Market Sizing

The top-down approach starts with a large, known number (like total population or GDP) and applies successive filters to narrow down to the target market. For example, estimating the U.S. market for dog food: start with 330M Americans, assume 45% of households own dogs, average 2.5 people per household, giving ~60M dog-owning households. Multiply by average annual dog food spend ($500) to get $30B. The bottom-up approach builds from the smallest unit: one customer's purchase behavior multiplied by the number of customers. Both methods should produce estimates in the same order of magnitude. Top-down is faster but less precise; bottom-up is more defensible because it's grounded in observable unit economics. The best analysts use both and triangulate.

TAM (Total Addressable Market) is the total revenue opportunity if a product achieved 100% market share with no constraints — the entire global or national market for the product category. SAM (Serviceable Addressable Market) is the portion of TAM that a company can realistically target given its business model, geography, and capabilities. SOM (Serviceable Obtainable Market) is the realistic near-term revenue a company can capture, accounting for competition, go-to-market capability, and execution constraints. Venture capitalists and investors evaluate startups using this hierarchy: the TAM must be large enough (typically $1B+) to support a venture-scale outcome, the SAM must be reachable, and the SOM must be credible. These market sizing concepts often intersect with revenue recognition when translating market opportunity into financial projections.

In consulting interviews, market sizing questions test your ability to structure ambiguous problems, make reasonable assumptions, and do mental math under pressure. Common formats include 'How many gas stations are in the U.S.?' or 'What is the market size for electric scooters in New York City?' The interviewer evaluates your approach (is it logical and MECE?), your assumptions (are they reasonable and clearly stated?), and your math (is it quick and accurate?). A strong answer segments the market into distinct groups, assigns reasonable numbers to each, and builds to a total. Always state your assumptions explicitly, round numbers for easier mental math, and sanity-check your final answer against known benchmarks. Understanding enterprise value and industry multiples can help you cross-reference your market size estimate with real company valuations.

The most common mistake is conflating TAM with realistic market opportunity — claiming a startup can capture a $100B market when it realistically addresses a $500M niche. Other pitfalls include using overly broad or narrow starting assumptions, forgetting key segments (e.g., sizing the consumer market but ignoring B2B), and failing to sanity-check. A good sanity check compares your estimate to known data points: if you estimate 200,000 restaurants in the U.S. but the actual number is about 1 million, your approach has a structural flaw. Another common error is not accounting for willingness to pay versus theoretical need — just because 50M people could use a product doesn't mean they'll pay for it.