Why the share price on its own tells you nothing. The number of shares a company has issued is a decision, not a fact about its size, so two companies of identical value can have wildly different prices per share. A share price is therefore a ratio of company value to an arbitrary denominator, and comparing two prices compares nothing. This is the single most common misreading the measure exists to correct.
Two different things move it, and only one of them is news. The first is the price, which moves continuously and takes the market capitalization with it. The second is the share count. Issuing new shares raises the count and ordinarily raises the total even though existing holders now own a smaller fraction each, and buying shares back lowers the count. So a change in a company's market capitalization can reflect a change in what the market thinks, a change in how the equity is divided, or both.
The headline figure and the figure indexes weight by are not the same number. The SEC's definition counts total shares outstanding. Most broad stock indexes weight companies by their float-adjusted market capitalization, which counts only the shares available to be bought and sold by the public and excludes large blocks held by founders, families, governments or other strategic holders. A company where a founding family holds half the shares therefore carries roughly half the index weight its headline size implies. The mechanics of that adjustment belong with the S&P 500, which is built on it, but the distinction is worth carrying: a news story and an index are frequently describing the same company with two different numbers.
US securities law does draw dollar lines by market value, and those lines are not what they look like. 17 CFR 240.12b-2 sets a company's filer status using public float, defined as the aggregate worldwide market value of the voting and non-voting common equity held by non-affiliates, measured on the last business day of the most recently completed second fiscal quarter. A large accelerated filer has a public float of $700 million or more. An accelerated filer has $75 million or more but less than $700 million. A smaller reporting company has a public float of less than $250 million, or alternatively annual revenues of less than $100 million together with either no public float or a public float of less than $700 million. The filer categories carry further conditions, including how long the company has been reporting and whether it qualifies as a smaller reporting company under the revenue test.
Those figures are worth knowing precisely because they are so easily misapplied. They determine filing deadlines and how much disclosure a company must provide. They are not the boundaries between small-cap, mid-cap and large-cap companies, they use float rather than total shares, and the conventional investing bands sit orders of magnitude higher. Anyone who has seen $250 million or $700 million described as a market-cap category boundary has seen a disclosure rule mistaken for an investment classification.
