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

Every investor deck has a market-size slide, and most of them collapse under the first serious question. Not because the number is too big or too small, but because nobody in the room can explain where it came from. This article covers what TAM, SAM and SOM actually mean, why the usual way of producing them fails, and what a figure that survives scrutiny looks like.

The three numbers, in plain terms

• TAM, Total Addressable Market: the full revenue opportunity if every possible customer bought your category of solution. It answers “how big is the prize?”
• SAM, Serviceable Addressable Market: the portion of TAM your product, geography and business model can actually reach. It answers “how big is the prize for a company shaped like yours?”
• SOM, Serviceable Obtainable Market: the share of SAM you can realistically win in a defined period, given competition, distribution and capacity. It answers “what will you actually take, and when?”

Investors rarely challenge the definitions. They challenge the path between the three numbers, and that path is where most decks have nothing to show.

Why top-down sizing fails

The classic shortcut is top-down: find one analyst headline (“the global market will reach $80B by 2030”), take a percentage, and call it a plan. It fails for three reasons. First, the headline number usually measures a broader category than the one you sell into. Second, the percentage is arbitrary: “we only need 1%” has ended more conversations than it has started. Third, it produces no intermediate facts anyone can check: one wrong assumption and the whole slide is dead.

Bottom-up sizing inverts the logic: count the customers you can identify, estimate what each is worth, and build the market from its parts. Number of mid-market logistics companies in France × share running legacy WMS × realistic annual contract value. Each factor is a claim someone can verify, which is exactly what makes the total credible.

What a defensible figure looks like

• Every input traces to a source: a filing, a census, a price page, a review count. Never to another slide.
• The segmentation is explicit: which customers are in, which are out, and why.
• Growth is justified by drivers (regulation, technology shifts, buying behaviour), not asserted as a CAGR.
• The SOM comes with a mechanism: which channel, which competitors lose the deal, at what pace.
• Someone reviewed it who was not motivated to make the number bigger.

That last point is the uncomfortable one. Founders are structurally optimistic about their own market. A number that has passed through independent review, where weak inputs were flagged and thin evidence was discarded, is worth more in a boardroom than a bigger number nobody audited.

The test your slide has to pass

Before you present a market size, ask one question: if the sharpest person in the room picks any number on this slide and asks “where does this come from?”, do you have an answer that ends with a source, or with a shrug? Markets are won by teams who knew the terrain before they committed to it. The sizing slide is simply the first proof that you do.

Aeryntis produces source-traced market analyses: TAM, SAM and SOM built bottom-up, checked end to end, delivered in three business days with every source attached. If you would rather defend numbers than defend guesses, submit a brief.