Cost guide · Updated July 2026
Fuel cost calculator: what a journey really costs
By the CarBudget team · Verified sources at the bottom of the page
To calculate fuel cost, divide the distance by your fuel economy and multiply by the price of fuel. In miles per gallon: fuel cost = (miles ÷ MPG) × price per gallon. In metric: fuel cost = (km ÷ 100) × consumption in L/100 km × price per litre. For example, a 300-mile trip in a car doing 45 MPG with fuel at £6.50 a gallon costs roughly £43.
Fuel is the cost you notice most, because you hand it over at the pump every week or two. Yet very few drivers can say what a particular journey actually costs them, or how much a full tank sets them back over a year. The maths is simple once you know the formula — and knowing it turns fuel from a vague drain into a number you can plan around, compare between cars, and split fairly on a shared road trip.
The fuel cost formula
Fuel cost always comes down to three ingredients: how far you drive, how efficiently your car uses fuel, and what fuel costs. In the UK and US, economy is usually quoted in miles per gallon (MPG), so the formula is:
Fuel cost = (distance in miles ÷ MPG) × price per gallon
The first part, distance ÷ MPG, tells you how many gallons the journey burns. Multiply that by the pump price and you have the cost. The one catch is the pump: in the UK fuel is priced per litre, so to use the MPG formula you first convert the litre price to a gallon price by multiplying by 4.55 (the number of litres in a UK gallon). Fuel at £1.43 a litre is therefore about £6.51 a gallon.
Worked example
Say you’re driving 300 miles to visit family, in a car that averages 45 MPG, with diesel at £1.43 a litre.
- Convert the price: £1.43 × 4.55 = £6.51 per gallon.
- Gallons used: 300 ÷ 45 = 6.67 gallons.
- Cost: 6.67 × £6.51 = £43.40 one way, or about £87 there and back.
Change any input and the answer moves in the obvious direction: a thirstier car (say 35 MPG) pushes the round trip past £110, while a hybrid doing 60 MPG brings it under £65. That sensitivity is exactly why fuel economy matters so much to running costs — see how to measure yours with our MPG calculator.
The litres and kilometres version
If you think in litres per 100 km — the standard almost everywhere outside the UK and US — the formula is even cleaner:
Fuel cost = (km ÷ 100) × consumption in L/100 km × price per litre
For a 480 km trip (roughly 300 miles) in a car using 6.3 L/100 km with fuel at £1.43/litre: (480 ÷ 100) × 6.3 × 1.43 = £43.20 — the same answer, as it must be. To move between the two systems, use L/100 km = 282.5 ÷ UK MPG, so 45 UK MPG is about 6.3 L/100 km. Watch the gallon trap: a US gallon is 3.79 litres, so US MPG figures look lower than UK ones for the same real economy.
Cost of a specific trip or a full tank
For a full tank, skip the distance entirely and just multiply tank size by price: a 50-litre fill at £1.43/litre is £71.50. To budget a year of driving, take your annual mileage, run it through the formula once, and you have your yearly fuel bill. Ten thousand miles a year at 45 MPG and £6.51 a gallon works out at roughly £1,447 a year — a figure worth knowing before you choose your next car, because a more efficient model can quietly save hundreds annually. To fold fuel into the full picture alongside insurance, tax and servicing, use the car cost calculator or read how much it costs to run a car in 2026.
Getting an accurate figure
Every calculation above is only as good as the MPG you feed it. Manufacturers quote lab-tested WLTP figures, but real driving — short cold-start journeys, town traffic, motorway cruising, a loaded boot, roof bars, cold weather and air-conditioning — routinely lifts consumption 10–25% above the official number. The honest way to budget is to base the calculation on your measured economy, not the brochure.
That’s the whole point of tracking fill-ups. Each time you log litres, price and your odometer reading in CarBudget, it calculates your true MPG (or L/100 km), your real cost per mile, and your monthly fuel spend — so the next trip you plan uses a number you can actually trust. You can then express it per mile with our cost per mile calculator.
Sources and methodology
The conversions and ranges in this guide use standard, widely published figures:
- Fuel-economy conversions (UK gallon = 4.546 litres; US gallon = 3.785 litres; MPG ↔ L/100 km constants): UK Government / official unit definitions.
- Real-world vs official (WLTP) economy gap: motoring associations’ economy testing (RAC, AA) and consumer economy studies.
- Pump prices are illustrative — check current UK average fuel prices (RAC Fuel Watch) or US prices (AAA / U.S. EIA) for live figures.
Know your real fuel cost, not a guess
Log each fill-up in CarBudget — litres, price and mileage — and see your true MPG, cost per mile and monthly fuel spend automatically. Free, no sign-up hurdles.