How to calculate cost per kilometre (and why it's the only car number that matters)

People obsess over price per litre at the pump. It’s the wrong number.

The number that actually predicts what your driving costs is cost per kilometre — and most people have never calculated it for their own car. The good news: you only need two figures from one fill-up.

The formula, in one line

Cost per kilometre = total price paid ÷ kilometres driven since the last fill.

That’s it. If you spent ₹2,500 to top up after driving 480 km, your cost-per-km on that tank was ₹5.21/km. Whether you drive a petrol hatchback or a diesel SUV, whether fuel is ₹95 or ₹105 per litre — the formula is the same.

If you want a one-off check right now, open the free mileage calculator — type the litres, the kilometres and the price, and you’ll see km/L, MPG, L/100km and cost-per-km all at once. No sign-up.

Why “cost per km” beats every other number

Three common alternatives — and why each is misleading:

Price per litre. Tells you about fuel pricing, not about your car. A diesel SUV at 12 km/L on ₹95 diesel costs you ₹7.92/km. A petrol scooter at 50 km/L on ₹105 petrol costs you ₹2.10/km. Same fuel-price headline, completely different reality.
Mileage (km/L or MPG). Better — at least it’s about your car. But you can’t pay anyone in km/L. You pay in money. Two cars with the same mileage but different fuel types (petrol vs diesel) have different cost-per-km.
Monthly fuel bill. Useful as a sanity check, useless for decisions. It depends on how much you drove that month. If your bill drops it could be because you drove less — not because your car got more efficient. Cost-per-km normalises out the distance.

Cost-per-km is the only number that survives a change in fuel price, vehicle, route, or season. That’s why it’s the right one for budgeting, for deciding whether to sell a car, and for comparing whether the bike really is “cheaper” than the auto-rickshaw.

Worked examples

Petrol hatchback in India

Fill-up: 32 L of petrol at ₹105.40/L. Total: ₹3,372.80.
Trip-meter delta since last fill: 512 km.

Mileage = 512 ÷ 32 = 16.0 km/L (about 37.6 MPG, or 6.25 L/100km). Cost per km = ₹3,372.80 ÷ 512 = ₹6.59/km.

Diesel sedan, mixed driving

Fill-up: 45 L of diesel at ₹92.10/L. Total: ₹4,144.50.
Trip-meter delta: 720 km.

Mileage = 720 ÷ 45 = 16.0 km/L (same headline mileage as the petrol car!). Cost per km = ₹4,144.50 ÷ 720 = ₹5.76/km.

Notice how the diesel sedan and the petrol hatchback both report 16 km/L — but the diesel is ₹0.83/km cheaper to drive. That’s the difference km/L hides and cost-per-km reveals.

110cc commuter motorcycle

Fill-up: 6.5 L at ₹105.40/L. Total: ₹685.10.
Trip-meter delta: 295 km.

Mileage = 295 ÷ 6.5 ≈ 45.4 km/L (about 107 MPG, 2.20 L/100km). Cost per km = ₹685.10 ÷ 295 = ₹2.32/km.

Almost a third of the diesel sedan, and a tenth of an Uber. Worth knowing the next time you’re deciding whether to ride the bike or call a cab.

What about “real” cost per km?

The pure-fuel cost-per-km we just computed is the number that changes month-to-month. The true cost-per-km also includes:

Service & maintenance — engine oil, tyres, brakes, chains.
Insurance — annual, but it amortises per kilometre.
Depreciation — the silent killer.
Parking, tolls, registration — sometimes lumpy, but real.

Add the annual total of all the above and divide by the kilometres you drove that year. The result is usually 2× to 3× the fuel-only cost-per-km for cars, and closer to 1.3–1.6× for motorcycles. You don’t have to track it daily; once a year is enough to keep yourself honest.

How FuelNote does this for you

Every fill-up you log in FuelNote feeds two rolling numbers on your home screen:

The 30-day average mileage (in your chosen unit — km/L, MPG or L/100km).
The 30-day cost-per-km (in your chosen currency).

You type litres and the kilometres since your last fill. FuelNote computes both. It works at the pump even when the cellular signal doesn’t. It’s free, no ads, and the math runs on your device — there’s nothing for us to sell back to you.

Or just bookmark the free mileage calculator for the next time you fill up. Either way, stop guessing what your car costs.