How UK fuel cost is calculated from miles, MPG and the pump price

The working order on this site is: take the trip in miles → turn that into litres from the economy figure → multiply by pounds per litre → optionally multiply by the number of trips. Cost per mile is the one-trip cost divided by miles. Nothing is stored; the helper does not look up a live national average.

With UK MPG: litres = (miles ÷ MPG) × 4.54609. That 4.54609 is the number of litres in one imperial gallon. With L/100km: convert miles to kilometres first (× 1.609344), then litres = (L/100km ÷ 100) × km. Enter the pump price in pounds per litre — 1.45 for 145 pence, not 145.

UK trip computers and older handbook figures use the imperial gallon of 4.54609 litres. A US gallon is about 3.785 litres — roughly 20% smaller. Type a US MPG number into a UK calculator and you overstate economy by about a fifth and understate the bill. If a US listing says 40 mpg, that is not 40 UK mpg.

Official WLTP figures are often quoted in L/100km, where lower is more efficient. 45 UK mpg is about 6.3 L/100km. 6.5 L/100km is about 43.5 UK mpg — the helper shows that equivalent when you work in L/100km. Dashboard MPG is usually a recent-driving average, not the brochure WLTP number, which is why two cars with the same official figure can cost different amounts on the same commute.

100 miles, 45 mpg, £1.45 a litre. The helper’s default. Fuel used: 10.10 litres. Cost: £14.65. Cost per mile: £0.146. That is the FAQ figure on the tool page.

250 miles, 6.5 L/100km, £1.50 a litre. A longer run using the metric economy figure: 26.15 litres, £39.23, £0.157 a mile, equivalent to 43.5 UK mpg.

30 miles, 40 mpg, £1.42 a litre. A one-way commute: 3.41 litres and £4.84, or £0.161 a mile. The next section scales that same trip to a return, a working week and a working month.

300 miles, 45 mpg, £1.45 a litre. Three times the default distance at the same economy and price: 30.31 litres and £43.95. Cost per mile stays £0.146 — only the total grows. That is the point of a pence-per-mile number: it lets you compare a petrol car with a train fare or a passenger split without redoing the whole sum.

Keep the 30-mile, 40 mpg, £1.42 trip and only change the number of trips. Each one-way leg is £4.84. A return (2 trips) is £9.68. Ten trips — five working days, there and back — is £48.42 and 34.10 litres. A rough working month of 22 days (44 one-way legs) is £213.03. Cost per mile does not move; it is still £0.161. What changes is how often you pay it.

That weekly £48 is the figure worth lining up against a rail season ticket or a lift-share, not the £4.84 one-way snapshot. The helper’s “number of trips” box is how you get there without multiplying on paper.

Hold distance and price at the default 100 miles and £1.45, and only change mpg. 35 mpg uses 12.99 litres and costs £18.83 (£0.188 a mile). 45 mpg is the £14.65 default. 50 mpg uses 9.09 litres and costs £13.18 (£0.132 a mile). The gap between 35 and 50 mpg on that same 100 miles is £5.65.

Hold economy at 45 mpg and raise the pump from £1.45 to £1.60. The same 100 miles becomes £16.16 — £1.51 extra. Fifteen pence on the litre is real money over a year of commuting, but on a single 100-mile trip it is smaller than the gap between a 35 mpg car and a 45 mpg car. That is why the helper asks for both numbers rather than a national average price alone.

What you pay at the pump already includes Fuel Duty and VAT. As of August 2026, duty on unleaded petrol and diesel is 52.95p per litre — the temporary 5p cut first made in March 2022, extended to 31 December 2026. VAT at 20% is charged on the VAT-exclusive price, which already includes duty, so VAT is one-sixth of the pump figure.

On this site’s £1.45 example that split is about 52.95p duty, 24.17p VAT and 67.88p for the fuel itself. The helper does not itemise the receipt. You type the full pounds-per-litre price. What happens to duty after 31 December 2026 is a Budget decision: GOV.UK currently shows a legislative default that starts returning toward the pre-2022 57.95p rate from January 2027, with rates to be confirmed. That is not a live pump forecast, and this site will not pretend it is.

From 6 April 2026, approved mileage allowance payments (AMAPs) for employees using their own car or van on business are 55p per mile for the first 10,000 business miles in the tax year, then 25p. Motorcycles stay at 24p and bicycles at 20p. That 55p is a tax-free reimbursement ceiling. It is meant to cover wear, insurance, servicing and depreciation as well as petrol or diesel.

On the 45 mpg · £1.45 example, fuel alone is £0.146 a mile — about 15p. The 55p AMAP is several times that, on purpose. Do not type 0.55 into the fuel calculator as if it were a pump price, and do not treat £0.146 as what an employer “should” pay. Company-car advisory fuel rates — pence per mile by engine size, for reimbursing fuel in a company car — are a third system again. This site models none of those HMRC tables.

Electricity for a battery car. That is pence per kilowatt-hour times kilowatt-hours per mile, not litres. Plug-in hybrids have two energy costs; this helper only prices liquid fuel. Congestion charging, ULEZ, Dartford, parking, ferries, insurance, tyres, servicing and Vehicle Excise Duty are standing or location costs, not pump maths — use the car-tax calculator for VED.

Real economy versus the number you typed. Speed, hills, cold starts, roof boxes, air-conditioning, passengers and a heavy boot all move mpg. A short school run in winter is not a 70 mph motorway average. Petrol versus diesel is only a price-per-litre difference here; the litres formula is the same.

Open the fuel cost calculator, pick MPG (UK) or L/100km, enter miles, economy and the pounds-per-litre price on the pump you actually use. Set trips to 2 for a return or 10 for a working week. Read total cost, litres and cost per mile. Nothing you type is stored.

If you are comparing two cars, run the same commute twice with different mpg figures, then add a year’s VED from the car-tax calculator. If the car is on finance, the loan-repayment calculator is the monthly standing cost; fuel is the usage cost. Inflation on a historic pump price belongs on the inflation tool — this page is “what does this trip cost today?”