Yahoo finance shows GOOG as having a market cap of $492.97B, while GOOGL has a market cap of $510.18B.
What's Google's total market cap? Is it really $1,003B?
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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 the 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.
From their 10-K pulled directly from Edgar:
As of October 22, 2015, there were 291,327,781 shares of Alphabet Inc.’s (the successor issuer pursuant to Rule 12g-3(a) under the Exchange Act as of October 2, 2015) (Alphabet) Class A common stock outstanding, 50,893,362 shares of Alphabet's Class B common stock outstanding, and 345,504,021 Alphabet's Class C capital stock outstanding.
From here just do the math.
The shares outstanding are listed on the first page of the 10-Q and 10-K reports.
Edit: I believe Class B shares in this instance are not traded on the market and therefore would not be included.