Market sizing answers one question: how big is the opportunity? The standard way to show it is three nested circles, each smaller and more real than the last. Get them right and you sound credible. Inflate them and any investor sees through it in seconds.
The three circles
- TAM — Total Addressable Market. The entire revenue opportunity if everyone who could buy your kind of product did. The global ceiling.
- SAM — Serviceable Addressable Market. The slice of TAM you can realistically reach with what you offer today — usually your geography and segment.
- SOM — Serviceable Obtainable Market. The share of SAM you can actually capture given competition and your constraints. Calculate it as a percentage of SAM: roughly 1–10% for large markets, 1–20% for small or niche ones.
TAM is the dream. SAM is the plan. SOM is the promise.
Why founders get it wrong
Two failure modes. The first is the trillion-dollar TAM — “the global wellness market is $1.5T” — which tells an investor nothing about you. The second is skipping straight to a SOM with no working. The credibility is in the chain: a believable TAM, a defensible SAM, and a SOM that’s a stated percentage of it, with your assumptions on show.
The AI shortcut
Sizing used to mean a week of trawling reports. AI collapses the desk research and does the arithmetic, while you supply the judgement and the verification. The discipline that makes it trustworthy: make it show every assumption and tell you exactly what to fact-check, because a number you can’t source is a number you can’t defend.
Help me size my market with TAM / SAM / SOM. My product: [what]. My customer: [who]. My geography to start: [where]. Walk through it step by step: estimate the global TAM (total revenue if everyone who could buy, did), then the SAM (the slice I can realistically serve with what I offer now), then a SOM (1-20% of SAM depending on how niche it is). For every number, state the assumption and the source you'd check, and flag which figures I must verify myself before putting them in a pitch.Never paste an AI’s market figure into a pitch unchecked. Use it to build the structure and find the sources fast — then confirm the headline numbers yourself. A wrong number on a slide costs you the room.
With the size proven, the next decision is how you’ll actually make money from it — your revenue model.
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