To size your market, work out three numbers: TAM (everyone who could buy the category), SAM (the slice your product and geography can actually serve), and SOM (the share you can realistically win in the near term). The honest one investors care about isn't the giant TAM — it's a credible, defensible SOM.
TAM, or Total Addressable Market, is the whole pie — total annual revenue if every possible buyer in the category bought. SAM, the Serviceable Addressable Market, narrows that to who your specific product and geography can serve. SOM, the Serviceable Obtainable Market, is the realistic share you can capture near-term, given competition and your reach.
Think of it as three concentric circles shrinking from "everyone who could" to "who we can serve" to "who we'll actually win first." Each one is more honest — and more useful — than the last.
Start top-down for TAM: find the category's total annual spend from industry reports or a simple build-up (number of potential buyers × average annual spend). For SAM, multiply by the realistic percentage your product and geography can serve — strip out segments you can't reach or sell to. For SOM, multiply SAM by the share you can plausibly win in the next year or two, grounded in your actual channels and competition.
Sanity-check top-down against bottom-up: how many customers can you realistically acquire per month through your real channels, at what price? If your bottom-up SOM and your top-down SOM disagree wildly, trust the bottom-up — it's tied to reality.
A huge TAM is easy to inflate and impresses no one who's paying attention. "It's a $50 billion market" tells an investor nothing about whether you can win — it's often a warning sign that the founder hasn't done the harder, narrower thinking. A credible SOM shows you understand exactly who you'll serve first and how.
The most fundable framing is a believable path: here's the realistic slice we can win in 24 months, here's how, and here's how it grows. Specific and defensible beats giant and vague every time.
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TAM is everyone who could buy the category, SAM is the slice your product and geography can actually serve, and SOM is the share you can realistically win near-term. They're three shrinking circles — from "everyone who could" to "who we'll actually win first."
Start with the category's total annual spend (TAM), multiply by the percentage your product and geography can serve (SAM), then by the share you can realistically capture near-term (SOM). Cross-check the result against a bottom-up estimate of how many customers you can actually acquire through your real channels.
Not on its own — a giant TAM is easy to inflate and often a red flag. Investors care more about a credible, defensible SOM: proof you know exactly who you'll serve first and have a realistic path to winning them.
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