAT on it.

Electric running costs shift the arithmetic. Home charging on an off-peak tariff at 8p per kWh, in a typical EV doing 4 miles per kWh, works out at 2p per mile. Public rapid charging at 60-80p per kWh costs 15-20p per mile, comparable to petrol, while workplace charging is often free or subsidised. For reference, a petrol car at 45 mpg and £1.72 a litre costs 17.4p per mile in fuel. Expect EV range to drop 20-30% in winter because of the heater, with summer range close to the rated figure. On total cost of ownership an EV typically comes out cheaper from year 4-5 onwards, helped by lower fuel and maintenance (road tax has been the same £200 standard rate as petrol since April 2025, and eVED adds 3p a mile from April 2028) and a benefit-in-kind rate of 4% in 2026/27, rising to 5% in 2027/28 and 7% in 2028/29. Salary sacrifice EV schemes can save 30-50% of the monthly cost for higher-rate taxpayers.

Driving style has a bigger effect on consumption than most people expect. Aggressive acceleration uses 30-40% more fuel than smooth driving. Tyres under-inflated by 0.5 bar cost 5-10% in fuel economy, and a roof rack or box left on when unused adds 10-25% to consumption on motorways through aerodynamic drag. Modern cars do not need warm-up time, and idling for 2 or more minutes wastes more fuel than restarting. Cruise control on motorways saves 5-10%, and planning routes to avoid stop-start traffic helps further. A faulty oxygen sensor can cost 40% in efficiency, so it pays to fix engine warning lights promptly. Combined, these habits save the average UK family £200-£400 a year.