Market capitalization, or market cap, is a measure of the market value of a publicly traded company, and can be used to discuss sectors or whole portions of the stock market. Market cap is based on the share prices of stocks, which fluctuate over time, so the market cap number you are discussing depends on when the share price was taken.
The market capitalization of a company is total value of a company as defined by its share price and outstanding shares. Market cap is calculated by multiplying the number of shares outstanding by the current share price. For example, at the time of publication, Wal-Mart had 3.485 billion shares outstanding and a share price of $52.50, giving the company a market cap of $183 billion.
During market hours, stock prices fluctuate, sometimes significantly. These price changes during the market day are referred to as intraday stock prices. For example, a stock might have opened at $50 and closed at $50, unchanged for the day. However, intraday the share price could have swung from a low of $45 to a high of $55. For most investors, the closing share price is enough information, but some traders work with intraday price changes as well.
Market Cap Listings
The listings for the market cap of a stock on the financial news websites use the previous daily closing stock price to calculate the current market value. The listed market cap of a stock does not change as the share price changes intraday. When the market has closed, and the closing share price is known, a new market cap is posted for a stock. Printed materials use market cap values for when they were written. Over a period of time -- days, weeks or months -- the market cap of a company can change significantly.
The intraday market cap of a stock can be calculated at any time using the current share price. Intraday market caps are usually noted when a stock price hits a milestone during the market day, and then drops back to close at a lower price. The most common example is when a company becomes the most valuable stock on the exchange, or even in the world with a intraday high price, then closes at a lower price, leaving the stock's market cap below the companies it passed with the intraday high.