Markets and business models
Market sizing: TAM, SAM and SOM
Top-down vs bottom-up market sizing with a worked example, and how to present a number investors will trust.
Investors do not fund "the AI market" or "the future of work." They fund a company that can sell a specific product to a countable set of customers at a price those customers will actually pay. Market sizing turns that claim into arithmetic: how many buyers exist, what they spend per year, and what share you can realistically win.
Done well, it forces clarity on who you sell to and which business model can reach venture scale. Done poorly, it becomes a slide with a "$50B TAM" from a Gartner report and no path to your first ten customers.
Concepts in this section
Read them in order. Each one builds on the one before.
| Step | Concept | What it answers |
|---|---|---|
| 1 | TAM, SAM and SOM | What do the three nested revenue numbers mean? |
| 2 | Bottom-up market sizing | How do you count customers × ACV, then filter? |
| 3 | Presenting market size | What belongs on the slide, and what fails the room? |
A CI analytics tool at $120/engineer/month, sold to ~18,000 mid-size US/Canada eng orgs, is a ~$3.1B TAM. GitHub/GitLab-only coverage cuts that to ~$1.3B SAM. A 900-customer year-5 plan is ~$155M SOM — about 12% of SAM. If early deals actually close at $48k ACV instead of $173k, the whole pyramid shrinks by ~3.6×. That arithmetic, not a pie chart, is the work.
Top-down ("enterprise software is $600B, we take 1%") is a sanity check after bottom-up exists. It is a terrible primary method: categories do not match your product, and 1% of $40B is a public company. If the two methods disagree by 10×, fix the customer definition or the ACV. Do not average them.
A large static market with entrenched incumbents can be worse than a smaller market growing 40%+ per year. When the category does not exist yet, size the adjacent spend you displace and show design-partner ACVs. A $200M TAM with high gross margins and network effects can be fundable; a $5B TAM of low-margin services work may not be. Pair the number with the business model. As an engineer, a feature that expands SAM (new cloud, new ICP, new geography) is a different bet from one that only deepens SOM inside the current wedge.
Common mistakes
- TAM as problem size. "Americans waste $1T on healthcare" is not revenue you can collect.
- Leading with top-down. There is no "how many demos next quarter" inside a Gartner pie chart.
- SOM that ignores the model. If SOM is $80M in year 5 and the financial model is $12M, one of them is fiction.
- Confusing beachhead with TAM. First SAM should be painfully narrow even when TAM is global.
Key takeaways
- TAM / SAM / SOM are nested revenue opportunities: all relevant customers, then those you can serve, then those you can win soon.
- Bottom-up (
customers × ACV) is the default; top-down reports are corroboration, not proof. - Pricing honesty dominates: wrong ACV multiplies into a fake market.
- A credible pitch shows formula, sources, filters and alignment between SOM and the financial plan.
Further learning
Short walkthrough of bottom-up TAM/SAM/SOM and the pitch-deck traps:
Y Combinator LibraryYC's library of startup advice: essays and videos that teach you how to start a company.YC Startup Library
Related
Business models
The main ways software companies make money, and the economics each implies for margin, sales motion and venture scale.
The pitch deck
A short slide narrative that sells the investment — get the meeting, frame the story, show why now and why you.
Startup ideas and why now
Where good ideas come from, which shapes to avoid, and how to judge an idea before you quit your job.