Three nested numbers: the biggest possible market, the slice you can plausibly serve, and the share you can credibly capture in three years.
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TAM, SAM, SOM
3 blocks · 3-row grid
"If your TAM is a trillion dollars, your SOM had better not be too."
What it is
Three nested sets. TAM is the total revenue opportunity if every potential buyer in the world bought your product. SAM is the subset of TAM you can actually serve with your current model, channel, language, and regulatory reach. SOM is the realistic share of SAM you can capture in a defined time window, given the team and capital you have.
Origin
Market sizing as a discipline grew out of corporate-development practice — the math venture investors and corporate boards used to evaluate whether a new bet was worth the capital. The three-layer TAM/SAM/SOM frame became the default because it forces a clear split between the size of the opportunity, the slice you serve today, and the share you can credibly claim by a given date.
When to reach for it
You're sizing a market for a fundraising deck or board memo.
You're evaluating whether a new product line or geography is worth the capital.
You're setting a credible three-year revenue plan and need to defend the number.
The blocks
Worked example uses Airbnb.
What good looks like
The whole pond. Compute it bottom-up: units × ARPU × segments. Cite your sources and your assumptions inline.
Example — Airbnb
~$1.4T total global travel-and-tourism spend in a given year — all lodging, flights, experiences, and ground travel worldwide.
What good looks like
The portion you can actually serve today. Filter TAM by geography, language, channel, regulation, and product fit — name each filter explicitly.
Example — Airbnb
~$200B short-term lodging spend in markets Airbnb operates in today: major-language countries, supported payment geographies, and cities without an outright short-term-rental ban.
What good looks like
A time-boxed share of SAM you can credibly capture. Tie it to the team, channel, and capital you actually have — not to the team you wish you had.
Example — Airbnb
~$80B in nights booked through marketplace platforms by 2027. At ~40% projected platform share, Airbnb captures ~$32B GBV → roughly $5–6B in net revenue.
How to use it
Compute TAM bottom-up when you can — units × ARPU × number of segments. Top-down "industry size" numbers are easy to inflate and harder to defend.
Cut TAM down to SAM by the constraints you actually live with: geography, language, regulation, sales motion, and product fit.
Set SOM to a credible market share by year 3. SOMs above 10% of SAM read as wishful thinking unless you can name the unfair advantage that earns them.
Show your assumptions. Investors discount a number with no math; they reward a number with a transparent worksheet.
Common mistakes
Citing only TAM ("we're going after a $1 trillion market") with no SAM or SOM behind it — every investor in the room will discount the number to zero.
Confusing GMV with revenue. For a marketplace, the take rate is the difference between a credible number and a fictional one.
Setting SOM as a flat share of TAM rather than tying it to a real go-to-market plan with a sales motion, channel, and headcount.
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