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What's the most correct way to calculate market cap for multi-class companies?
Some companies issue multiple classes of shares.  Each share may have different ratios applied to ownership rights and voting rights.  Some shares classes are not traded on any exchange at all.  Some share classes have limited or no voting rights.   Voting rights ratios are not used when calculating market cap but the market typically puts a premium on shares with voting rights. Total market cap must include ALL classes of shares, listed or not, weighted according to thee ratios involved in the company's ownership structure.  Some are 1:1, but in the case of Berkshire Hathaway, Class B shares are set at an ownership level of 1/1500 of the Class A shares. In terms of Alphabet Inc, the following classes of shares exist as at 4 Dec 2015: When determining market cap, you should also be mindful of other classes of securities issued by the company, such as convertible debt instruments and stock options.  This is usually referred to as "Fully Diluted" assuming all such instruments are converted.