Fuel Cost Calculator

What a journey costs in fuel

Deciding whether to drive or take the train, splitting a road trip between passengers, claiming mileage back from an employer, or comparing two cars before buying one — all of them need the same number, and none of them are served by a vague sense that fuel is expensive. Give this calculator a distance, a fuel economy figure and a price, and it returns the fuel used, the total cost, the cost per mile or kilometre, and the amount each if you are splitting it.

A worked example: 300 miles at 32 mpg

A 300-mile trip in a car doing 32 mpg, with fuel at $3.45 a US gallon: 300 ÷ 32 = 9.375 gallons, at a cost of $32.34, or 10.8 cents a mile. Split between three people, that is $10.78 each.

The same car expressed in metric terms is 7.35 litres per 100 km, which is the figure a European specification sheet would quote. Running metric directly: 500 km at 7.5 L/100 km is 37.5 litres, and at $1.80 a litre that is $67.50.

The imperial case is where the arithmetic quietly diverges. A 300-mile trip at 40 mpg in imperial gallons uses 7.5 imperial gallons — but that same car is only 33.3 mpg in US terms, because an imperial gallon is about 20% larger. Comparing a UK economy figure against a US one without converting overstates the car by a fifth.

The conversions used

Fuel used is distance ÷ economy in imperial or US units, or distance × consumption ÷ 100 in metric. Everything is normalised internally to kilometres and litres per 100 km so that one code path serves all three systems, then converted back for display.

Two constants do the work. Miles per gallon and litres per 100 km are reciprocals, not a scale: L/100km = 235.214583 ÷ mpg, and the same constant converts back. US and imperial gallons differ by a factor of 1.20095, so imperial mpg ÷ 1.20095 gives the US equivalent. Because the two economy measures are reciprocal, a given improvement in mpg is worth much more at the bottom of the range than at the top — going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50.

Why the real cost will be higher

The published or dashboard economy figure is an optimistic starting point. Real consumption is worse in cold weather, in traffic, at motorway speeds above about 60 mph, with a roof box or a full car, with under-inflated tyres, and on short trips where the engine never reaches operating temperature. A useful habit is to calculate your own figure from a few full-tank fills rather than trusting the brochure.

The calculation covers fuel and nothing else. Tolls, parking, congestion charges, servicing, tyres, insurance and depreciation are all real costs of the journey — which is why mileage reimbursement rates, such as the US IRS standard rate or the UK HMRC rate, are several times the pure fuel cost. If you are comparing driving against another mode of transport, fuel alone understates the case considerably.