TAM, SAM and SOM size a market in three narrowing layers — total demand, the slice you can serve, and the share you can win.
Bottom-up TAM = number of potential customers × average annual revenue per customer; SAM = TAM × share you can serve; SOM = SAM × share you can realistically capture
40,000 UK companies could use your product, each paying about £6,000 a year. You can serve roughly 50% with your current product and realistically win 5% of that in three years.
TAM = 40,000 × £6,000 = £240M; SAM = £240M × 0.50 = £120M; SOM = £120M × 0.05 = £6M
TAM, SAM, and SOM are three nested estimates of market size, each narrower than the last. Total addressable market (TAM) is the total annual revenue if every possible customer on earth bought your product. Serviceable addressable market (SAM) narrows that to the portion you can actually reach given your geography, segment, and product. Serviceable obtainable market (SOM) narrows again to the share you can realistically capture in the near term, given competition and your own capacity.
TAM is usually estimated one of two ways. Top-down starts from an industry analyst figure and carves out your slice; bottom-up — the more credible method for founders — multiplies the number of potential customers by what each would pay per year. A bottom-up TAM built from real pricing and a real customer count is far harder to inflate and far more persuasive to investors than a top-down number lifted from a market report.
The point of the three layers is honesty about scale. A huge TAM proves the opportunity is big enough to matter; a credible SAM and SOM prove you have thought about who you can actually sell to and how much you can win. Investors discount a giant TAM with no path to capturing it, so the SOM — your realistic near-term target — is often the figure that carries the most weight in a fundraise.
TAM, SAM, and SOM frame whether a business is worth building and funding at venture scale, which is why they anchor almost every pitch deck and market-sizing exercise. A credible bottom-up TAM shows the prize is large enough; a thought-through SAM and SOM show you understand who you can actually reach and win. Inflated or top-down-only numbers erode trust fast, so the discipline is to build them from real pricing and a real customer count.
There is no universal target, but venture investors typically want a TAM in the billions and a credible, bottom-up path to a $100M+ (~£80M+) ARR business to back a fund-returning outcome; a bottom-up TAM grounded in real pricing is treated as far more credible than a top-down analyst figure (sources: a16z and Bessemer market-sizing guidance).
TAM is the total market if everyone bought; SAM is the portion you can serve given your geography, segment, and product; SOM is the share you can realistically capture in the near term. Each narrows the one above it, moving from total opportunity to a realistic target.
Multiply the number of potential customers by the average annual revenue each would pay. A bottom-up TAM built from a real customer count and real pricing is far more credible to investors than a top-down figure carved out of an industry report.
A large TAM proves the opportunity exists, but a credible SOM proves you can actually win a meaningful slice of it. Investors discount a giant TAM with no realistic path to capture, so the SOM — read alongside your acquisition economics — often carries the most weight in a raise.
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