How to Calculate Fuel Cost for a Trip (L/100 km Formula)

Updated October 11, 2026

To calculate the fuel cost of a trip, multiply the distance in kilometres by your car’s fuel consumption in litres per 100 km, divide by 100, then multiply by the price per litre. A 500 km trip in a car that uses 6.5 L/100 km needs 32.5 litres, which costs €58.50 at €1.80 a litre.

This guide gives you the formula, a cost table, a simple way to measure your own consumption and a conversion to miles per gallon. To skip the maths, use our fuel cost calculator (L/100 km), which works in euros, pounds and dollars.

Last updated: October 2026.

The fuel cost formula

Litres needed = distance (km) x consumption (L/100 km) / 100
Trip cost = litres needed x price per litre

You can also work out the cost per kilometre, which makes planning easy:

Cost per km = consumption (L/100 km) x price per litre / 100

Worked example: a 500 km trip

Your car uses 6.5 L/100 km. Petrol costs €1.80 a litre.

1. Litres needed: 500 x 6.5 / 100 = 32.5 litres
2. Trip cost: 32.5 x €1.80 = €58.50
3. Cost per 100 km: 6.5 x €1.80 = €11.70
4. Cost per km: €0.117

If four people share the trip, each pays about €14.63.

The €1.80 price is an example. Fuel prices change often and differ between countries, so enter today’s price at the pump.

Fuel cost table: €1.80 per litre

A car that uses 9 L/100 km costs twice as much to run as one that uses 4.5 L/100 km on the same trip.

How the fuel price changes the cost

Here is the same car (6.5 L/100 km) on a 500 km trip at three prices:

Every 20 cents on the price adds about €6.50 to this trip.

How to measure your own fuel consumption

Your car’s brochure figure is usually lower than real life. To find your real number:

1. Fill the tank completely and reset the trip meter (or note the odometer).
2. Drive normally until the tank is about half empty or lower.
3. Fill the tank completely again and note the litres added and the distance driven.
4. Use this formula: litres added / kilometres driven x 100

Example: you drive 520 km and add 34 litres. Your consumption is 34 / 520 x 100 = 6.54 L/100 km. Repeat this over a few tanks and take the average, since one fill-up can vary.

Convert L/100 km to miles per gallon

Some people, especially in the UK, still think in miles per gallon (mpg). The conversion differs because the UK gallon is bigger than the US gallon.

• UK mpg = 282.481 / (L per 100 km)
• US mpg = 235.215 / (L per 100 km)

Note that a lower L/100 km is better, while a higher mpg is better. If you work in US gallons and miles, use our fuel cost calculator (MPG) instead.

How much does fuel cost per year?

Add up your yearly distance. For 12,000 km a year at 6.5 L/100 km and €1.80 a litre:

• Fuel used: 12,000 x 6.5 / 100 = 780 litres
• Yearly cost: 780 x €1.80 = €1,404
• Monthly cost: about €117

Even a small change matters. If you cut your consumption by just 1 L/100 km, you use 120 fewer litres a year and save about €216 at the same price.

What the formula does not include

Fuel is only part of what a trip costs. For a full budget, also think about:

• Tolls and vignettes: many European countries charge for motorways, in different ways.
• Parking and congestion or low-emission zone charges in cities.
• Wear, tyres, servicing and insurance, which add to the cost per kilometre.
• Fuel price differences: prices can vary a lot from one country to the next, so check the price where you will actually fill up.

8 ways to spend less on fuel

1. Check your tyre pressure. Under-inflated tyres increase consumption.
2. Drive smoothly. Hard acceleration and braking use more fuel.
3. Slow down on motorways. Consumption rises noticeably at higher speeds.
4. Remove roof boxes and racks when you are not using them.
5. Clear out heavy items from the boot.
6. Plan your route to avoid stop-and-go traffic and detours.
7. Compare prices before you fill up. A few cents per litre adds up over a year.
8. Service your car on time. A well-maintained engine runs more efficiently.

Petrol or diesel versus electric: how to compare

You can use the same idea for an electric car. Cost per 100 km = consumption in kWh per 100 km x price per kWh.

Example with made-up figures: a car using 17 kWh/100 km at €0.30 per kWh costs €5.10 per 100 km. The petrol car in our example costs €11.70 per 100 km. Real electricity prices vary a lot, especially between home charging and public fast chargers, so use your own figures. The result also ignores the car’s purchase price.

Planning a bigger purchase?

If you are thinking about buying or leasing a car, see what it adds to your monthly budget with the loan payment calculator and the car lease calculator. To see how much of your income goes on transport, try the 50/30/20 budget calculator. And if you want to know how rising prices affect your budget over the years, use the inflation calculator.

Frequently asked questions