Back of the envelope

A quick calculation from a few remembered quantities, done to establish the size of an answer before any detail.

Estimating an Indian consumer category from 140 crore people, a 30% addressable share and ₹2,000 of annual spend gives about ₹84,000 crore, which is enough to know whether a company's claim is plausible.

The skill is in choosing quantities you actually know and being explicit about each step, so the estimate can be corrected rather than merely disbelieved.

Speed matters less than being able to show the chain.