What they are
Three nested market-size numbers, from broadest to narrowest:
- TAM (Total Addressable Market). The whole pie. Everyone in the world who could ever buy a product like yours. A ceiling, not a target.
- SAM (Serviceable Addressable Market). The slice of TAM you could realistically reach with your product, channels, language, and business model.
- SOM (Serviceable Obtainable Market). The chunk of SAM you can actually win in the next one to three years. This is the number that decides whether to build.
TAM ≥ SAM ≥ SOM, always. The relationship looks roughly like:
TAM (global, 10-50× SAM) → SAM (reachable with current model, 5-20× SOM) → SOM (winnable in 1-3 years).
Why SOM matters most
Founders default to leading with TAM (“the market is $200B”) because the number is big and impressive. Investors learned to ignore the TAM line a decade ago. They look at SOM.
SOM is the number that decides whether the business sustains itself. If your ten-customer or hundred-customer version of the business cannot exist, no TAM number rescues it. Build SOM first, work up.
Top-down vs bottom-up
Two common approaches. One is honest, the other is theater.
| Approach | What it does | What it produces |
|---|---|---|
| Top-down | Start with an analyst’s TAM number, multiply by an assumed market share | A defensible-looking number that means nothing operationally |
| Bottom-up | Count specific buyers, estimate reach, multiply by price | A defensible SOM that you can actually go after |
Bottom-up is harder and the number comes out smaller. That’s the point. The smaller, harder number is the one you can sell to.
Worked example (bottom-up)
A solo founder builds a tool that helps early-stage B2B SaaS founders in Europe validate ideas before they raise.
SOM (year 1-2):
- Crunchbase + LinkedIn: ~2,000 pre-seed B2B SaaS founders in Europe right now.
- Realistic reach with content + community + 1 paid channel: 10% in year 1 = 200 founders see the product.
- Realistic conversion from awareness to paying customer: 5%. That’s 10 paying customers.
- Average annual contract: $300/year.
- SOM revenue = $3,000 in year 1.
That number tells the founder this is a side project, not a venture business, unless something changes (price, geography, or product).
SAM (3-5 years):
- Same buyer profile expanded to all of North America + Europe + Asia: ~30,000 buyers.
- Annual contract increased to $500 once the product has more value: 30,000 × 5% × $500 = $750,000.
- SAM ≈ $750K-$1.5M annual.
TAM (long-term ceiling):
- All founders globally who validate ideas pre-launch (broaden buyer to include indie hackers, students, MBA founders): ~500,000.
- At $500 average: $250M ceiling.
- TAM ≈ $250M. Big enough for a business, not big enough for a venture-scale outcome.
The honest TAM ($250M) is two orders of magnitude smaller than the typical pitch deck TAM ($25B+), but it’s the one the founder can actually defend.
What they are NOT
| What TAM/SAM/SOM actually is | What people make them into |
|---|---|
| Buyer counts × realistic price × realistic reach | ”$200B market × 1% capture = $2B opportunity” |
| Bottom-up, defensible | Top-down, copied from a Gartner slide |
| Built around a specific ICP | Vague “the market is everyone” framings |
| Updated as the buyer or product evolves | A one-time deck slide |
A 2018 case study from a16z: most failed seed-stage decks had TAMs above $10B. Most successful ones had defensible SOMs of $5-50M and a clear path to expansion. The big number didn’t help; the small, real one did.
Common mistakes
1. Leading with TAM. A $200B TAM impresses no one in 2026. Lead with SOM. Earn the right to talk about TAM.
2. Top-down math. “1% of the $200B market.” 1% capture is a fantasy unless you’re already a category leader. Use bottom-up buyer counts.
3. Ignoring substitutes. Spreadsheets, “do nothing,” and free alternatives are part of the market. A 40,000-buyer SAM where 95% will keep using spreadsheets is a 2,000-buyer SOM.
4. SOM with no reach plan. “We’ll win 10% of the SOM” is meaningless without naming the channels that get you there. Tie SOM to a real GTM motion.
Further reading
- a16z, How Big is the Market. The investor view on why bottom-up beats top-down.
- Market Research spoke. The full validation flow that produces an honest SOM.
- TAM/SAM/SOM calculator. Plug in your numbers and see what your SOM actually supports.
Related
Keep exploring
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Run nine framework-backed decisions in order before writing code: define the buyer, prove the pain is painful, name the winning angle, scope V1 to the smallest test of the hypothesis, get behavioral evidence (paid pre-orders, signed letters of intent, or credit cards on file from a Fake Door Test), then ship. Most failed startups skipped at least three of those nine. Plan to spend two to four weeks on this. It saves six to nine months of building the wrong thing.
For indie hackers who've wasted months on dead ideas. ShipFit forces 9 decisions before you write a line of code. Proven frameworks, exports to Cursor.
If you want a conversation partner, Buildpad. If you want to stop researching and ship, ShipFit. Both solve different problems for different founders. Don't pick on hype.
Ready to make your next product a success?
9 decisions between your idea and a product worth building.