Also called: Top-down TAM, Analyst sizing
TL;DR
Estimating market size from a published total (analyst report, government data) and applying assumed share percentages.
Top-down sizing starts from a published market total and applies share assumptions to arrive at TAM, SAM, and SOM. It's quick, but the published total is usually defined differently from the company's product, and the share assumption is hand-waved.
Top-down is best used as a triangulation against bottoms-up, if the two are within an order of magnitude, the sizing is probably defensible. If they diverge wildly, one of them (usually top-down) is wrong.
Gartner pegs the global CRM market at $80B (2024). The startup claims a 0.5% capture in 5 years = $400M ARR. This is fast to cite, but investors will only accept it if a bottoms-up build corroborates the same order of magnitude.
Top-down is acceptable as a one-line context number on the Market slide; bottoms-up should be the primary calculation.
Calculating market size by counting the actual eligible customers and multiplying by realistic per-customer revenue.
Total Addressable Market, the total revenue opportunity if the product captured 100% of every customer who could conceivably buy it.
Serviceable Addressable Market, the portion of the TAM that the company's product, geography, and channels can realistically serve.
Serviceable Obtainable Market, the realistic share of SAM the company can capture in a defined planning horizon.
A precise definition of the buying organization that gets the most value from your product and is the cheapest to acquire.
Long-form deep dives on the slides Top-Down Market Sizing most often shows up on.
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