Car & Vehicle

CNG vs Petrol Calculator

Compare running on petrol with running on CNG, and see how far it takes to recover the cost of the kit.

8 inputs Free, no sign-up

Your figures

What the car returns on petrol.

Kilometres per kilogram, from the car's own display or your own measurement.

A retrofit kit, or the difference in on-road price between the petrol and factory CNG versions. Leave blank if there is none.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

CNG is priced per kilogram and petrol per litre, so the only fair comparison is per kilometre. That is what this page does: what each fuel costs to cover one kilometre, over the distance you actually drive, and how long the kit takes to pay for itself.

No price and no economy figure is stored here. CNG rates differ sharply between cities and change often, and what a particular car returns on gas is something its owner measures.

How to use this calculator

  1. Enter your yearly distance. High mileage is what makes CNG worthwhile, so this matters more here than almost anywhere else on the site.
  2. Enter the petrol mileage and the petrol price.
  3. Enter the CNG figure in kilometres per kilogram, and the price per kilogram.
  4. Enter the cost of the kit, or the price difference if you are buying a factory CNG car.

The formula

Petrol per km = Price per litre / km per litre
CNG per km    = Price per kg / km per kg
Yearly cost   = Cost per km x Distance
Break-even km = Kit cost / (Petrol per km - CNG per km)

Kilograms and litres never meet in this arithmetic. Each fuel is reduced to rupees per kilometre on its own terms, and only then compared — which is why the page does not offer to convert km/kg into mpg.

SymbolMeaningUnit
Petrol mileage Kilometres per litre km/L
CNG mileage Kilometres per kilogram km/kg
Kit cost Retrofit or price gap ₹

Worked example

20,000 km a year; petrol 15 km/L at ₹105, CNG 25 km/kg at ₹76; kit ₹70,000:

Petrol = 105 / 15 = 7.00 per km -> 1,40,000 a year
CNG    =  76 / 25 = 3.04 per km ->   60,800 a year
Saving = 79,200 a year
Break-even = 70,000 / 3.96 = 17,677 km

Under a year at that distance. At 6,000 km a year it would be nearly three.

Notes and limits

  • A CNG cylinder takes most of the boot. On a small hatchback that is a real cost that does not appear in any of these numbers.
  • Filling is slower than petrol and stations are far fewer, so the practical cost includes queueing. In some cities that is the deciding factor rather than the arithmetic.
  • A retrofit affects the warranty on most vehicles and has to be endorsed on the registration certificate and the insurance. Check both before committing.
  • Performance on gas is noticeably down on petrol, and the engine reverts to petrol when the cylinder empties — so the petrol tank does not become redundant.
  • CNG prices have moved a great deal in recent years. Re-run this when they do.

What this calculation assumes

  • Fuel only. Boot space, refuelling time, warranty, servicing and resale are not compared.
  • Every figure is one you entered. No CNG price or km/kg figure is stored in this page.
  • The comparison assumes the vehicle runs on CNG for the whole distance.

Frequently asked questions

Is CNG cheaper than petrol?

Per kilometre it is usually much cheaper, because a kilogram of CNG costs less than a litre of petrol and takes the car further. Whether that repays the cost of the kit depends on how far you drive, which is what this page works out.

How do I compare km/kg with km/L?

You do not compare them directly - a kilogram of gas and a litre of liquid are different quantities. Convert each into rupees per kilometre using its own price, and compare those.

Is a CNG kit worth it?

It depends entirely on your distance. At 20,000 km a year a kit commonly pays back inside a year; at 5,000 km it may take four or five, by which time other considerations matter more. Enter your own figures rather than trusting a general claim.

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