The formula, and a worked example
There is no mystery in the math and it is worth showing so you can sanity-check the output. Liters needed equals distance divided by one hundred, multiplied by consumption. Total cost is that liter figure multiplied by the price per liter. That is the whole calculation.
With the values the calculator starts on, a 300 kilometer trip in a car that uses 7 liters per 100 kilometers, at a price of 1.80 per liter, the calculation is 300 divided by 100, giving 3, multiplied by 7, which is 21.00 liters. At 1.80 per liter that comes to 37.80 for the trip.
A second example for a heavier vehicle: 450 kilometers at 8.5 liters per 100 kilometers is 38.25 liters, and at 1.65 per liter that is 63.11. Both figures update the moment you change an input, so you can drag a number and watch the effect rather than recalculating.
- Liters = distance / 100 x consumption
- Cost = liters x price per liter
- 300 km at 7 L/100km = 21.00 liters, and at 1.80 per liter that is 37.80
Getting the consumption figure right
The consumption number is where the answer is really decided, and the manufacturer figure is usually optimistic. Official consumption is measured in a controlled test cycle, not in traffic with a roof box and the air conditioning on. The number on your own trip computer, averaged over a few tanks, is closer to reality.
The honest way to measure it yourself is the tank method. Fill to the click, note the odometer, drive normally until the tank is well down, fill to the click again, and divide the liters you just put in by the kilometers driven, then multiply by one hundred. That gives liters per 100 kilometers directly, and it accounts for your actual roads, your actual load, and your actual right foot.
Conditions move the number more than most people expect. Motorway cruising at 120 rather than 100 can cost ten percent or more, cold weather raises consumption until the engine warms, a loaded roof rack is a real penalty at speed, and stop-start city driving is worse than any highway figure. For a budget, add a margin rather than assuming the best case.
What the calculator does not include
This is a fuel calculator, not a cost-per-kilometer calculator. It says nothing about tolls, parking, ferries, congestion charges, tires, servicing, or depreciation, all of which are real costs of driving somewhere. For a genuine comparison against a train ticket or a flight, those belong in the total and this figure is only one line of it.
It also assumes a single steady consumption figure for the whole distance. A trip that is half city and half motorway is really two trips with different consumption, and the accurate way to handle that here is to run the calculator twice and add the results, rather than averaging the consumption and hoping.
The inputs are metric throughout: kilometers, liters per 100 kilometers, and a price per liter. There is no miles-per-gallon mode and no unit switcher, so if you are working in miles and gallons you need to convert first. A mile is about 1.609 kilometers and a US gallon is about 3.785 liters.
- Fuel only: no tolls, parking, tires, servicing, or depreciation
- One consumption figure for the whole trip, so split mixed journeys in two
- Metric only: no MPG mode and no unit switcher