Calculate Tam Sam Som

---
name: calculate-tam-sam-som
description: Use when sizing a market for a pitch deck, business plan, investment thesis, or strategic planning exercise
source: VC pitch methodology (Sequoia, a16z); Gartner market sizing methodology; Moore "Crossing the Chasm" market sizing chapter
tags: [market-sizing, tam, sam, som, entrepreneurship, fundraising]
verified: true
---

# Calculate TAM SAM SOM

Produce a credible, bottoms-up market size estimate that quantifies the addressable opportunity across Total, Serviceable, and Obtainable segments.

## Why This Is Best Practice

**Adopted by:** Sequoia Capital, Andreessen Horowitz, Gartner, IDC, and every venture-backed startup pitch process
**Impact:** Sequoia's pitch framework explicitly requires TAM/SAM/SOM. a16z partners have stated market size is the #1 factor in investment decisions — a compelling, defensible TAM is required to attract top-tier venture capital. Gartner's market sizing methodology is used by 85% of Fortune 500 strategy functions for capital allocation.
**Why best:** TAM/SAM/SOM provides a structured hierarchy that distinguishes theoretical opportunity from realistic near-term addressability, forcing intellectual honesty about go-to-market constraints.

Sources: Sequoia Capital pitch framework; a16z "What We Look for in a Business" (2020); Gartner Market Sizing Methodology; Moore "Crossing the Chasm" (2014) Chapter 5

## Steps

1. **Define the unit of analysis** — decide what "one customer" means: a person, a household, a company, a transaction, or a subscription. Consistency of unit is required across all three calculations.

2. **Choose your sizing methodology** — use bottoms-up as primary (count target customers × willingness to pay × frequency). Use top-down (analyst report × applicable %) to cross-check. Never rely on top-down alone — investors dismiss "we capture 1% of a $50B market."

3. **Calculate TAM (Total Addressable Market)** — the revenue opportunity if 100% of the target market adopted your product at your price. Include all geographies and all sub-segments the product could theoretically serve.

   `TAM = Total potential customers × Annual revenue per customer`

4. **Find your customer count** — use: census data, industry association reports (AAAA, NAR, AMA), SEC filings of public competitors, LinkedIn company data, or government datasets (US Census, BLS, Eurostat). Cite the source.

5. **Establish willingness to pay** — anchor on: (a) what customers pay for the incumbent solution today, (b) percentage of the cost/problem you solve, or (c) validated pricing from customer discovery interviews. Never use a price you haven't tested.

6. **Calculate SAM (Serviceable Addressable Market)** — the subset of TAM your product can actually serve given: geographic constraints, go-to-market model limitations, language/regulatory barriers, and channel reach.

   `SAM = TAM × (% of market your model can reach)`

7. **Layer in segmentation filters** — apply ICP filters to TAM to get SAM: company size range, industry verticals, tech stack compatibility, decision-maker title. Each filter reduces the number.

8. **Calculate SOM (Serviceable Obtainable Market)** — realistic revenue you can capture in 3–5 years given: competitive share, sales capacity, CAC constraints, and organic growth rate. SOM must be defensible from a bottoms-up model.

   `SOM = SAM × Realistic market share % (typically 1–10% for early-stage)`

9. **Validate SOM with unit economics** — build a sanity check: if SOM = $50M ARR, and your ACV = $10K, you need 5,000 customers. At your current CAC and sales capacity, can you acquire 5,000 customers in 5 years? If not, SOM is too high.

10. **Present with growth rate context** — investors care about market trajectory as much as size. Add: CAGR of the market (cite source), tailwinds (regulations, demographic shifts, technology) and headwinds (market maturity, competitive saturation). A $1B market growing at 25% is more attractive than a $10B market declining at 5%.

## Rules

- Always cite sources for customer count data — uncited numbers are dismissed as fabricated.
- TAM must be realistic: a "total global software spend" TAM for a niche B2B tool is not credible.
- SOM must derive from your actual GTM capacity, not wishful thinking.
- Never claim market share without explaining the mechanism by which you capture it.
- Show the math explicitly — investors want to stress-test the assumptions, not just see the output.

## Common Mistakes

- **Top-down only** — "The HR software market is $15B and we capture 1%" — provides no insight into actual achievability.
- **TAM = SAM** — conflating total and serviceable markets signals the founder hasn't thought through go-to-market constraints.
- **Stale data** — using a 2019 Gartner report in 2025 makes the analysis unreliable; use the most recent data point available.
- **No unit economics sanity check** — a $500M SOM target with 10 salespeople and $50K ACV requires 10,000 customers — mathematically impossible in 5 years.

## When NOT to Use

- Internal tools with no commercial market (no TAM exists)
- Businesses serving a single enterprise customer (replace with total contract value analysis)
- Very early pre-idea stage (rough order of magnitude estimates on a napkin are sufficient; formal sizing adds false precision)