Why does Bottom-Up Market Sizing matter?
It is the difference between a number you can defend and one you cannot. A bottom-up estimate exposes its own assumptions: how many buyers exist, what they pay, how often. Each of those is separately checkable, and an investor who disagrees can argue with a specific input rather than dismissing the whole figure. A top-down estimate offers nothing to argue with, which is why it persuades nobody who has seen it before.
What does Bottom-Up Market Sizing look like in practice?
Rather than starting from a market-research figure for the industry, count the entities you would sell to and multiply by what you would charge. If there are roughly 200,000 practices that match your ICP and your plausible annual price is in the low thousands, the arithmetic gives you a defensible order of magnitude and, more usefully, tells you which assumption to go verify first. Usually that is price, because founders tend to know their customer count better than their pricing power.
What are the common mistakes with Bottom-Up Market Sizing?
- Using a price you have never charged anyone. If pricing is unvalidated, say so. The estimate is still useful, but its weakest input should be labeled.
- Counting every entity that exists rather than every entity matching the ICP.
- Stopping at the number. The purpose is to surface which assumption is load-bearing so you can go test it.
- Quietly switching to top-down when bottom-up produces a smaller market than hoped. A smaller, real market is more fundable than a large, invented one.
Related concepts
- TAM, SAM, and SOMThree nested estimates of market size: everyone who could ever buy this kind of product (TAM), the portion you could realistically serve (SAM), and the portion you could plausibly win in the near term (SOM).
- Ideal Customer Profile (ICP)A description of the specific kind of customer your product serves best, precise enough that you can tell whether any given company or person qualifies.