Market capitalization

Market capitalization — market cap — is the total dollar value of a company's outstanding shares. It is the standard way to size a public company and the primary axis used to bucket equities into categories like large-cap and small-cap.

Formula

market_cap = current_share_price * shares_outstanding

A company trading at $50 with 200 million shares outstanding has a $10 billion market cap.

Size buckets

The thresholds vary by source, but the broadly accepted ranges are:

• Mega-cap — > $200B. The largest names in the index. Tend to dominate sector and index returns.
• Large-cap — $10B to $200B. The bulk of the S&P 500.
• Mid-cap — $2B to $10B. Russell Midcap territory.
• Small-cap — $300M to $2B. Russell 2000 territory.
• Micro-cap — $50M to $300M. Often illiquid.
• Nano-cap — < $50M. Frequently OTC-traded with significant liquidity risk.

Why size matters in portfolio risk

Smaller-cap stocks have historically delivered higher expected returns and higher volatility — the small-cap premium. They also carry higher idiosyncratic risk: a single bad quarter or a missed earnings number can move a small-cap 30% overnight, where a mega-cap might move 5%. Concentration in mid- or small-cap positions is a different risk profile than the same dollar amount in mega-caps.

Limitations

• Float vs total shares. Some indexes use free-float market cap — only shares available to public investors. Insider-held shares are excluded.
• Says nothing about debt. Two companies with identical market caps can have very different enterprise values once debt is included.
• Price-driven. Market cap rises and falls with the stock price. A company can move from large-cap to mid-cap on a bad year without anything changing operationally.