Fuel Cost: The Three-Variable Formula
Auto · 5 min read · Last updated September 2026
The formula
Cost = fuel needed × price per unit
Worked example: 15,000 km at 15 km/L needs 1,000 L; at 100 per litre that is 100,000 for the year. In US units: 12,000 miles at 25 mpg needs 480 gallons; at $3.50/gal that is $1,680.
What efficiency changes are worth
| Efficiency | Fuel for 15,000 km | Cost @ 100/L |
|---|---|---|
| 12 km/L | 1,250 L | 125,000 |
| 15 km/L | 1,000 L | 100,000 |
| 18 km/L | 833 L | 83,333 |
The jump from 12 to 15 saves 25,000 a year; 15 to 18 saves 16,667. Efficiency improvements have diminishing returns — which is why the biggest lever is usually total distance driven, not the last km/L.
Where real-world numbers drift
Manufacturer figures are lab-cycle numbers; city driving, short trips, roof racks and cold starts commonly cost 10–25% vs the brochure. Measure your own consumption over two or three full tanks (fill to the same point, note litres and km), then use your figure in the Fuel Cost Calculator — and the efficiency converter if you need to move between km/L, mpg and L/100km.
Try the calculators
Worked example: Fuel Cost Calculator at its default inputs
Take the default setup on Fuel Cost Calculator: no fields changed, nothing tuned to a nice round number. Run in the site's own engine, those values produce the answer below.
| Input | Default |
|---|---|
| Distance (km) | 500 |
| Mileage (km/L) | 15 |
| Fuel Price ($/L) | 1.5 |
| Toll/Parking ($) | 0 |
Result: $50.00
Distance (km): half, as given, and half again
The table below varies distance (km) alone — half, the default, and half again.
| Distance (km) | Result |
|---|---|
| 250 | $25.00 |
| 500 | $50.00 |
| 750 | $75.00 |
Read the three rows as a range: the spread between them is how much the answer depends on distance (km) alone, with everything else held still.
Related guides
Mileage is the input people guess wrong
Most drivers quote the figure from the brochure, not from their own car. Measure it at the pump: fill the tank, reset the trip meter, drive normally, fill again at the same pump, and divide distance by the litres (or gallons) it took to refill. Two or three tanks averaged is enough.
City driving with short trips, air-conditioning and cold starts can be 20-30% worse than the highway number. If most of your driving is a school run, use the city figure or the calculation will understate the cost of every trip.
Price per litre is not the price you pay
The pump price changes weekly and varies by station, so a trip costed at last month's price is already stale. Cost the round trip at the highest price you have seen recently: the difference is a few percent, and it is the direction the error usually runs.
For a recurring commute, calculate the monthly cost rather than the per-trip cost. Monthly fuel is a fixed line in most household budgets even though the underlying maths is variable, and seeing it that way makes carpooling or a route change comparable.
What the fuel number leaves out
Fuel is roughly a third to a half of what a car costs to run. Tolls, parking, servicing, tyres, insurance and depreciation do not appear in a fuel calculator, and on a long commute the depreciation per kilometre can exceed the fuel per kilometre.
That does not make the fuel figure useless — it is the right number for "what will this trip cost me today". It is the wrong number for "can I afford this car", which needs the ownership total.
Idling and load are real costs
An engine idling in traffic burns fuel for zero distance, so a route with fewer stops often beats a shorter route. Air-conditioning adds a measurable load, more so in small engines and in stop-start traffic than on the highway.
Roof racks, full boots and under-inflated tyres all raise consumption. None of them are inputs in a fuel calculator, which is why the tool is a good estimate and a poor substitute for your own three-tank measurement.
Frequently asked questions
How do I calculate the fuel cost of a trip?
Distance ÷ efficiency × fuel price. A 500 km trip at 15 km/L with fuel at 100/L: 500 ÷ 15 = 33.3 L × 100 = 3,333.
How much does driving 15,000 km a year cost in fuel?
At 15 km/L and 100 per litre: 15,000 ÷ 15 × 100 = 100,000 per year. Substitute your own efficiency and fuel price for your real number.
Is L/100km better than km/L?
L/100km is linear: halving consumption halves the figure, which makes savings easier to reason about. km/L grows without bound. Either works if you run the formula consistently.
Why does my real fuel cost exceed the calculation?
Measured consumption is usually worse than brochure figures: idling, short trips, traffic and load all reduce efficiency. Meter your own tanks and use that number.
Last updated