Fuel Cost Calculator UK
Example: petrol 170.71p/L, diesel 198.19p/L — DESNZ weekly road fuel prices, week commencing 9 November 2026. Change it to your local price.
Model: litres = miles ÷ MPG × 4.54609 (litres in a UK gallon); cost = litres × price per litre. With litres/100 km, litres = miles × 1.60934 × L/100km ÷ 100. Monthly and annual costs use your miles per month.
How fuel cost is worked out
Three numbers decide what a journey costs: how far you drive, how much fuel your car uses, and what a litre costs. In the UK, fuel economy is usually quoted in miles per gallon (MPG), and a UK gallon is 4.54609 litres — not the smaller US gallon of 3.785 litres, which is a common source of confusion when comparing cars online. To find the litres for a trip, divide the miles by the MPG to get gallons, then multiply by 4.54609. A 100-mile trip at 40 MPG uses 2.5 gallons, which is 11.37 litres. Multiply by the pump price — 170.71p a litre for petrol in the week commencing 9 November 2026 — and the trip costs £19.40.
Newer cars often quote litres per 100 km instead, which is the figure on the manufacturer’s spec sheet. The maths is the same idea in reverse: convert miles to kilometres (multiply by 1.60934), then work out the litres from the L/100km figure. A car using 7 litres per 100 km needs 11.27 litres for 100 miles, costing £19.23 in petrol. The two methods should agree closely for the same car; if they do not, one of the figures is probably a laboratory test figure rather than real-world economy.
UK fuel prices in November 2026
Fuel prices in the UK are published every week by the Department for Energy Security and Net Zero (DESNZ). In the week commencing 9 November 2026, the UK average was 170.71p a litre for unleaded petrol and 198.19p a litre for diesel. Diesel had been above 198p for weeks and briefly averaged over 200p a litre in the week of 12 October 2026 — the first time the weekly diesel average had passed £2. Prices at individual forecourts vary by several pence a litre, and supermarket forecourts are usually cheaper than motorway service stations, where a litre can cost 10p to 20p more.
About half of what you pay is tax. Fuel duty is a fixed 52.95p a litre on both petrol and diesel, and VAT at 20% is charged on top of the whole pump price, including the duty. On a litre of petrol at 170.71p, that is 52.95p of duty plus 28.45p of VAT — about 81p of tax in every litre. The Autumn Budget on 28 October 2026 extended the 5p fuel duty cut and the freeze until 1 July 2027, so duty stays at 52.95p until then. Wholesale oil prices take about two weeks to reach the pumps, according to the RAC, which is why pump prices fall more slowly than oil prices.
What a mile really costs
| Car economy | Petrol cost per mile | Diesel cost per mile | 800 miles a month (petrol) |
|---|---|---|---|
| 30 MPG | 25.9p | 30.1p | £206.95 |
| 40 MPG | 19.4p | 22.5p | £155.21 |
| 50 MPG | 15.5p | 18.0p | £124.17 |
| 60 MPG | 12.9p | 15.0p | £103.47 |
Cost per mile is the most useful number for comparing cars, because it strips out the trip distance. A car doing 30 MPG costs two-thirds more per mile than one doing 50 MPG — on 800 miles a month, that is £82.78 a month, or £993 a year, just from fuel economy. But remember that fuel is only part of the cost of running a car. Insurance, tax, servicing, tyres and depreciation usually cost more than fuel over a year. A car that is cheap on fuel but expensive to insure may not be cheap overall.
Five ways to spend less on fuel
- Slow down on the motorway. Driving at 70 mph instead of 80 mph can cut fuel use by around 10% on a long journey, because air resistance rises sharply with speed.
- Check tyre pressures monthly. Under-inflated tyres increase rolling resistance and fuel use. The correct pressures are on a sticker inside the driver’s door or fuel cap.
- Take the roof box off. A roof box or roof bars left on when empty can add 10% or more to fuel use at motorway speeds.
- Combine short trips. A cold engine uses far more fuel for the first few miles. Two errands in one trip cost less than two separate cold starts.
- Compare forecourts. A few pence a litre adds up: saving 5p a litre on 100 litres a month is £5 a month, £60 a year. Motorway service stations are almost always the most expensive place to fill up.
Petrol, diesel or electric: cost per mile compared
The same mile costs a different amount in each fuel, and the gap is bigger than most people expect. At the November 2026 averages, a petrol car doing 40 MPG costs 19.4p a mile, and a diesel doing the same 40 MPG costs 22.5p a mile — diesel is dearer per litre, though diesel cars usually do more miles per gallon, which narrows the gap in practice. An electric car is a different calculation altogether: it uses electricity, not litres, so the cost is the unit rate divided by the miles per kWh. At the October 2026 cap rate of 26.32p per kWh, an electric car doing 3.5 miles per kWh costs about 7.5p a mile on home charging — less than half the petrol cost. Public rapid charging is dearer, often 60p to 80p per kWh, which works out at 17p to 23p a mile, close to petrol. So the fuel saving from an electric car depends almost entirely on where you charge it.
That is also why comparing cars on MPG alone can mislead. A diesel that does 55 MPG at 198.19p a litre costs 16.4p a mile, which beats a petrol car doing 40 MPG at 19.4p. And a hybrid that does 60 MPG on petrol costs 12.9p a mile, which beats both. The calculator’s cost-per-mile figure is the fair way to compare them, because it turns every car into the same unit: pence per mile. Use it when you are choosing between two cars, and remember to add the other costs — insurance, tax, servicing and depreciation — before you decide which car is actually cheaper to own.
Related: Company Car Tax Calculator · PCP vs HP Calculator · Take Home Pay Calculator
Sources: Department for Energy Security and Net Zero, Weekly road fuel prices, week commencing 9 November 2026 (petrol 170.71p/L, diesel 198.19p/L); RAC on fuel duty 52.95p/L and wholesale lag; HM Treasury, Autumn Budget 2026 (duty freeze to 1 July 2027). Checked 13 November 2026.