The total market value of a company: share price multiplied by shares outstanding.
Market cap is the simplest measure of company size. Rough US categories: mega-cap above $200B, large-cap above $10B, mid-cap $2B to $10B, small-cap $300M to $2B, micro-cap below that.
Size affects behavior. Large caps have deep liquidity and tight spreads; micro caps can move 30% in a day on modest volume.
Example: a stock at $40 with 500 million shares outstanding has a market cap of $20 billion. A $2 move in the share price changes the market cap by $1 billion.
Original diagrams for the ideas on this page. Illustrative, not real market data.
The bid-ask spread. Buy orders sit below, sell orders above, and the gap between the best bid (50.01) and best ask (50.04) is the spread you pay to cross. Bar length shows the size resting at each price.
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