Size a market the way investors actually evaluate them.
The triangulation method: build a top-down estimate from a published source, a bottom-up estimate from unit economics, and let the delta tell you whether your assumptions hold. Reconcile live, below.
Top-Down
Start with a published industry total. Filter down with named assumptions.
Bottom-Up
Build from unit economics. Every input auditable to a named source.
The delta tells the story
Run both calculations. If they agree within 30%, your defensible SOM is the lower of the two numbers. If they disagree by more than 30%, one of your assumptions is wrong — find the error before the number goes in front of anyone.
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› Methodology notes
The 30% rule. If top-down and bottom-up SOM numbers disagree by more than 30%, one of your assumptions is materially wrong. Either the top-down source includes categories you should not count, or your bottom-up inputs are too narrow, or both. Find the error before the number goes anywhere.
Why investors prefer bottom-up. Bottom-up numbers reveal how the founder thinks about unit economics and go-to-market. Top-down numbers tell an investor that the founder can cite Gartner. Bottom-up numbers tell an investor that the founder understands the business.
Present the lower number. When top-down and bottom-up agree within 30%, use the lower of the two as your defensible SOM. Explain the delta in a footnote. Credibility comes from conservatism backed by traceable inputs.