Total Addressable Market (TAM, SAM, SOM) Explained
TAM, SAM, and SOM answer 'how big is this opportunity, really?' Here's how to calculate each, the two methods that matter, and how to use them in a market-sizing case.
When a founder pitches an investor, or a candidate sizes a market in a case interview, the same question is on the table: how big is this opportunity, really? TAM, SAM, and SOM are the three nested answers — and confusing them is one of the most common mistakes in business analysis.
Here's how to get them right.
The three nested markets
Picture three circles inside each other.
TAM — Total Addressable Market. The total revenue available if you captured 100% of the market for your product or service. It's the whole pie. If you sold a product everyone on earth who could want it actually bought, TAM is what you'd make.
SAM — Serviceable Addressable Market. The slice of TAM you can actually serve given your business model, geography, and product. A meal-kit company's TAM might be "all groceries," but its SAM is "people who buy prepared/convenience food, in the regions we deliver to."
SOM — Serviceable Obtainable Market. The realistic share of SAM you can win in the near term, given competition and your resources. This is the number you'll actually plan around. It's the smallest circle, and the most honest one.
The discipline of the framework is forcing you from the grand number (TAM) to the real one (SOM) through explicit, defensible narrowing.
The two ways to calculate TAM
Top-down. Start from a large published figure (an industry report says "the global X market is $80B") and narrow by relevant percentages. Fast, but fragile — you're trusting someone else's number and your narrowing percentages are often guesses. Investors are skeptical of top-down because it's easy to inflate.
Bottom-up. Build the number from the unit economics:
TAM = (number of potential customers) × (annual revenue per customer)
Example: a B2B software tool priced at $1,200/year, targeting 500,000 small businesses in a region → TAM = 500,000 × $1,200 = $600M.
Bottom-up is almost always more credible because every assumption is visible. If someone challenges your number, you can point to exactly which input to debate. This is the method to default to in interviews and in real planning.
A worked example
Say you're sizing a premium dog-food subscription in the US.
- TAM: All money spent on dog food in the US. ~90M dogs × ~$400/year on food ≈ $36B.
- SAM: Dog owners who buy premium food online and would consider a subscription. Maybe 20% of dogs, at a higher ~$700/year price → 18M × $700 ≈ $12.6B.
- SOM: Realistically obtainable in 3 years given competition and marketing reach — perhaps 1–2% of SAM → $130M–$250M.
Notice how each step is a reasoned narrowing, not a wild guess. That structure is the whole skill.
Practice this framework
Work through the Dropbox 2008: Growth When Ads Don't Work case with AI coaching.
How to use it in a market-sizing case interview
Market sizing is one of the most common case interview components, especially at McKinsey, where it's often embedded inside a larger case. Interviewers are not checking whether you get the "right" number — there isn't one. They're checking whether you can:
- Structure the estimate logically (top-down or bottom-up, stated explicitly).
- Make reasonable assumptions and flag them as assumptions.
- Do clean mental math without getting lost.
- Sanity-check the final number against reality.
A strong approach: state your method, build the estimate step by step out loud, round aggressively to keep the math clean, and end with a sanity check ("$36B — that feels right for a category as big as US dog food"). You can pressure-test your own estimates with our free market sizing tool.
Common mistakes
- Confusing TAM with SOM. Pitching your TAM as if you'll capture it. No one captures their TAM. Investors hear it as naïveté.
- Top-down hand-waving. "It's a $1 trillion market and we just need 1%." The 1% is doing all the work and it's unexamined.
- Forgetting to narrow. A TAM with no SAM/SOM is a vanity number. The narrowing is where the credibility lives.
The bottom line
TAM, SAM, and SOM turn "is this big enough?" into a structured, defensible estimate. Default to bottom-up, make every assumption explicit, and always narrow to the number you'd actually bet on. That discipline is what separates a credible market estimate from a hopeful one.
BoardroomIQ helps you build the quantitative judgment that case interviews and investors reward. Try the market-sizing tool and case library at boardroomiq-ai.com.