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How to calculate the fuel cost of a trip

Working out the fuel cost of a trip is a simple multiplication: the litres you will use times the price per litre. And the litres are the distance divided by 100, times your car’s consumption.

3 min readReviewed By Thorben Rasmus IdelReviewed by Nahar Geva

TL;DR

To calculate the fuel cost of a trip, multiply the litres you will use by the price per litre. The litres are the distance divided by 100, times your car’s consumption in litres per 100 km. For example, 100 km in a car that does 6 l/100 km uses 6 litres; at €1.80 a litre, that is €10.80. For a round trip, double the distance; for the cost per person in a carpool, divide the total by the number of people.

The fuel cost formula

Working out the fuel cost of a trip is no mystery: it is a multiplication. You need three numbers, the distance, your car’s consumption and the fuel price, and you do two sums.

First, the litres you will use:

litres = distance ÷ 100 × consumption (l/100 km)

Then the cost:

cost = litres × price per litre

For example, 100 km in a car that does 6 litres per 100 km uses 6 litres. If fuel is at €1.80 a litre, the trip costs €10.80. You can do this sum directly in the fuel cost calculator.

What consumption in l/100 km means

Consumption in litres per 100 km is the number that most affects the result. It is the amount of fuel the car uses to drive 100 kilometres: a car that does 6 l/100 km uses 6 litres every 100 km, and the lower that figure, the more economical the car.

The spec sheet quotes the combined WLTP consumption2, but real consumption depends on driving style, speed, traffic, load and even the tyres. The catalogue value is measured in ideal conditions and tends to be optimistic. For a reliable estimate, use the average consumption your trip computer shows over several weeks.

The fuel price

The price per litre changes almost every day and varies between stations and between petrol and diesel. That is why it is a value you enter, not a fixed table: use the current price of the fuel your car takes. In Portugal you can check the daily average price on the DGEG fuel-price portal1.

Round trip, cost per km and carpools

If the trip is there and back, the distance is double: always count the total distance driven. The cost per kilometre (the total cost divided by the distance) is useful for comparing routes and seeing what each kilometre you drive costs. And if you carpool, divide the total cost by the number of people to find out how much each one pays. On a €60 trip with four people, that is €15 each.

Fuel is only part of the cost of a car

Fuel cost helps you plan a trip, but it is not the total cost of owning a car. Tolls, wear (tyres, oil, servicing), insurance and the annual tax all weigh on the budget. For the annual tax, see the IUC calculator; if you are thinking of financing the purchase, the car loan calculator shows the monthly payment and the total cost of the financing. For the fuel cost of each trip, use the fuel cost calculator.

Common mistakes

  • Using the catalogue consumption instead of the real one

    The WLTP figure on the spec sheet is measured in ideal conditions and tends to be optimistic. Use the consumption your trip computer shows over several weeks for a reliable estimate.

  • Forgetting the return leg

    If you drive there and back the same day, the distance is double. Always count the total distance driven, not just the outward trip.

  • Treating fuel cost as the cost of running a car

    Fuel is only one part. Tolls, wear, servicing, insurance and the annual vehicle tax (IUC) are separate costs that also weigh on the car budget.

Frequently asked questions

How do you calculate the fuel cost of a trip?
Multiply the litres used by the price per litre. The litres are the distance divided by 100, times the consumption in litres per 100 km. For example, 200 km at 6 l/100 km uses 12 litres; at €1.80 a litre, that is €21.60.
What is consumption in litres per 100 km?
It is the amount of fuel a car uses to drive 100 kilometres. A car that does 6 l/100 km uses 6 litres every 100 km. The lower it is, the more economical the car.
How much does it cost to drive 100 km?
It depends on the car’s consumption and the fuel price. At 6 l/100 km and €1.80 a litre, 100 km costs €10.80. At 8 l/100 km it costs €14.40.
How do you split fuel cost in a carpool?
Divide the total cost by the number of people in the car. On a €60 trip with four people, that is €15 each.

Related reading & calculators

Sources

  1. 1.Fuel prices (daily average price) — Direção-Geral de Energia e Geologia (DGEG) · retrieved 26 Jun 2026
  2. 2.Vehicle fuel labelling and consumption — Automóvel Club de Portugal (ACP) · retrieved 26 Jun 2026

Author / Reviewed by

Author

Thorben Rasmus Idel

Co-founder & writer

Co-founder of Calculadora Capital and the writer behind the methodology on every calculator and article. An entrepreneur and active investor, Thorben founded Idel Versandhandel GmbH, an international trading company operating across 16 countries, and invests across stocks, ETFs and cryptocurrency. He writes the methodology and verifies the math behind each page, drawing on hands-on business and investing experience to keep the tools and explanations grounded in how money, markets and taxes actually work for everyday people in Portugal.

Reviewed by

Nahar Geva

Co-founder & reviewer

Co-founder of Calculadora Capital and the independent reviewer behind every calculator and article. An entrepreneur and active investor, Nahar brings a data- and product-driven mindset together with hands-on experience in the markets — investing across stocks and ETFs as well as cryptocurrency and other digital assets, alongside broader personal finance and real estate. On each page Nahar reviews the methodology and double-checks the math and figures, pressure-testing how the tools and explanations hold up against the way money, markets and taxes actually work for everyday investors.

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