The formula
Every fuel-cost calculation comes down to two steps: how much fuel the distance needs, and what that fuel costs.
Fuel (litres) = distance (km) × consumption (L/100 km) ÷ 100
Fuel (gallons) = distance (miles) ÷ miles per gallon
Cost = fuel × price per litre (or per gallon)
The calculator converts between kilometres and miles, litres and gallons, so you can mix units: for example a distance in miles, a car rated in L/100 km and a price per litre.
Worked examples
A 300 km drive in a car using 6.5 L/100 km
Fuel = 300 × 6.5 ÷ 100 = 19.5 litres. At 1.80 per litre the trip costs 19.5 × 1.80 = 35.10. A return trip doubles it to 70.20, or 35.10 each if two people share.
A 200-mile trip at 30 mpg (US)
Fuel = 200 ÷ 30 = 6.67 US gallons. At 3.50 per gallon the cost is 6.67 × 3.50 = 23.33.
US gallons, UK gallons and litres
A US gallon is 3.785 litres; a UK (imperial) gallon is 4.546 litres, about 20% more. So "30 mpg" means something different in the US and the UK: 30 US mpg equals about 36 UK mpg. Using the wrong one makes your estimate 20% off, so the calculator asks which one you mean. Most of the world uses L/100 km, where a lower number is better, or km/L, where a higher number is better.
| L/100 km | km/L | mpg (US) | mpg (UK) |
|---|---|---|---|
| 4 | 25 | 58.8 | 70.6 |
| 6 | 16.7 | 39.2 | 47.1 |
| 8 | 12.5 | 29.4 | 35.3 |
| 10 | 10 | 23.5 | 28.2 |
| 12 | 8.3 | 19.6 | 23.5 |
Which fuel-economy figure should you use?
Official figures (WLTP in Europe and many other countries, EPA in the US) are measured in standard test cycles. Real-world consumption is often 10–20% higher, especially for short trips, in cold weather and at motorway speeds. The best number is your own: fill the tank, drive normally, fill it again and divide. The fuel economy calculator does this for you.
What changes your fuel use on a trip
- Speed: air resistance rises with the square of speed, so driving at 130 km/h (80 mph) instead of 110 km/h (70 mph) can increase consumption noticeably.
- Load and roof boxes: extra weight matters in stop-start traffic; roof boxes and bike racks add drag at speed.
- Traffic and hills: stop-start driving and climbs use more fuel. Add a safety margin of 10–15% for unknown roads.
- Tyre pressure: under-inflated tyres increase rolling resistance. Check them before a long trip.
- Air conditioning: uses some extra fuel, but at high speed it can be better than open windows, which add drag.
Sharing the cost fairly
For a shared trip, divide the fuel cost by the number of people. Some groups also add tolls and parking, or a little for wear on the car. Agreeing before you leave avoids awkward conversations later.
Common mistakes
- Mixing US and UK gallons (a 20% error).
- Treating L/100 km like km/L. 8 L/100 km is good economy; 8 km/L is quite thirsty.
- Forgetting the return trip. Choose "return" if you will drive back.
Cost per kilometre and per mile
Fuel cost per km = L/100 km × price per litre ÷ 100. At 1.80 per litre, a car using 5, 7 or 9 L/100 km costs 0.09, 0.126 or 0.162 per km. Multiply by your yearly distance to see the annual fuel bill: 15,000 km at 7 L/100 km is 1,890.
Comparing with an electric car
For an electric vehicle, use kWh/100 km × electricity price. At 16 kWh/100 km and 0.25 per kWh, 100 km costs 4.00, against 12.60 for the 7 L/100 km petrol car above. Public fast charging can cost several times the home rate, so use the mix of charging you expect.
Is it cheaper to drive or take the train?
For one person, fuel alone may look cheaper than a train ticket, but add tolls, parking and wear on the car. For two or more people sharing a car, driving often becomes the cheaper option. Compare door-to-door totals rather than fuel alone.
Frequently asked questions
How do I calculate fuel cost for a trip?
Divide the distance by your fuel economy to get the fuel needed (or multiply km by L/100 km and divide by 100), then multiply by the fuel price.
How much fuel does 100 km use?
Exactly your L/100 km figure. A car rated at 6.5 L/100 km uses 6.5 litres every 100 km.
How do I convert mpg to L/100 km?
Divide 235.2 by US mpg, or 282.5 by UK mpg. 30 US mpg is about 7.8 L/100 km.
Why is my real fuel use higher than the official figure?
Official tests use standard driving cycles. Higher speeds, short trips, cold weather, traffic and extra load all increase real-world consumption.
How do I calculate fuel cost per km?
Multiply L/100 km by the price per litre and divide by 100. 7 L/100 km at 1.80 per litre costs 0.126 per km.
What is my yearly fuel cost?
Fuel cost per km × kilometres per year. At 0.126 per km and 15,000 km, that is 1,890.
Sources
- US Environmental Protection Agency and US Department of Energy, fueleconomy.gov (fuel economy ratings and driving tips).
- UNECE Worldwide harmonised Light vehicles Test Procedure (WLTP), Global Technical Regulation No. 15.
- NIST Handbook 44, Appendix C: US gallon = 3.785411784 L; UK imperial gallon = 4.54609 L.
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.