Fuel Cost Calculator
Enter the journey distance, your car's fuel economy (mpg) and the price per litre to see the journey's fuel cost, your cost per mile and an optional per-person split. Real-world consumption is the biggest unknown, so measure your true mpg for the most accurate result.
How it works
Cost = distance ÷ mpg × 4.546 × price per litre (4.546 converts gallons to litres). A 200-mile trip at 45 mpg and £1.45/litre costs about £29.30.
Worked example with a split: a 240-mile round trip at 40 mpg and £1.50 a litre uses 240 ÷ 40 = 6 gallons, or 6 × 4.546 ≈ 27.3 litres, costing about £40.91 — £13.64 each between three people. Remember to enter the full round-trip distance: quoting the one-way mileage is the most common way journeys get under-budgeted by half.
The odd-looking 4.546 is the UK (imperial) gallon in litres: fuel economy is quoted in miles per gallon while pumps price per litre, so the conversion bridges the two. Cost per mile falls out of the same arithmetic — 4.546 × price per litre ÷ mpg — which at 45 mpg and £1.45/litre is about 14.6p per mile, a useful figure for comparing cars or sharing driving costs.
Real-world economy differs from official figures — motorway speed, town driving, load and air-con all move mpg by 10-30%. Measure your true mpg brim-to-brim for accurate budgeting.
To find your true mpg: brim the tank, drive normally, brim it again, and divide the miles covered by the litres refilled ÷ 4.546. Repeat over two or three tanks for a stable figure, because a single motorway-heavy or town-heavy tank can mislead either way.
The result is the fuel cost only — add tolls and parking separately, and remember insurance, servicing, wear and depreciation are not in the number, so the true per-mile cost of running a car is considerably higher than its fuel cost alone. For business mileage, HMRC's approved rate (55p/25p per mile in 2026/27) covers fuel AND wear — your pure fuel cost per mile is usually well below it, which is what makes claiming worthwhile.
FAQ
How do I calculate fuel cost for a journey?
Divide miles by your mpg to get gallons, multiply by 4.546 to get litres, then by the pump price: 200 miles ÷ 45 mpg × 4.546 × £1.45 ≈ £29.30. Add tolls and parking separately, and double the distance for a return trip.
What is my fuel cost per mile?
Roughly 4.546 × price per litre ÷ mpg: at 45 mpg and £1.45/litre that is about 14.6p per mile. Compare that with HMRC's 55p approved mileage rate and you can see how much of the allowance covers wear, insurance and depreciation rather than fuel.
How can I improve my car's mpg?
Smooth acceleration, keeping to ~60-65 mph on motorways, correct tyre pressures, removing roof boxes and shedding boot weight each add a few percent — together often 15-20%. Short cold-engine trips are the biggest economy killer; combining errands into one run genuinely saves fuel.
How much does a 100-mile journey cost in fuel?
At a typical 45 mpg and £1.45 a litre: 100 ÷ 45 × 4.546 × 1.45 ≈ £14.65, or about 14.6p a mile. A thirstier car at 30 mpg pays roughly £21.97 for the same run — economy differences translate directly into cash, with that 15-mpg gap costing about £7.30 every 100 miles.
How do I measure my real-world mpg?
Use the brim-to-brim method: fill the tank to the brim, drive normally, fill it to the brim again, then divide the miles covered by the gallons used (litres refilled ÷ 4.546). Repeat over two or three tanks for a stable average, since one unusually motorway-heavy or town-heavy tank can distort the figure by a wide margin.
Does driving slower really save fuel?
On motorways, yes — keeping to around 60-65 mph is one of the biggest single economy gains, and smooth acceleration, correct tyre pressures, removing roof boxes and shedding boot weight each add a few percent more. Together the habits are often worth 15-20%: on the £29.30 example trip, that is roughly £4.40 to £5.86 back in your pocket.