From distance to money in three steps
Any fuel cost estimate needs three facts: how far you will drive, how much fuel the car uses over a given distance, and what that fuel costs. The calculator asks for each one with its unit:
- Distance in km or miles. Tick Round trip (double the distance) when you will drive back the same way.
- Fuel economy as L/100 km (used in Europe, Canada, Australia and much of the world), mpg (US), mpg (UK) or km/L (common in parts of Asia and Latin America).
- Fuel price per liter, per US gallon or per imperial gallon, in whatever currency you pay.
Inside, everything is turned into kilometers, kilometers per liter and price per liter. The distance divided by km/L gives the liters needed; liters times price per liter gives the cost. The result shows the fuel in liters, US gallons and imperial gallons, the cost of the trip, the share for each of the People sharing the cost, and the cost per kilometer or mile.
The trip cost formula
All unit conversions are exact: a mile is 1.609344 km, a US gallon is 3.785411784 L and an imperial gallon is 4.54609 L. The only uncertain number is the fuel economy, which is an estimate before the trip.
Worked examples
- Round trip: 350 × 2 = 700 km.
- Fuel: 700 × 6.5 ÷ 100 = 45.5 L.
- Cost: 45.5 × 1.85 = 84.175, shown as 84.18.
- Per person: 84.175 ÷ 2 = 42.09. Per km: 84.175 ÷ 700 = 0.12.
- Fuel: 240 ÷ 28 = 8.571 US gallons.
- Cost: 8.571 × 3.40 = 29.14.
- Per mile: 29.14 ÷ 240 = 0.121.
- 45 mpg (UK) means 45 miles per imperial gallon. 180 ÷ 45 = 4 imperial gallons, or 18.18 L, which costs 26.37 at 1.45 per liter.
- Read as 45 mpg (US), the same 180 miles need 4 US gallons, or 15.14 L, costing 21.96.
- Picking the wrong gallon understates this trip by 4.41, about 17%.
Official ratings versus the road
The economy printed in a brochure or on a window label comes from a standardized laboratory test. That makes it good for comparing one model with another, but both the US fueleconomy.gov site and Natural Resources Canada state plainly that your own consumption will differ. Driving style, speed, traffic, weather, air conditioning, the car’s condition and even the fuel blend all move it.
For a planning estimate, the most reliable figure is the one you have measured on your own car with a full-tank check (see the gas mileage calculator). If you only have the official rating, it helps to see how sensitive the cost is to it: the 700 km round trip above costs 84.18 at 6.5 L/100 km, but 97.13 at 7.5 L/100 km, a gap of nearly 13 for one extra liter per 100 km.
Where trip estimates go wrong
- Mixing US and UK gallons. Both are called gallons, but the imperial gallon is about 20% bigger. In the UK, mpg figures use the imperial gallon even though fuel is sold by the liter; US figures use the US gallon. The third example shows the effect.
- Treating L/100 km like mpg. In L/100 km a lower number is better; in mpg and km/L a higher number is better. Entering a consumption figure in the wrong unit box produces a nonsense cost.
- Forgetting the way back. A one-way distance from a map app is often what people type. The round-trip box doubles it.
- Using highway economy for a city trip, or the reverse. Many labels show separate city, highway and combined figures. Use the one that matches the drive, or your own measured figure.
- Leaving out tolls and parking. The calculator covers fuel only; tolls, parking, and wear on the car are separate costs.
Electric cars are rated in kWh per 100 km or miles per kWh rather than by fuel volume, so this calculator does not fit them. The same logic works by hand: kWh per 100 km × distance ÷ 100 gives the energy, and energy × price per kWh gives the cost.
Questions people ask
How do I share fuel fairly when someone rides only part of the way?
Use the cost per kilometer or mile. In the first example it is about 0.12 per km, so a 300 km stretch costs about 36 (19.5 L at 1.85). Split that stretch between everyone in the car for it, and the rest between those who drove the whole way.
Does a heavier load or a roof box change the result?
Yes, extra weight and drag usually raise consumption, but by how much depends on the car and the speed. If you regularly drive loaded, measure the economy on a loaded trip and use that figure here.
Sources
- Your Mileage Will Vary, U.S. Department of Energy and U.S. Environmental Protection Agency (fueleconomy.gov).
- Understanding fuel consumption ratings, Natural Resources Canada.
- Weights and Measures Act 1985, Schedule 1 (definition of the gallon), legislation.gov.uk (The National Archives).
- Guide for the Use of the International System of Units (SI), NIST SP 811, Appendix B, National Institute of Standards and Technology.