market-sizing-methods

name	market-sizing-methods
description	TAM/SAM/SOM estimation, ICP definition, and customer segment sizing. Use when estimating market opportunity, defining ideal customer profiles, or sizing addressable segments.

Market Sizing Methods

Market sizing is frequently done wrong — either too optimistically (top-down from industry reports) or too vaguely (hand-waving about "a large and growing market"). This skill covers rigorous methods for estimating opportunity size and defining who to target.

When to Use This Skill

• Sizing the opportunity before committing to a product direction
• Defining the ideal customer profile (ICP) for a new product or segment
• Estimating TAM/SAM/SOM for business planning or investor materials
• Validating whether a niche is large enough to build a business on
• Segmenting an existing user base by value and growth potential

Core Concepts

TAM / SAM / SOM

Term	Definition	Question It Answers
TAM — Total Addressable Market	Everyone who could theoretically buy this	How big is the universe?
SAM — Serviceable Addressable Market	The portion you can realistically reach with your current model	How much of TAM can you access?
SOM — Serviceable Obtainable Market	What you can realistically capture given competition and constraints	What can you actually win?

TAM is for context. SAM is for planning. SOM is for forecasting.

Top-Down vs. Bottom-Up

Top-down: Start from industry size data, apply percentages down to your segment.

• Fast but often inaccurate — percentages compound errors
• Use as a sanity check, not a primary estimate

Bottom-up: Start from individual customer unit economics, build up.

• More work but far more defensible
• Forces explicit assumptions about pricing and adoption

Always do both and compare. When they diverge significantly, investigate why.

Bottom-Up TAM Estimation

Steps

1. Define the unit: What's one customer? (one business, one user, one seat)
2. Define the universe: How many units exist that match your target definition?
3. Apply revenue per unit: What would each unit pay per year?
4. Multiply: Universe × ARPU = TAM

Example

Product: B2B expense management tool
Target: SMBs with 10-200 employees in the US

Step 1 — Universe:
  US businesses with 10-200 employees: ~1.5M (Census Bureau data)
  Subset with travel/expense activity: ~60% = 900K businesses

Step 2 — ARPU:
  Base price: $15/seat/month × avg 25 seats = $375/month = $4,500/year

Step 3 — TAM:
  900K businesses × $4,500/year = $4.05B TAM

SAM (reachable via digital/PLG model):
  Businesses with >5 employees that use SaaS tools: ~40% = 360K
  360K × $4,500 = $1.62B SAM

SOM (Year 3 target):
  0.5% market share = 1,800 customers × $4,500 = $8.1M ARR

Transparency Requirements

For every market size estimate, document:

• The data source for the universe size
• The assumptions behind any percentages applied
• The revenue per unit assumption and how it was derived
• Confidence level: High (primary research), Medium (secondary data), Low (estimation)

ICP Definition

The Ideal Customer Profile describes the type of customer most likely to buy, retain, and expand. It's not a persona (who they are) — it's a firmographic and behavioral profile (what kind of organization they are and what they're doing when they buy).

ICP Template

# Ideal Customer Profile: [Segment Name]

## Firmographics (B2B) / Demographics (B2C)
- Company size: [employees / revenue range]
- Industry: [specific verticals]
- Geography: [regions / markets]
- Tech maturity: [description]

## Behavioral Signals
- What triggers them to look for a solution like ours:
- What they're currently using (or doing) instead:
- How they evaluate and buy:

## Jobs to Be Done
- Primary job: [what they're trying to accomplish]
- Constraints: [what limits their current approach]

## Value Realization
- Time to first value: [how quickly they see benefit]
- Expansion pattern: [how usage grows after initial purchase]
- Retention risk: [what causes churn]

## Negative ICP (who to avoid)
- [Characteristics of customers who churn, are unprofitable, or require excessive support]

ICP Validation Signals

An ICP definition is a hypothesis until validated by:

• Win rate: Deals matching the ICP close at a higher rate
• Time to value: ICP customers reach activation faster
• Retention: ICP customers churn less
• NPS / satisfaction: ICP customers are more satisfied
• Expansion: ICP customers expand their usage

If your best customers don't match your stated ICP, update the ICP.

Segment Sizing and Prioritization

When multiple segments exist, score each on:

Dimension	Question	Score
Size	How many potential customers?	1-5
Willingness to pay	How much would they pay?	1-5
Accessibility	How easy to reach and sell?	1-5
Strategic fit	How well does this segment align with our strengths?	1-5
Growth trajectory	Is this segment growing?	1-5

Rank segments by total score. Focus initial go-to-market on the top 1-2 segments.

Common Sizing Mistakes

Mistake	Why It's Wrong	Fix
"1% of a $10B market"	Doesn't explain HOW you get that 1%	Build a bottom-up model instead
Using industry reports uncritically	Reports define markets differently than your product	Redefine from first principles
Conflating TAM with SAM	Not all of TAM is reachable with your GTM model	Apply accessibility filters
No negative ICP	Targeting everyone means optimizing for no one	Define explicitly who you're NOT targeting
Static sizing	Markets change; a 3-year-old estimate may be wrong	Date estimates and refresh annually

Best Practices

1. Show your work — every number needs a source or a stated assumption
2. Use ranges, not point estimates — $400M–$700M is more honest than $550M
3. Start with SAM, not TAM — TAM impresses; SAM is what you can actually build a business on
4. Test the ICP against real customers — your best customers define the ICP, not the other way around
5. Update annually — market structure changes; stale sizing leads to wrong decisions