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TAM, SAM, SOM: How to Size a Market Correctly

January 29, 2026 · 6 min read
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“We only need 1% of a $100 billion market.”

If you’ve sat through enough pitch decks or earnings calls, you’ve heard some version of this line. It sounds modest. It sounds achievable. And it’s almost always misleading, because the $100 billion number is usually the wrong reference point.

Market sizing is one of the most abused exercises in business and investing. Done poorly, it flatters every thesis. Done well, it’s one of the most revealing tools for understanding whether a company’s ambitions are grounded in reality.

The difference comes down to three numbers: TAM, SAM, and SOM. Most people treat them as interchangeable. They’re not, and confusing them leads to fundamentally wrong conclusions about growth potential, competitive dynamics, and valuation.

Three numbers, three very different questions

Total Addressable Market (TAM) is the theoretical ceiling, the total revenue opportunity if a company captured every possible customer with no constraints. It’s useful as a boundary, not as a target.

Serviceable Addressable Market (SAM) narrows the lens to reality. It’s the portion of TAM a company can actually reach given its current product, geography, pricing, and go to market capabilities.

Serviceable Obtainable Market (SOM) is what matters for near term planning. It’s the realistic share of SAM the company can capture over a defined period, typically three to five years, given competitive dynamics, sales capacity, and execution constraints.

The relationship is always TAM ≥ SAM ≥ SOM. And the discipline lies in how honestly each number is derived.

Two ways to calculate TAM, and why they often disagree

Top down: start from industry data, segment downward

Begin with a broad market statistic from an industry report, government data, or trade association, then narrow it through successive filters.

Example: A company selling project management software to European mid market companies.

  1. Global enterprise software market: EUR 500B (industry report)
  2. Project management software specifically: ~12% = EUR 60B
  3. European market: ~25% of global = EUR 15B
  4. Mid market segment (50-500 employees): ~30% = EUR 4.5B

TAM = EUR 4.5B

The approach is straightforward, but every segmentation assumption compounds. An inaccurate percentage at step two cascades through the entire calculation. And the broader the starting point, the more room for self serving definitions.

Bottom up: start from unit economics, aggregate upward

Build from the ground level: how many potential customers exist, and what could you realistically charge each one?

Same example:

  1. Mid market companies in Europe: ~200,000
  2. Those with project intensive operations (tech, consulting, construction): ~60,000
  3. Average annual contract value: EUR 15,000
  4. TAM = 60,000 x EUR 15,000 = EUR 900M

This is dramatically different from the top down EUR 4.5B, and likely more accurate. The top down figure was inflated by including large enterprises and segments the product doesn’t actually serve.

Bottom up sizing is almost always more credible. It’s also harder to do and easier to scrutinize, which is precisely why many presentations default to top down.

From TAM to SAM: applying real world constraints

SAM answers: of the total theoretical market, what portion can this company actually address today?

The constraints are practical:

  • Geographic: Which countries or regions can the company sell in right now?
  • Product: Which customer segments does the product genuinely serve well?
  • Regulatory: Are any markets excluded by law or compliance requirements?
  • Channel: Can the company reach all potential customers through its existing go to market?
  • Pricing: What willingness to pay exists at the company’s price point?

Continuing the example: The software company operates in English and German only, serves SMBs and mid market well but isn’t competitive in enterprise, and has no presence outside Europe.

SAM = ~EUR 400M (from a bottom up TAM of EUR 900M)

This is the number that defines the medium term opportunity, and it’s far more useful than the TAM for assessing growth potential.

From SAM to SOM: what can actually be captured

SOM is the most important number and the most frequently left unexamined. It asks: given competition, sales capacity, customer acquisition dynamics, and execution realities, what share can this company realistically win?

The inputs:

  • Current market share and growth trajectory
  • Sales capacity (headcount, quota, close rate)
  • Competitive displacement rate (how fast can share be won from incumbents?)
  • Whether growth comes from new market creation or conversion of existing spend

Rules of thumb: A well funded startup in a fragmented market might realistically target 2-5% of SAM in five years. A venture with strong network effects could reach higher. Mature market leaders often hold 20-40%.

Continuing the example:

  • Current ARR: EUR 8M
  • Growth rate: 80% year over year
  • Projected ARR in five years (with declining growth): ~EUR 100M
  • SOM = EUR 100M = ~25% of SAM

That’s aggressive but within the plausible range for a high growth B2B software company with strong product market fit. The key question is whether the growth rate assumptions are realistic.

The five most common mistakes in market sizing

1. Defining the market by the desired conclusion. This is the most common manipulation: defining the market broadly enough that the target share looks modest. “We only need 1% of $100B” almost always means the $100B is the wrong reference class.

2. Double counting. Top down analyses frequently count the same revenue twice when combining overlapping segmentation dimensions.

3. Inflating or ignoring growth. Static TAM works for mature markets. For growing ones, specify whether the figure is current or projected, and project it honestly, not aspirationally.

4. Confusing total spend with addressable spend. Not all spending in a sector is addressable. If you’re selling a specific workflow tool, your TAM is not “all enterprise IT spend.” It’s the spending that could plausibly be replaced by what your product does.

5. Leaving SOM unconstrained. What limits market capture? Sales capacity? Customer acquisition cost? Competitive intensity? Regulatory process? Without explicit constraints, SOM is wishful thinking.

What market sizing reveals for investors

When evaluating a company, properly constructed market sizing answers several critical questions:

  • Growth runway: Is there enough SAM to sustain current growth rates for years, or is the company approaching saturation?
  • Market structure: What share does the company hold? Is the market concentrated around a few leaders, or fragmented across hundreds of small players?
  • Competitive dynamics: A fragmented market with no dominant player represents a different investment thesis than one where a leader controls 60%+.
  • TAM expansion potential: Is the company expanding its addressable market over time, through new geographies, products, or customer segments?

The most compelling business outcomes typically come from companies that established dominance in a well defined niche, built competitive advantage within it, and then expanded the market itself, not just their share of it.

For most businesses, the discipline of bottom up sizing, realistic SAM constraints, and honest SOM forecasting is what separates credible analysis from narrative. The numbers should challenge the thesis, not confirm it.

THETA generates AI powered market and company analysis reports that include addressable market assessment as part of the analytical framework. This article is educational content only and does not constitute investment advice.