Conduct a TAM/SAM/SOM re-calibration

TAM Theater: Why Most Market Size Numbers Collapse Under Scrutiny

Top-down TAM numbers sourced from analyst reports are nearly always wrong for a specific product — they describe the category, not the serviceable slice. Paul Graham's essay on startup ideas treats TAM inflation as a common fundraising reflex that backfires at the Series B when growth stalls against a number nobody can reconcile. Bottom-up TAM — eligible units × units per customer × ACV — forces every input to be evidence-backed and defensible. Stripe's own press content and Andreessen Horowitz's perspective on AI markets both argue that credible sizing starts with unit economics, not analyst estimates.

How the Conduct a TAM/SAM/SOM re-calibration Prompt Works

The prompt walks through each layer — TAM, SAM, SOM — with a mandatory citation per exclusion. The sanity check step compares SOM against current ARR and competitor revenue to catch the most common errors: SOM smaller than existing competitor revenue, or a 5-year plan larger than SOM.

When to Use It

• Growth is stalling and the board is asking why against a big TAM.
• Fundraising is starting and the old TAM slide will not survive due diligence.
• A new segment pivot needs a freshly sized TAM.
• Sales is setting targets off numbers that do not reconcile.
• A new PM is joining and wants to understand the addressable opportunity.

Common Pitfalls

• Top-down sourcing. Quoting "$50B category" tells a board nothing about how much of that you can capture. Bottom-up or bust.
• Missing exclusions. If your SAM equals your TAM, you have not done the work. Real products exclude geographies, segments, and use cases.
• SOM bigger than plausible. Claiming 30% of SOM in 3 years is fantasy unless you have category-leader GTM. Triangulate against competitor revenue.

Sources

• How to Do Great Work — Paul Graham
• How to Get Startup Ideas — Paul Graham
• The Product Strategy Stack — Reforge
• Y Combinator Library — Y Combinator