Market sizing is the work of quantifying the demand available in a target market, commonly expressed as Total Addressable Market (TAM), Serviceable Addressable Market (SAM), and Share of Market (SOM) — the realistic portion an offering actually captures. A defensible market size is built bottom-up from explicit, segment-level assumptions — how many customers, what share have the unmet need, what each pays, and how many adopt over what period — with uncertain inputs carried as ranges so the sensitive ones are visible, rather than carved top-down from a big published figure by a chain of unsupported percentages. It bounds the opportunity and disciplines a strategy against wishful 'if we capture 1% of a huge market' reasoning.
Market sizing sits in the Financial Logic dimension of the BRI Strategy Framework and informs the Viability judgment, building up from unit economics and from the target markets and unmet needs on the demand side. In Growth Forge® Software it is a segment-based, bottom-up revenue-forecast tool — uncertain inputs run through Monte Carlo simulation — usable without specialist finance expertise. For the full treatment, see the Market Sizing pillar at /supporting/market-sizing. Related Terminology Index entries: Total Addressable Market; Serviceable Addressable Market; Share of Market; Financial Logic; Target Markets; Unit Economics; Viability.
