Market capitalization

In plain words

A company is split into shares. If it has 1 billion shares and each trades at $50, buying every single one would cost $50 billion. That number is its market capitalization, or market cap.

Market cap is the quickest way to compare company sizes: a firm worth $3 trillion is a giant, one worth $300 million is small. It changes every second the market is open, because the share price does.

Add up all companies and you get the size of a whole stock market. The shares of US nonfinancial companies were worth about $83.1T at the end of Q2 2026.

Example

One level deeper

Investors sort stocks by size: large caps (roughly above $10 billion), mid caps, and small caps (roughly $300 million to $2 billion). Index makers draw the lines a little differently and move them as markets grow.

Most big indexes, including the S&P 500, are weighted by market cap: the bigger the company, the bigger its share of the index. A handful of giants can therefore move the whole index.

Market cap is not the money a company has, nor what it would cost to buy it outright. A buyer also takes on its debts and gets its cash, which is why analysts use enterprise value = market cap + debt − cash. Many indexes count only the free float, the shares that actually trade.

The US stock market against the size of the economy

The market value of all US nonfinancial companies’ shares divided by GDP. Above 100%, the market is worth more than a year of the economy’s output. Known as the Buffett indicator, it is now about 255%.

Why it matters

It tells you how big a company is, which shapes how risky it is and how widely it is owned.

It decides how much of an index fund goes into each company: in a market-cap-weighted fund the biggest firms get the most money.

Set against profits (the P/E ratio) or against the size of the economy, it shows how expensive a market is.

A common mistake