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Documentation Index

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Market capitalization (market cap) is the total market value of a company’s outstanding shares. It’s the primary measure of company size in investing and forms the basis of major stock indices.

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Beginner

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What It Means

Market cap tells you what the market thinks a company is worth. It’s calculated by multiplying the stock price by the total number of shares outstanding. 
Market Cap = Share Price × Shares Outstanding

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Example

CompanyShare PriceShares OutstandingMarket Cap
Company A$502 million$100 million
Company B$200500,000$100 million
Both companies have the same market cap despite very different share prices. Share price alone doesn’t tell you company size.

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Size Categories

CategoryMarket Cap RangeExamples
Mega-Cap>$200 billionApple, Microsoft, Amazon
Large-Cap$10B - $200BMost S&P 500 companies
Mid-Cap$2B - $10BRegional leaders
Small-Cap$300M - $2BGrowing companies
Micro-CapLess than $300 millionVery small public companies

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Why It Matters

Market cap determines:
  • Which indices include a stock (S&P 500, Russell 2000, etc.)
  • How much of an index a stock represents
  • Liquidity and trading characteristics
  • Risk and return expectations

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Advanced

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Market Cap vs. Other Size Measures

MeasureDefinitionUse Case
Market CapPrice × SharesMost common size measure
Enterprise ValueMarket Cap + Debt - CashAcquisition value, better for comparisons
RevenueAnnual salesSize of operations
Total AssetsBalance sheet assetsFor financials, regulated industries

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Float-Adjusted Market Cap

Most indices use float-adjusted market cap: 
Float-Adjusted Market Cap = Price × Float Shares

Float Shares = Total Shares - Insider Holdings - Restricted Shares
This counts only shares actually available for trading, excluding:
  • Insider holdings (executives, founders)
  • Government holdings
  • Strategic holdings by other companies
  • Restricted stock

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Index Weighting

Market cap determines portfolio weights in cap-weighted indices: 
S&P 500 Example (simplified):
- Apple: $3T market cap → ~7% of index
- Small company: $50B market cap → ~0.1% of index
Cap-weighted indices are “self-rebalancing” - as stocks rise, their weight naturally increases. No trading required to maintain weights.

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The Size Factor

Academic research documents the “size premium” - historically, smaller companies have outperformed larger ones:
PeriodSmall-Cap Premium
1926-2023~2-3% annually
1980-2000Significant
2000-2020Near zero or negative
The size premium has been inconsistent in recent decades. Small-cap outperformance is not guaranteed and comes with higher volatility.

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Market Cap Characteristics by Size

CharacteristicLarge-CapSmall-Cap
VolatilityLowerHigher
LiquidityHighLower
Analyst CoverageExtensiveLimited
Information EfficiencyHighLower
Trading CostsLowHigher

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Dynamic Nature

Market cap changes continuously:
  • Stock price movements
  • Share buybacks (reduces shares, may increase price)
  • Stock issuance (increases shares, may decrease price)
  • Stock splits (no effect on market cap)

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Sector Distribution by Size

Different sectors dominate different size segments:
SegmentDominant Sectors
Mega-CapTechnology, Healthcare
Large-CapFinancials, Industrials
Mid-CapIndustrials, Consumer
Small-CapFinancials, Healthcare, Industrials

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Limitations

  • Price-Based: Reflects market sentiment, not intrinsic value
  • Volatile: Can change dramatically with price swings
  • Debt-Blind: Doesn’t account for leverage (use Enterprise Value)
  • Industry Differences: Hard to compare across sectors

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Related Terms

Beta

Small caps typically have higher beta

Factor Investing

Size is a key factor

Diversification

Across size segments