Every investor has seen the slide: "The market is worth $80 billion. If we capture just 1%, we're a unicorn." And every investor has learned to ignore it, because a top-down fantasy number says nothing about whether this company can win any of it.
Market sizing done honestly is one of the most useful exercises in validation — not because the number impresses anyone, but because the process forces you to define exactly who your customer is and how you reach them. Here is how to do it properly.
The three numbers, defined
- TAM — Total Addressable Market. Total demand for the category if you had 100% share, no constraints. It answers: is this pond big enough to matter?
- SAM — Serviceable Addressable Market. The slice your actual product, business model, language and geography can serve today. Not everyone with the problem — everyone you can plausibly sell to.
- SOM — Serviceable Obtainable Market. What you can realistically win in 3–5 years given competition, your channels and your budget. This is the number serious investors interrogate, because it is a claim about execution, not about the industry.
Top-down vs bottom-up
There are two ways to produce these numbers, and they are not equally credible.
Top-down starts from an analyst headline ("the global CRM market is $70B") and slices downward with assumed percentages. It is fast, and it is how most bad market slides get made — every assumption is someone else's, and the percentages are guesses dressed as math.
Bottom-up starts from your unit economics and builds upward:
- Count your potential customers (real segments, not "everyone with a phone")