"How big is this market" is really three questions stacked inside one phrase, and mixing them up is a classic novice tell. TAM (Total Addressable Market) is the entire category, nationally or globally, regardless of who could actually serve it. It's the whole pie, sized with the techniques in market sizing methods. SAM (Serviceable Addressable Market) is the slice a specific business model could realistically reach, filtered by the geography the company actually operates in, the segment its product fits, the channel it can sell through. SOM (Serviceable Obtainable Market) is the slice of SAM the company could realistically win, this year or over a stated horizon, given its actual capacity, brand, and distribution. That last number is bounded far more by the company's own capability than by the size of the market.
Why does it matter which number you quote?
Why does this matter for a guesstimate? Because TAM is the number every market sizing technique builds. SAM and SOM are business-specific filters applied afterward. Quoting TAM alone as the answer to "what could we sell" is a separate error from a bad sizing build. You'd be answering an easier question than the one you were asked. Worked Example 1 sizes the Indian room-air-conditioner category as a whole: TAM, roughly ₹10,000–10,500 crore of annual flow. Now picture a specific mid-premium split-AC brand selling only through organized urban retail. It would apply a SAM filter first. Split-AC is 65% of TAM, and 80% of split-AC volume moves through organized urban channels the brand can reach, so SAM comes out near ₹5,200–5,460 crore. Then comes a SOM filter, bounded by the brand's own realistic obtainable share this year, and 5% of SAM lands near ₹260–273 crore. That's three different numbers, each correctly derived, each answering a different question. When a prompt doesn't say which one it wants, ask. When you're building for your own purposes, say which one you're building and why, every time.
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How do you get from TAM to SAM and SOM?
Watch the direction of travel. TAM is built with the market-sizing techniques. SAM and SOM come from applying the company's own reach and capability as successive filters on a TAM you've already defended. Get the TAM wrong and every filtered number downstream inherits the error, multiplied by whatever filters you apply. That's one more reason the base build deserves full triangulation before anything gets filtered on top of it.