Market sizing is often where pitches collapse. VCs want to see that you understand TAM, SAM, and SOM and can defend each with real logic — not an analyst report headline.

TAM — Total Addressable Market

The total revenue opportunity if your product captured 100% of the market globally.

• Calculated as: Total target customer count × average revenue per customer.
• Used to show the upper bound of ambition.

SAM — Serviceable Addressable Market

The portion of TAM reachable given product, geography, language, regulatory constraints, and GTM.

• Calculated as: TAM filtered by your realistic serving constraints.
• Examples: SMB vs enterprise, US-only initially, English-only.

SOM — Serviceable Obtainable Market

The share of SAM you can realistically capture in the near term.

• Calculated as: SAM × realistic market share (often 1–10% in a meaningful timeframe).
• Used to show the near-term revenue ceiling at current GTM.

Top-down vs bottom-up

Top-down (discouraged):

• “IDC says the market is $200B. We’ll take 1%.”
• Weak because IDC numbers rarely map to your monetization.

Bottom-up (preferred):

• “There are 50,000 mid-market manufacturers in North America.”
• “Our ACV is $50K/year.”
• “SAM = 50,000 × $50K = $2.5B.”
• “Realistic 5-year SOM at ~3% market share = $75M ARR.”

Investors trust bottom-up because it’s defensible and falsifiable.

Worked example: Vertical SaaS for veterinary clinics

Bottom-up:

• 32,000 veterinary clinics in the U.S.
• ACV: $12,000/year on average.
• TAM (US) = 32,000 × $12K = $384M.
• Global expansion: 4x over 5–10 years → **$1.5B global TAM**.
• SAM: 20,000 clinics using cloud software = $240M.
• SOM: 5% share over 5 years = $12M ARR.

A VC will weigh this against the team’s realistic growth rate and category defensibility.

Red flags in market sizing

1. “$1 trillion market” without any grounded customer logic.
2. Stacked adjacencies: “Healthcare + life sciences + pharmaceuticals + adjacent” — too broad.
3. Consumer markets without ARPU math.
4. No path to SOM — handwaving “we’ll take 1%” without sales motion.

How top VCs stress-test market size

• Customer interviews: “How much do you pay today for the problem?”
• Bottom-up counts: From industry directories, census data, or vertical associations.
• Comparable company revenues: What do category leaders actually earn?
• Pricing sensitivity: Would a 2x price change market size meaningfully?

Practical takeaway

1. Founders: Build your TAM/SAM/SOM from customer counts and ACV, then compare to top-down for sanity.
2. Investors: Ignore consultant-report numbers; pressure-test bottom-up logic instead.
3. Operators: Refresh market sizing annually — market maturity changes the “realistic SOM” calculation.

Further reading

• NVCA model documents: https://nvca.org/model-legal-documents/