Fuel Cost Calculator
Work out how much a UK car journey really costs. Enter your distance, MPG and price per litre — the calculator shows the trip cost, the cost per person if you are sharing, an annual cost projection, and a side-by-side petrol vs diesel vs electric comparison for the same journey.
£17.34 round trip
£17.34 per person (1 sharing) · 11.4 litres used
Estimated annual fuel cost: £1,733.62 at £10,000.00 miles/year
Petrol vs diesel vs electric — same trip compared
| Type | Cost for this trip | Cost per mile |
|---|---|---|
| Petrol | £17.34 | 17.3p |
| Diesel | £15.18 | 15.2p |
| Electric (home charging) | £6.53 | 6.5p |
Comparison uses default assumptions: petrol 40 MPG at 152.54p/L, diesel 50 MPG at 167p/L, EV 4 mi/kWh at 26.11p/kWh (home charging, standard tariff) — edit the fields above to compare your own numbers.
Formula: miles ÷ MPG × 4.546 (litres per imperial gallon) × price per litre. Default prices: RAC average UK pump prices, £1.53/L petrol and £1.67/L diesel (17 July 2026); EV price from the Ofgem standard-tariff price cap. All prices are editable. How we calculate →
How the fuel cost formula works
UK fuel economy is measured in miles per (imperial) gallon, but fuel is sold by the litre — so the calculator first converts your MPG into litres used, then multiplies by the price per litre. The formula is: miles ÷ MPG × 4.546 (litres per imperial gallon) × price per litre. For example, a 40 MPG car doing 100 miles at 152.54p/litre uses 100 ÷ 40 = 2.5 gallons, or 2.5 × 4.546 = 11.365 litres, costing 11.365 × £1.5254 — the full breakdown is shown above once you enter your own numbers.
4.546 litres per imperial gallon is an exact conversion (not a rounded estimate) — it is the UK gallon, not the smaller US gallon (3.785 litres), which is why American MPG figures are not directly comparable to UK ones without converting.
Current UK petrol and diesel prices
The calculator defaults to the RAC's reported UK average pump prices as of 17 July 2026: 152.54p/litre for petrol and 167p/litre for diesel. Pump prices move week to week with wholesale oil costs, so these defaults are a starting point — the price fields are fully editable, and it is worth checking the price board at your usual station or a live fuel-price site before a long trip.
Diesel has swung noticeably in 2026, at one point dropping by the largest monthly amount on record before climbing again — which is exactly why the calculator lets you overwrite the default with today's actual price rather than locking you into a fixed figure.
Splitting fuel cost between passengers
For a shared trip — a lift to the airport, a group heading to a festival, a car-share to work — the calculator divides the total trip cost by the number of people sharing it, including the driver if they are chipping in too. This is a simple even split; adjust manually if you want to exclude the driver or weight the split differently.
Annual fuel cost: the number that matters more than one trip
A single journey's cost is useful for planning a trip, but your true running cost is driven by how many miles you cover in a year. Enter your typical annual mileage to see the full-year fuel bill at your MPG and price — useful for comparing cars, deciding whether to switch fuel type, or working out if a longer commute is actually affordable.
Petrol vs diesel vs electric: the real comparison
The table above runs the same trip through three cost models side by side: petrol at 40 MPG, diesel at 50 MPG (diesel engines are typically more fuel-efficient per mile, which is why the default MPG is higher), and an electric car at 4 miles per kWh charged at home on a standard tariff. Home charging is consistently the cheapest per mile of the three at current prices, though the gap narrows a lot on public rapid chargers, which often charge considerably more per kWh than a home tariff.
These are default assumptions, not your car's actual figures — real MPG and mi/kWh vary by model, driving style and conditions, so treat the comparison as directional rather than exact for your specific vehicle.
Frequently asked questions
How do I calculate the fuel cost of a trip?
Divide the distance in miles by your car's MPG to get gallons used, multiply by 4.546 to convert to litres, then multiply by the price per litre. For example, 100 miles at 40 MPG at 152.54p/litre costs about £17.34.
What are the current average UK petrol and diesel prices?
As of 17 July 2026, the RAC reported average UK pump prices of 152.54p/litre for petrol and 167p/litre for diesel. These change regularly with wholesale prices, so use the calculator's editable price field to enter today's local price for an accurate result.
Is diesel cheaper to run than petrol?
It depends on both the price gap at the pump and how much more efficient your diesel engine is. Diesel is typically pricier per litre than petrol but the engines usually return higher MPG, so the true cost-per-mile gap is often smaller than the pump price difference suggests — the comparison table above works this out for you using default MPG figures for each fuel type.
How much cheaper is an electric car to run than petrol or diesel?
On a standard home electricity tariff, an EV is typically the cheapest of the three per mile, because electricity converts to miles more efficiently than burning fuel. The gap shrinks a lot if you rely on public rapid chargers, which usually cost more per kWh than charging at home overnight.
Why is the imperial gallon conversion factor 4.546 and not 3.785?
4.546 litres is the UK (imperial) gallon, the unit UK fuel economy figures (MPG) are based on. The US gallon is smaller, at 3.785 litres — mixing the two up is a common error when comparing an American car's MPG rating to a UK one.
How is the annual fuel cost calculated?
The same formula as a single trip, run against your typical annual mileage instead of a one-off distance: annual miles ÷ MPG × 4.546 × price per litre. It assumes your MPG and the fuel price stay constant across the year, so treat it as an estimate that you can update as prices change.
Also driving a newly registered or expensive car? Check what you will owe with our UK Car Tax (VED) Calculator.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →