How to calculate fuel cost

Fuel cost for a journey comes down to three numbers: how far, how thirsty the car is, and the price. Here is the formula, how to handle mpg and litres per 100 km without tripping up, and how to turn one trip into a yearly figure.

By the Calculator Den team · Updated 2026-10-22

Working out what a journey costs in fuel is one of the most useful bits of everyday arithmetic, whether you are deciding if a trip is worth it, splitting costs with passengers, or pricing mileage for work. It rests on just three numbers: the distance, how far your car goes on a unit of fuel, and the price of that fuel. The only thing that trips people up is units, because fuel economy is measured two completely different ways around the world. A free fuel cost calculator does the sums instantly, but once you see the method you can sanity-check any figure it gives you.

The fuel cost formula

At its simplest, fuel cost is fuel used × price per unit. The fuel used depends on distance and economy, so the whole thing in one line is:

Fuel cost = (distance ÷ fuel economy) × fuel price

That works as long as your distance, economy and price all use the same units. That last clause is where the care is needed, because "fuel economy" means opposite things in different countries.

Watch the units: mpg versus litres per 100 km

There are two families of fuel economy, and they run in opposite directions.

• Distance per fuel (miles per gallon, or km per litre). A bigger number is better: 50 mpg is more efficient than 30 mpg.
• Fuel per distance (litres per 100 km, the standard across most of Europe). A smaller number is better: 5 L/100km is more efficient than 8 L/100km.

The maths is slightly different for each, so you have to know which one you are holding. With miles per gallon or km per litre you divide the distance by the economy to get the fuel used. With litres per 100 km you multiply: fuel used = distance ÷ 100 × the L/100km figure. Mixing these up is the single most common mistake, and it is why a calculator that asks for your units is safer than a half-remembered formula.

One more unit trap: the US gallon and the UK (imperial) gallon are not the same size. A UK gallon is about 4.55 litres and a US gallon about 3.79 litres, so "40 mpg" in Britain is not the same efficiency as "40 mpg" in America. If you convert between systems, convert the gallon too.

To move between the two scales at a glance, divide 282.5 by a UK mpg figure, or 235.2 by a US mpg figure, to get litres per 100 km:

Worked example in miles and gallons

Say you are driving 300 miles, your car does 40 miles per (UK) gallon, and fuel is 1.50 per litre in your own currency. The economy is in gallons but the price is per litre, so first get them to agree. Fuel used is 300 ÷ 40 = 7.5 gallons, which is 7.5 × 4.55 = about 34.1 litres. At 1.50 a litre the trip costs 34.1 × 1.50 = about 51.15. If your pump price is already per gallon, skip the conversion and just do 7.5 × the price per gallon.

A US reader driving the same 300 miles at 40 US mpg burns 7.5 US gallons too, but a US gallon is smaller, so that is 7.5 × 3.79 = about 28.4 litres, or about 42.60 at 1.50 a litre. Same mpg number, less fuel, because "mpg" is not the same unit on each side of the Atlantic.

Worked example in kilometres and litres

Now the metric version, which is cleaner because everything is already in litres. You drive 300 km, the car uses 6.5 litres per 100 km, and fuel is 1.50 per litre. Fuel used is 300 ÷ 100 × 6.5 = 19.5 litres. The cost is 19.5 × 1.50 = 29.25. Notice the multiply, not divide: with L/100km the economy figure is already fuel per distance, so you scale it by how many hundreds of km you drove.

Cost per mile or per kilometre

A single trip number is useful, but a cost per mile or per km lets you compare journeys and cars on equal footing. Just divide the trip cost by the distance. In the metric example above, 29.25 ÷ 300 km = about 0.098 per km, just under 10 cents a kilometre. In the imperial one, 51.15 ÷ 300 miles = about 0.17 per mile. This per-distance figure is the one worth remembering, because you can multiply it by any trip length later without redoing the whole calculation.

From one trip to a yearly fuel bill

Scale the same method up and you get your annual fuel spend, which is usually the number that actually matters for a budget. If you drive about 1,000 km a month at 0.098 per km, that is roughly 98 a month, or about 1,170 a year. Seeing the yearly figure is often what makes a more efficient car, or a change in driving habits, feel worth it. A small slip in economy, say from 6.5 to 8 L/100km, looks trivial per trip but adds a few hundred a year at that mileage.

Fuel is also one running cost you can influence day to day. If the yearly number stings, our guide on how to save money on fuel covers the habits that cut it, with the real percentage each one saves, and the cost of ownership calculator puts fuel alongside depreciation, insurance and maintenance so you see the full picture.

Common fuel cost mistakes

• Dividing when you should multiply. With litres per 100 km you multiply; with mpg or km per litre you divide. Check which economy figure you have first.
• Mixing gallons and litres. If economy is in mpg but the price is per litre, convert one of them before multiplying, and use the right gallon (UK or US).
• Using the optimistic figure. The economy on the brochure is a best case. Real-world economy is usually worse, especially in town, so use what your car actually returns if you can.
• Forgetting the return leg. A "300 km trip" is often 600 km there and back. Price the whole journey.

Get the units straight and the rest is one division and one multiplication. When you want it done in a second, including the mpg-to-litres conversion, the free fuel cost calculator takes your distance, economy and price and shows the trip cost and litres, so you can compare routes, cars and driving styles without the arithmetic.