Fuel Cost Calculator

What this calculator does. It turns three numbers — trip distance, your car's fuel economy and the price you're paying at the pump — into a single pounds-and-pence estimate of what a journey will cost in fuel. It handles UK and US units side-by-side, so a US car rated in mpg and a UK driver pricing fuel per litre can use the same tool without doing any maths in their head.

The formula in plain English. Fuel cost = (distance ÷ economy) × price per unit. Behind the scenes we convert your distance into kilometres, work out how many litres of fuel that distance will use given your car's economy, apply a small journey-type adjustment, then multiply by your fuel price — converting between price-per-litre and price-per-gallon as needed.

A worked UK example. A 250-mile trip in a car doing 45 mpg (UK) at £1.45 per litre works out as follows. 250 miles ÷ 45 mpg = 5.56 UK gallons. 5.56 × 4.546 litres = 25.3 litres. 25.3 × £1.45 = £36.70 for the round trip one-way, or around £73 there and back. If most of that drive is motorway, the estimate drops by about 10%; if it's mostly city driving, it rises by about 20%.

Units made simple. 1 mile is 1.609 km, a UK gallon is 4.546 litres, and a US gallon is 3.785 litres. That last difference matters: a US car rated at 30 mpg is actually doing about 36 mpg in UK terms for the same fuel consumption. Switch any unit in the form and the maths re-runs instantly.

Why this matters. Fuel is one of the few costs where small upfront decisions — which route to take, whether to make a separate trip or combine errands, whether a weekend away is realistic on the current budget — translate directly into money you keep or money you spend. Knowing the cost of a journey before you set off also makes it easier to split fairly between passengers, claim the right amount on expenses, or compare driving against train and coach tickets honestly. For a typical UK household, deciding to consolidate one long supermarket run a week instead of three short ones can save £150–£250 a year in fuel alone.

When to use it. Planning a road trip, budgeting a regular commute, comparing two cars before buying, splitting petrol money with friends, or working out whether driving or taking the train is genuinely cheaper for a specific journey.

When not to rely on it alone. For business mileage claims, HMRC uses fixed per-mile rates (currently 45p for the first 10,000 miles and 25p after) that cover wear, insurance and depreciation as well as fuel — those rates aren't the same as fuel cost. For total cost of ownership, you also need to factor in servicing, tyres, insurance and depreciation, which can easily double the per-mile figure.

Common mistakes. Mixing UK and US gallons is the most expensive one — it skews results by around 20%. Using the manufacturer's headline mpg rather than your actual long-term trip-computer figure is the second; real-world economy is typically 10–20% worse than the official combined number, especially in winter or with a heavy load. Forgetting that pump prices are usually quoted per litre in the UK and per US gallon in the US is the third — pick the matching price unit in the form.