Fuel Cost Calculator — MPG, L/100km, Trip Cost and Splits

Calculate exactly how much gas you will need for your trip, the total fuel cost, cost per kilometre or mile, toll charges, and per-passenger split.

Created by Ashar PervaizLast updated 24 October 2026

Your Car's Efficiency Is Quoted in One of Four Units

Fuel cost arithmetic is simple. Getting the units straight is not, and that is where almost every wrong answer comes from. Four systems are in common use, two of them share a name, and they are not interchangeable.

Two traps follow from that table. The first is that L/100 km runs backwards relative to every other unit — a car at 5 L/100 km is thriftier than one at 9, which catches people comparing a European spec sheet against an American one.

The second is the gallon. A US gallon is about 3.785 litres and an imperial gallon about 4.546, so the same car is quoted at roughly 20% higher MPG in Britain than in America without being any more efficient. A car rated 30 MPG in the US is about 36 MPG in the UK. Comparing a US figure against a UK figure as though they were the same unit makes the British car look substantially better when it may be the identical model.

Converting Between Them

L/100 km = 235.2 ÷ MPG (US)
L/100 km = 282.5 ÷ MPG (imperial)
km/L = 100 ÷ L/100 km
MPG (imperial) = MPG (US) × 1.201

A car rated 30 MPG (US) is 235.2 ÷ 30 = 7.8 L/100 km, which is 100 ÷ 7.8 = 12.8 km/L. All four describe the same car.

Trip Cost, Whichever Unit You Have

The structure is the same in every system: work out how much fuel the distance needs, then multiply by what fuel costs. Only the middle step changes.

From L/100 km: Fuel = Distance in km × L/100 km ÷ 100
From km/L: Fuel = Distance in km ÷ km/L
From MPG: Fuel = Distance in miles ÷ MPG
Then: Cost = Fuel × Price per litre or gallon

A 450 km drive in a car returning 7.5 L/100 km, with fuel at 1.60 a litre: 450 × 7.5 ÷ 100 = 33.75 litres, and 33.75 × 1.60 = 54.00.

The one rule that prevents most errors is to keep distance and efficiency in the same family. Miles go with MPG, kilometres go with L/100 km or km/L. Converting the distance and the efficiency in opposite directions produces an answer that is out by roughly a factor of 1.6 and still looks plausible.

Cost Per Mile Is the Number Worth Knowing

Trip cost answers one journey. Cost per unit distance answers every journey, and it takes one division:

Cost per km = (L/100 km ÷ 100) × Price per litre
Cost per mile = Price per gallon ÷ MPG

At 7.5 L/100 km and 1.60 a litre, that is 0.075 × 1.60 = 0.12 per kilometre. Once you hold that single figure, any journey is a multiplication you can do in your head: a 40 km round trip is 4.80, and a 300 km weekend drive is 36.

It also reframes decisions that are otherwise argued about vaguely. A twenty-kilometre detour to a cheaper supermarket costs about 2.40 in fuel before anything else is counted, which is a concrete number to weigh against the saving rather than a feeling.

What a Commute Costs Over a Year

Commuting cost surprises people because the per-trip figure is small and the repetition is not.

Monthly = Round trip distance × Working days per month × Cost per unit distance

A 24 km round trip, 21 working days a month, at 0.12 per kilometre comes to 504 km and about 60 a month — roughly 726 a year in fuel alone. That figure is the honest input to any decision about moving closer, changing car, or negotiating a day at home. Note that it excludes insurance, tax, servicing, tyres and depreciation, all of which are real costs of the same journey.

Splitting Fuel Money Fairly

Shared journeys tend to be settled by guesswork or by whoever remembers. The fair figure is easy to establish.

Each person pays = Total fuel cost ÷ Number of people

A 54 trip cost with four people aboard is 13.50 each. Whether the driver pays a share is the part worth agreeing in advance: they are also absorbing wear, tyres and depreciation, so many groups split fuel among the passengers only. Neither convention is more correct, but deciding it before the journey avoids the conversation at the end of it.

For one-way trips where only some people travel both legs, work out the cost of each leg separately rather than halving the total. Our bill split calculator handles uneven shares for the rest of a trip. If you are converting between MPG and litres per 100 km by hand, that relationship is one of the awkward ones — it inverts rather than scales — and it is set out in our formula library alongside the miles-to-kilometres and litres-to-gallons factors.

Why the Real Figure Is Always Worse Than the Sticker

Manufacturer economy figures come from standardised test cycles, which exist so that different cars can be compared on equal terms. They are not predictions of what you will get, and the gap is systematic rather than random.

• Speed. Air resistance rises sharply with velocity, so motorway cruising at higher speeds costs disproportionately more fuel than the same distance driven more slowly.
• Short journeys. A cold engine runs richer until it warms. A run of two-kilometre trips can consume far more per kilometre than one long drive covering the same total.
• Stop-start traffic. Energy spent accelerating a mass is lost to the brakes every time you stop, which is why urban figures trail motorway ones in conventional cars.
• Load and roof racks. Weight matters, but an empty roof rack is worse than its mass suggests because it spoils the airflow across the whole car.
• Tyre pressure. Under-inflated tyres increase rolling resistance measurably, and it is the one item on this list that costs nothing to fix.

The practical response is to calculate with your own observed consumption rather than the brochure figure. Fill the tank, note the odometer, drive normally, then fill again and divide. Two or three tanks of that gives a number worth budgeting from.

Does a More Efficient Car Pay for Itself?

The comparison people skip is how far you have to drive to recover a higher purchase price. The answer is a division.

Saving per 100 km = (Old L/100 km − New L/100 km) × Price per litre
Break-even distance = Extra purchase cost ÷ Saving per 100 km × 100

Replacing a car using 8 L/100 km with one using 6, at 1.60 a litre, saves 2 litres or 3.20 every 100 km. If the more efficient car costs 3,000 more, the break-even is 3,000 ÷ 3.20 × 100, which is about 94,000 km.

Whether that is a good trade depends entirely on your annual mileage. At 20,000 km a year it takes nearly five years to break even on fuel alone; at 40,000 it takes under two and a half. The efficiency difference is only half the calculation, and the half people quote is usually not the deciding one.

Running costs rarely arrive one at a time. Fuel sits next to the energy bill, the weekly shop and whatever the car is worth now, and the everyday calculators cover the ones that turn up most often.

If the driving is for work rather than your own errands, the pence per mile you can claim is set centrally and has nothing to do with what the fuel actually cost you. What the HMRC mileage rate covers explains why the published rate includes wear and depreciation, not just petrol, and when claiming it leaves you out of pocket.

Fuel Cost Questions