TAM, SAM, SOM: market sizing without kidding yourself

Market sizing exists to answer one question: is this opportunity big enough to be worth building, but specific enough that you can actually win it? The three-layer model — TAM, SAM, SOM — is the standard tool, and it's where founders most often either inflate a fantasy or undersell something real.

The three layers

TAM (Total Addressable Market) — everyone in the world who could conceivably buy a solution to this problem. The whole pie.

SAM (Serviceable Addressable Market) — the slice you can actually reach with your current business model, geography, and channel. You're not selling enterprise software to consumers; SAM narrows TAM to who you can realistically serve.

SOM (Serviceable Obtainable Market) — the portion of SAM you can plausibly capture in the next 18–24 months, given your resources and competition. This is the number that actually matters for planning.

Bottom-up beats top-down

The fantasy version of market sizing is top-down: "the market is $50 billion, and if we get just 1%, that's $500 million." This number is meaningless — "just 1%" hides the entire problem of how you'd reach those customers. Investors have seen it a thousand times and discount it instantly.

Do it bottom-up instead: count the actual customers you can reach, multiply by what they'd pay. "There are roughly 40,000 Shopify merchants in our niche; at $50/month, capturing 5% in two years is 2,000 customers and $1.2M ARR." That's defensible because every number traces to something real.

Small markets are often deep

Don't let a modest TAM scare you off. Niche markets are frequently deeper than they appear — a micro-SaaS serving only "Shopify merchants selling pet food" can be a multi-million-dollar business if the problem is specific and painful enough. A great product for a tiny market is a fine lifestyle business; the question isn't "is the market huge" but "is the obtainable slice enough to fund the business you want to build."