TAM, SAM, SOM: how to size a market investors will actually believe

Every pitch has a market-sizing slide, and most of them get ignored. Not because investors don't care about market size — they care enormously — but because the number on the slide usually can't be defended for thirty seconds. The fix is to show your work.

The three layers

• TAM (Total Addressable Market) — the entire revenue opportunity if you had 100% of everyone who could conceivably buy.
• SAM (Serviceable Addressable Market) — the slice your product and business model can actually serve today.
• SOM (Serviceable Obtainable Market) — the realistic share you can win in a defined timeframe given competition and reach.

Top-down vs. bottom-up

Top-down starts from a big industry figure and narrows it with assumptions. Bottom-up builds from units: number of customers × price × frequency. Do both. When a top-down and a bottom-up estimate land in the same neighborhood, the number becomes credible. When they don't, you've found an assumption worth examining.

Investors don't fund the size of your TAM. They fund whether they believe the assumptions that produced it.

Make assumptions explicit

The single biggest upgrade to any market sizing is to surface the assumptions — pricing, adoption, frequency, reachable segment — as named, checkable inputs, then show how the answer moves when they change. That sensitivity is what turns a number into an argument. It's also exactly how Gevara frames market sizing: TAM, SAM, and SOM with the math shown and the assumptions exposed.