How to Calculate Fuel Cost for a Trip (Simple Formula and Examples)

Before a road trip, a move to a new city or a carpool with colleagues, one question always comes up: how much will the fuel cost? Guessing usually goes wrong in both directions. You either take too little cash or argue about who owes what at the end.

The good news is that the maths is short. You need three numbers: the distance, how much fuel your vehicle uses, and the price of fuel. This guide shows the formula for every common unit, works through real examples, and explains how to get a fuel average you can actually trust.

Fuel Cost CalculatorTrip fuel cost from distance, mileage and fuel price.

The formula in one line

Fuel cost = fuel needed × price per litre (or gallon)

The only part that changes is how you work out "fuel needed", because countries measure fuel economy in different ways.

Notice the difference: with km/l and mpg a bigger number means a more economical car. With L/100 km a smaller number is better, because it tells you how much fuel is burned, not how far you go.

Worked examples

Kilometres per litre. A 300 km trip in a car that does 15 km/l, with fuel at 1.80 per litre:

• Fuel needed: 300 ÷ 15 = 20 litres
• Fuel cost: 20 × 1.80 = 36.00

Litres per 100 km. A 450 km trip in a car that uses 6.5 L/100 km, same fuel price:

• Fuel needed: 450 × 6.5 ÷ 100 = 29.25 litres
• Fuel cost: 29.25 × 1.80 = 52.65

Miles per gallon. A 250-mile trip in a car that does 30 mpg, with fuel at 3.50 per gallon:

• Fuel needed: 250 ÷ 30 = 8.33 gallons
• Fuel cost: 8.33 × 3.50 = 29.17

The currency doesn't matter. Use the price you pay at your local pump, in rupees, roubles, euros or dollars, and the answer comes out in the same currency.

Return trips and splitting the cost

Two quick additions turn this into a real trip budget:

• Going and coming back? Double the distance before you start. The 300 km example above becomes 600 km, or 72.00 in fuel.
• Sharing the car? Divide the total by the number of people. Three friends sharing that return trip pay 24.00 each.

It also helps to know the cost per kilometre: 36.00 ÷ 300 km = 0.12 per km. With that one number you can price any future trip in your head, or compare driving against a bus or train ticket.

The calculator above does all of this at once. Tick "Return trip", enter the number of people, and it shows the total, the litres or gallons needed, the cost per person and the cost per km or mile.

How to find your car's real fuel average

The figure in the brochure is measured in a lab and is almost always better than what you get on the road. Your own number is easy to measure with the full-tank method:

1. Fill the tank completely and reset the trip meter to zero.
2. Drive as you normally do until the tank is at least half empty.
3. Fill up to full again and note how many litres it took.
4. Divide the distance on the trip meter by those litres. That is your real km per litre.

For example, 412 km driven and 31.5 litres to refill gives 412 ÷ 31.5 ≈ 13.1 km/l. Repeat it two or three times, on both city and highway driving, and you will have a figure you can rely on.

Converting between km/l, L/100 km and mpg

If your car's manual uses a different unit from your country's fuel prices, convert first:

• L/100 km = 100 ÷ km/l. So 15 km/l is 6.67 L/100 km.
• km/l = 100 ÷ L/100 km. So 8 L/100 km is 12.5 km/l.
• US mpg to km/l: multiply by 0.425. So 30 mpg is about 12.75 km/l, or 7.84 L/100 km.

Watch out for the gallon. A US gallon is about 3.785 litres, but a UK (imperial) gallon is about 4.546 litres, so "30 mpg" in the UK is a more economical car than "30 mpg" in the US. Use the same kind of gallon for the fuel average and the fuel price. For miles, kilometres, litres and gallons, the unit converter does the conversion for you.

Unit ConverterConvert length, weight, temperature, area, volume, speed and more.

Why real trips cost more than the calculation

The formula gives you the fuel cost under normal driving. On the day, a few things push it up:

• Speed. Fuel use climbs quickly at high motorway speeds because air resistance grows with speed.
• Air conditioning. Running the AC uses noticeably more fuel, especially in slow city traffic.
• Traffic and idling. Stop-and-go driving and waiting with the engine running burn fuel without moving you anywhere.
• Load. Passengers, heavy luggage and especially a roof box or roof rack increase consumption.
• Low tyre pressure. Soft tyres roll less easily. Check them before a long trip.
• Short trips with a cold engine. An engine uses more fuel until it warms up.

A simple habit is to add 10% to the result for a long trip as a buffer. If you prefer a different margin, the percentage calculator works out any percentage of your total.

Percentage CalculatorSolve percent of, percent change, increase, decrease and more.

Also remember that fuel isn't the only cost of driving. Tolls, parking and wear on the car add up too, so include them when you compare driving with other ways of travelling.

Frequently asked questions

How do I calculate petrol cost for a trip? Divide the distance by your car's km per litre to get the litres needed, then multiply by the price per litre. For L/100 km, multiply the distance by the consumption and divide by 100 instead.

How much fuel does a car use per 100 km? It depends on the car and how it is driven. Small petrol cars often use around 5–7 litres per 100 km, while large SUVs can use 10 litres or more. Measure your own car with the full-tank method for an accurate number.

Does it work for motorbikes and diesel cars? Yes. The formula is the same for any vehicle and any fuel. Just use your vehicle's own average and the price of the fuel it uses.

How do I split fuel cost fairly between passengers? Divide the total fuel cost by the number of people in the car, including the driver. Some groups let the driver pay less because they also cover wear and tear on the car.

Why is my real fuel cost higher than the calculator says? The result is only as accurate as the fuel average you enter. Use your measured average, not the manufacturer's figure, and add a small buffer for traffic, AC and speed.

Plan the numbers once before you leave, and the only surprise on your trip will be the view.