How to calculate fuel cost for a trip (mpg or L/100 km)

How to · Updated 2026-11-10

Divide the distance by your fuel economy to get the fuel used, then multiply by the price. Worked US and metric road-trip examples, split between passengers.

To find the fuel cost of a trip, work out how much fuel the distance needs from your car’s fuel economy, then multiply that by the price per litre or gallon. The fuel cost calculator does it in one go, takes mpg (US or UK), L/100 km or km/L as they come, and can double the distance for a round trip and split the cost between passengers.

What you need

• The distance. A map app gives this; use the route you will really drive, not the straight line.
• Your fuel economy. The trip computer on the dashboard is a good source. Better still, fill the tank, note the distance until the next fill-up, and divide. The figure in a brochure is usually a little optimistic.
• The fuel price, per litre or per gallon, from a recent receipt or a station sign.
• How many people will share, if anyone is chipping in.

Step by step

The formula depends on how your economy is written. With miles per gallon:

Fuel used (gallons) = distance (miles) ÷ mpg

Cost = fuel used × price per gallon

With litres per 100 km:

Fuel used (litres) = distance (km) × L/100 km ÷ 100

Cost = fuel used × price per litre

Two things trip people up. First, mpg is “higher is better”, while L/100 km is “lower is better”, so the formulas run in opposite directions: you divide by mpg but multiply by L/100 km. Second, a US gallon (about 3.79 L) is smaller than a UK gallon (about 4.55 L), so the same car shows a higher mpg figure in the UK.

With the calculator:

1. In Your trip, type the Distance and choose km or miles.
2. Type the Fuel economy and choose its unit: L/100 km, km/L, mpg (US) or mpg (UK).
3. Type the Fuel price and say whether it is per litre, per US gallon or per UK gallon. Mixing units is fine; the page converts everything to litres and kilometres behind the scenes.
4. Tick Round trip if you are coming back the same way. It doubles the distance.
5. Set People sharing the cost. With more than one, the result line adds a per-person share.
6. The answer appears straight away: a big cost figure, then a line with the total distance in km and the litres needed.

Further down, Convert fuel economy changes one economy figure into all four units, handy when you are comparing a hire car’s figure with your own.

A worked example

Three friends drive 240 miles each way to a weekend away and come back the same route. The car does 32 mpg (US), and fuel costs 3.40 per US gallon.

1. Round trip distance: 240 × 2 = 480 miles.
2. Fuel used: 480 ÷ 32 = 15 US gallons.
3. Cost: 15 × 3.40 = 51.00.
4. Each person: 51.00 ÷ 3 = 17.00.

The page shows “51.00 fuel cost” and underneath “772 km needs about 56.8 litres. Shared between 3: 17.00 each.” That is the same trip in metric: 480 miles is 772 km, and 15 US gallons is 56.8 L.

The same sum in metric units: a couple drive 350 km each way in a car that uses 6.5 L/100 km, with fuel at 1.65 per litre.

1. Round trip: 700 km.
2. Fuel: 700 × 6.5 ÷ 100 = 45.5 L.
3. Cost: 45.5 × 1.65 = 75.08, or 37.54 each.

If the friends also paid for parking, food and a cabin, the trip splitter settles all the shared costs at the end with the fewest payments. For a daily drive to work, the commute cost calculator compares the car with transit and cycling over a month or a year.

Check your answer

• Cost per mile or km. In the first example, 51.00 ÷ 480 miles is about 0.11 per mile. Price per gallon divided by mpg gives the same figure (3.40 ÷ 32 = 0.106), so the two routes agree.
• Tank check. Most cars hold roughly 40 to 70 litres (about 10 to 18 US gallons). If your answer says a short trip uses several tanks, a unit is probably wrong, such as km typed with miles selected.
• Gallon type. If your result looks about 20 % off from what you expected, check you picked the right gallon. The gap between US and UK gallons is roughly that size.

Questions people ask

Does the cost include tolls, parking or wear on the car? No, only fuel. Add those separately, or put the whole trip’s costs into a splitter.

Why is my real fuel use higher than the estimate? Motorway speed, air conditioning, a roof box, heavy luggage, cold weather and hills all raise consumption. Adding 10 % as a margin is a sensible habit on a long drive.

How should passengers split the cost fairly? Equal shares are simplest. Some groups also add a little for the driver’s time or the car’s wear; agree on it before you set off.