A city gas distributor grows volumes 9% a year at a Rs 9 per scm margin. Input gas costs rise Rs 5 per scm and it can pass on only 60% this year. What happens to its margin and to the stock pitch?

The margin falls from Rs 9 to Rs 7 per scm, and EBITDA falls about 15% despite 9% volume growth. Volume would need to grow 29% to make up the gap. The pitch survives only if the Rs 2 is a lag that is recovered next year; if CNG's price advantage over petrol caps pricing for good, the stock was valued on a margin it no longer earns.

A tea stall whose milk costs Rs 5 more a litre, but which dares raise its price by only Rs 3 because the next stall is cheaper, keeps selling the same cups at a thinner margin. A gas distributor's margin is the gap between what it charges and what it pays for gas, so a cost rise it cannot fully pass on comes straight out of that gap. Rs 9 less the Rs 5 rise plus the Rs 3 it passes on leaves Rs 7 per scm, a 22% cut.

No, and the arithmetic shows why. EBITDA is volume times margin, so a 22% margin cut needs 29% more volume just to stand still. At 9% growth, volume reaches 3,270 million scm and EBITDA is 3,270 x Rs 7, about Rs 2,289 crore, against Rs 2,700 crore last year. The enterprise value moves from 11.9x EBITDA to 14.0x on this year's number without the share price changing at all.

It changes the question the pitch must answer. The long case was a volume story; this year shows that a gas distributor's good and bad years are decided by how quickly it passes on costs, not by volume. If the Rs 2 is a lag, recovered as prices catch up next year, EBITDA rebounds to about Rs 3,208 crore and today's dip is a chance to add. If the cap is structural, because CNG must stay cheaper than petrol to keep conversions coming, the margin may settle near Rs 7, and a stock valued on Rs 9 is overvalued at the same price.

So the update to the portfolio manager is short: the volume thesis is intact, the margin thesis is now the swing factor, and the evidence to watch is the price gap between CNG and petrol and management's next two price revisions. Say what would make you exit: a second cost rise absorbed the same way would suggest the cap is permanent. The limit of this analysis is that it treats the whole margin as one number; industrial and household tariffs often reprice on different schedules, so split them if the data allow.