Also called: Bottom-up market sizing, Bottom-up TAM
TL;DR
Calculating market size by counting the actual eligible customers and multiplying by realistic per-customer revenue.
Bottoms-up sizing builds the TAM from observable reality: how many companies fit the ICP, what each could plausibly pay, and what the time horizon looks like. The result is grounded and defensible, you can argue with the inputs but not the structure.
It's the preferred method for institutional investors because the assumptions are visible and the company's operating plan flows from the same numbers. Top-down sizing, by contrast, is mostly used as a sanity check.
A construction SaaS: 730k US construction firms (Census) × 60% addressable size band × 20% target persona density × $4,800 avg ACV = $420M bottoms-up TAM, defensible because every assumption maps to a citable source.
The Market slide presents a bottoms-up calculation with the eligible-customer count and the assumed ACV both visible.
Estimating market size from a published total (analyst report, government data) and applying assumed share percentages.
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.
A precise definition of the buying organization that gets the most value from your product and is the cheapest to acquire.
Annual Contract Value, the recurring revenue value of a single customer contract on a per-year basis, a standard B2B SaaS deal-size metric.
Long-form deep dives on the slides Bottoms-Up Market Sizing most often shows up on.
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