> Machine-readable version of https://bharometer.com/guides/cng-mileage-km-per-kg.html — a Bharometer guide (Bharometer: fuel & mileage tracker for India, https://bharometer.com/).

# CNG mileage explained: km per kg, and how it compares with petrol

**The short answer:** CNG is sold by the **kilogram**, not the litre, so CNG mileage is **km/kg**: kilometres driven ÷ kilograms filled, measured full-tank to full-tank exactly like petrol. A typical CNG hatchback delivering 25 km/kg at ₹75/kg costs **₹3 per km** — usually around half the per-km cost of the same car on petrol.

## Why kilograms and not litres

CNG is a gas compressed to around 200 bar. Its volume depends on pressure and temperature, so "a litre of CNG" isn't a fixed quantity of fuel — but a kilogram is a kilogram at any pressure. Dispensers therefore meter by mass, receipts show kilograms, and the only meaningful mileage unit is **kilometres per kilogram**. If an app or spreadsheet forces CNG into litres, its numbers are fiction.

## Calculating km/kg

The method is identical to the [full-tank method for petrol](https://bharometer.com/guides/how-to-calculate-mileage.html): fill to cutoff, note the odometer, drive, fill to cutoff again, divide kilometres by kilograms dispensed at the second fill. One CNG-specific caveat: a CNG "full" depends on the station's dispensing pressure — a low-pressure fill holds less than a true full tank. Filling at the same station, or at stations with healthy pressure, keeps readings consistent; treat an obviously short fill as a partial.

## Comparing CNG with petrol: use ₹/km

Km/kg and km/l cannot be compared directly — different units, different fuels. Convert both to **cost per kilometre**:

- **Petrol:** ₹100/l ÷ 15 km/l ≈ **₹6.7/km**
- **CNG:** ₹75/kg ÷ 25 km/kg = **₹3.0/km**

Same car, same roads — CNG at less than half the running cost. Against that saving sit the cylinder's boot space, slightly reduced power, periodic cylinder testing, and hunting for a working pump on long routes. The ₹/km gap, tracked over months, tells you whether the trade-off is paying for itself — see the [cost-per-km guide](https://bharometer.com/guides/fuel-cost-per-km.html).

## Bi-fuel cars: track two tanks, honestly

Most CNG cars in India are bi-fuel — they start on petrol and can switch any time. That means **two separate fuel systems with two separate mileages**: km/kg for the CNG side, km/l for the petrol side, each with its own full-to-full chain. Adding a petrol fill into a CNG calculation (or vice versa) poisons both numbers, since litres and kilograms can't be summed. A proper log keeps the chains separate and shows each fuel's own ₹/km — plus how much the CNG side saves every month.

## CNG done right, automatically

Bharometer (fuel & mileage tracker for India, iPhone) tracks CNG in kg with true km/kg mileage, supports bi-fuel (CNG + petrol) vehicles as separate tanks, and shows each fuel's cost per km — including how much you're saving versus petrol. https://bharometer.com/ · Free on the App Store: https://apps.apple.com/in/app/bharometer-fuel-kmpl-costs/id6784579909

## Related guides

- [How to calculate your real mileage (km/l)](https://bharometer.com/guides/how-to-calculate-mileage.html)
- [Fuel cost per km: what your vehicle really costs to run](https://bharometer.com/guides/fuel-cost-per-km.html)
- [Kitna deti hai? Finding your true average](https://bharometer.com/guides/kitna-deti-hai-real-average.html)