The three numbers
- TAM (Total Addressable Market): the total revenue if every possible customer used your product.
- SAM (Serviceable Addressable Market): the slice TAM you can actually reach with your current product, geography, and channels.
- SOM (Serviceable Obtainable Market): the slice of SAM you can realistically capture in 3 years.
The trap is that founders almost always inflate TAM, ignore SAM, and pretend SOM is whatever they want it to be. The fix is to invert the order and start from SOM.
Why top-down sizing usually lies
"It's a $50B market and we just need 1%" is a red flag because it tells you nothing about which customers, what they pay, or how you reach them. The same logic justified hundreds of failed startups in every category. Pattern-matching investors hear "1% of a giant market" and immediately discount the deck.
A better bottom-up method
- Identify a specific customer profile (industry, company size, role).
- Estimate how many of them exist (LinkedIn filters, public databases, association reports).
- Multiply by the realistic ARPU based on competitor pricing.
That product is your SAM, and it is honest. SOM is then a discount on SAM based on channel realism, usually 1–10% of SAM in years 1–3 for a focused early-stage company, sometimes less.
Worked example
Imagine a tool for independent veterinary clinics in the United States. There are ~30,000 such clinics. Comparable practice-management add-ons sell for ~$120/month, or $1,440/year. Your SAM is 30,000 × $1,440 = ~$43M ARR. If you realistically expect to reach 5% of clinics in 3 years, your SOM is ~$2.2M ARR. That is a credible bottom-up story, small enough to be honest, large enough to be a real business.
When TAM matters
TAM matters mostly to investors who need to believe a 50× return is possible. For a bootstrapper, SAM and SOM are the only numbers that should drive decisions. If you are building for early profitability, you can ignore TAM entirely and report SAM × realistic capture rate.
How to source your numbers
The least-bad sources for sizing are: Statista (good headline, expensive deep), industry association reports (free, often outdated), Crunchbase competitor revenue, and direct LinkedIn counts of your buyer persona. Stitching two or three sources together with explicit assumptions is more credible than a single big number from a research firm.
Common sizing mistakes
- Using TAM as if it is reachable revenue.
- Mixing global and regional numbers in the same paragraph.
- Forgetting to multiply by paid penetration rate (most "users" never pay).
- Citing a market study without checking the methodology: many cited numbers are vendor-funded.
- Pretending bottom-up rigor is unnecessary because "the market is obvious."
Key takeaways
- TAM is the marketing number, SAM is the operating number, SOM is the planning number.
- Top-down sizing ("1% of a $50B market") is a red flag; bottom-up sizing is the credible standard.
- A SAM of ~$50M and a SOM of ~$2M ARR is a perfectly fundable story for many SaaS categories.
- For bootstrappers, SAM × realistic capture rate is the only number that matters.
- Cite every source with its methodology: single numbers without methodology are noise.
Related reading
Pair sizing with How Vibe Ideas scores startup ideas and Finding your first 100 customers for distribution realism. The Runway Calculator helps you turn SOM into a 12-month plan.
A common mistake is treating the TAM as the goal. Almost no company captures more than 5% of even a narrowly defined market. The useful number is the SOM - what can you realistically reach in 18-24 months given your sales motion, your geography, and your team size. A $500M TAM with a realistic $2M SOM in year one is a fine foundation for a bootstrapped business. A $50B TAM with a $200M SOM in year one needs venture capital and a large sales organisation. Know which type of business you are building before you build your financial model. Use the Runway Calculator to work backwards from your SOM to your monthly target.
Breaking your market size down this way also helps you prioritise. Start with the SOM, win it completely, and use that dominance to expand into the SAM over the following years.