Three nested market-sizing numbers. TAM (Total Addressable Market) is the entire global demand for the category. SAM (Serviceable Addressable Market) is the slice you could serve given your channels and geography. SOM (Serviceable Obtainable Market) is what you could realistically capture in 1-3 years. Most founders inflate TAM and skip SOM. The honest move is the opposite: start at SOM, work up. The ten-customer version of your business is more useful than the $10B TAM slide nobody believes.
The fast version
Three nested market-sizing numbers. From outside in:
| What it is | How to compute | |
|---|---|---|
| TAM | Total Addressable Market: total global demand for the category | Industry reports, public market data |
| SAM | Serviceable Addressable Market: the slice you could serve | TAM filtered by channel + geography reach |
| SOM | Serviceable Obtainable Market: what you can realistically capture in 1-3 years | Bottom-up: buyer count × conversion × price |
Most founders do this in the wrong order. The pitch-deck order (TAM → SAM → SOM) inflates the numbers and produces a SOM that’s still wishful thinking. The honest order is the reverse: start at SOM, work up.
The bottom-up SOM math
For a SaaS targeting a specific buyer:
- Count specific buyers that match your defined persona AND are reachable through your year-1 channels.
- Example: “B2B SaaS founders running pre-seed startups in Europe who’ve raised but haven’t shipped” → ~2,000 people (Crunchbase + LinkedIn Sales Navigator search).
- Apply a realistic conversion rate.
- Cold outbound: 1-3%
- Warm-contact (you know them or their network): 5-15%
- PLG with strong virality: 10-25%
- Multiply by your price.
- Annual revenue from each customer = price × 12 (if monthly).
- The product is your year-1 SOM revenue.
In the example: 2,000 people × 5% conversion × $228/year = $22,800 year-1 SOM revenue. That’s the floor. If the floor doesn’t clear your minimum viable business, the buyer segment is too small or the price is too low.
Why founders inflate TAM
Three failure modes:
1. The “1% of a $10B market” calculation. Treats market capture as if you could just dial in a slider. You can’t. 1% of a $10B market is $100M, which sounds reasonable; the path to capture even 0.001% of most $10B markets typically takes 5-10 years and dozens of millions in capital.
2. Including buyers you can’t reach. TAM counts buyers in markets you have no channel into. If you’re an indie hacker in London, your effective SAM is bounded by the channels you have today (Twitter, LinkedIn, niche communities). Including the entire German enterprise IT spend in your TAM is a fiction.
3. Mixing categories. “We’re project management + CRM + invoicing” isn’t a market; it’s three markets you’d need three GTM motions for. TAM math that adds them together overstates your real opportunity.
What to use TAM/SAM/SOM for
| Number | Useful for |
|---|---|
| TAM | Convincing an investor the category is big enough to be venture-scale |
| SAM | Showing your channel + geography focus is intentional |
| SOM | Running your business; deciding whether the year-1 numbers add up |
If you’re not raising, you mostly only need SOM. If you are raising, all three; but compute SOM honestly first and walk back to SAM/TAM from there.
How ShipFit handles this
ShipFit’s stage 1 (Worth Building?) computes SOM honestly from your buyer persona inputs. The system forces you to start small (specific reachable buyer count) and only computes SAM/TAM as context after SOM is locked. If your SOM doesn’t clear the minimum viable business threshold, ShipFit suggests pivots (different buyer segment, different price) before recommending you walk away.
Further reading
- Pre-launch market research spoke, the full bottom-up SOM workflow.
- The 9-step playbook, where market sizing fits in the wider validation flow.
- How to validate a business idea, the question this market math feeds.
Related
The Lean Startup
Validated learning, the build-measure-learn loop, what an MVP actually is, the three engines of growth, and the ten pivots Ries names rather than one.
Market Research
Most founder market research is a TAM slide that nobody believes. The numbers that actually matter are smaller, harder to defend, and tell you whether the market exists for the ten-customer version of your business.
Product-Market Fit
The state in which a product satisfies a strong market demand such that demand pulls the product through the company. Coined by Marc Andreessen in 2007. Most rigorous measure: 40%+ of active users would be 'very disappointed' to lose the product (Sean Ellis test).
Frequently asked questions
How do I calculate SOM honestly?
Why is TAM the least useful number?
What's a 'good' SOM for a pre-revenue startup?
Should I use top-down or bottom-up market sizing?
Keep exploring
The 9-step playbook from market verdict to ship-ready spec.
Validated learning, the build-measure-learn loop, what an MVP actually is, the three engines of growth, and the ten pivots Ries names rather than one.
Four survey questions, four cumulative curves, four intersections. How to run the Van Westendorp price sensitivity meter, plot it, and read the price range.
Most founder market research is a TAM slide that nobody believes. The numbers that actually matter are smaller, harder to defend, and tell you whether the market exists for the ten-customer version of your business.
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Use Rahul Vohra's Superhuman PMF Engine. (1) Survey active users with the Sean Ellis question 'how would you feel if you could no longer use this?'. (2) Segment respondents by 'very disappointed' / 'somewhat disappointed' / 'not disappointed'. (3) Profile your fans (the very disappointed) to find your real ICP. (4) Build a roadmap that's half doubling-down on what fans love and half closing the on-the-fence blockers. (5) Re-run quarterly. The score should rise.
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