Petrol Cost Calculator
Work out what any journey costs in petrol or diesel — in miles, MPG and pence per litre, with the cost per mile worked out for you.
Fuel Type
Measure economy in
— UK MPG uses the imperial gallon (4.546 litres).
UK average as of — enter the price on the forecourt sign, e.g. 161.9
Journey Cost
£0.00
Cost per Mile
0.0p
Litres Needed
0.0
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Petrol Costs, Explained
Working out the fuel cost of a journey means turning three numbers into one: how far you're going, how much your car drinks, and what you're paying at the pump. Divide the miles by your MPG to get gallons, convert to litres, and multiply by the price per litre. Britain's slightly awkward hybrid — distances in miles, fuel sold in litres — is exactly why this is worth having a calculator for.
Enter the price exactly as it appears on the forecourt sign. UK petrol stations quote pence per litre to one decimal place, almost always ending in .9, so a board reading 161.9 means 161.9p — not £161.9 and not £1.62. That convention holds everywhere in the UK, Northern Ireland included.
The cost per mile figure is the one worth remembering. It's the number that makes journeys comparable, tells you what a commute really costs over a year, and gives you a fuel-only baseline to set against a business mileage claim. Bear in mind it covers fuel and nothing else — tyres, servicing, insurance and depreciation are all on top.
If you think in L/100km rather than MPG — common if you've driven on the continent or have a newer car that displays both — switch the toggle and the figure you've already typed converts across automatically.
FAQ
How do I work out the petrol cost of a journey?
Divide your journey distance in miles by your car's MPG to get gallons, multiply by 4.546 to convert to litres, then multiply by the price per litre. This calculator does all of that for you — just enter miles, MPG and the pence per litre shown on the forecourt.
Why is diesel more expensive than petrol in the UK?
Diesel typically runs around 20p per litre above petrol in the UK. It carries the same fuel duty, but refining margins, winter heating-oil demand and lower forecourt competition all push the pump price higher. Diesel cars usually return better MPG, which can offset some of the gap over a long journey.
Is UK MPG the same as American MPG?
No, and the difference is significant. UK MPG uses the imperial gallon (4.546 litres) while US MPG uses the US gallon (3.785 litres). A car rated 50 MPG in Britain would be roughly 42 MPG under American figures. This calculator uses imperial gallons throughout.
What is E10 petrol and does it affect fuel economy?
E10 has been the standard grade of unleaded petrol in the UK since 2021 and contains up to 10% bioethanol. Because ethanol carries slightly less energy than petrol, most drivers see a small drop in MPG — typically around 1%. Super unleaded (E5) contains less ethanol but costs noticeably more per litre.
How does cost per mile compare with HMRC's mileage allowance?
They measure different things. The cost per mile shown here is fuel only. HMRC's approved mileage allowance — 55p per mile for the first 10,000 business miles from April 2026, then 25p — is intended to cover running costs as a whole, including insurance, servicing, tyres and depreciation. That is why the allowance is normally well above your pure fuel cost.
Why is fuel more expensive at motorway services?
Motorway service areas pay far higher site costs and face very little nearby competition, so prices are often 15-20p per litre above a supermarket forecourt. On a long journey it is usually worth filling up before you join the motorway.