The table below lists the historical market caps of the largest public U.S. companies. Currently (9/30/2020), the most valuable public company is Apple Inc with a market cap of $1,984,491.90 million, followed by Microsoft Corp ($1,592,408.43 M) and Amazon.com Inc ($1,577,513.73 M). The combined market cap of Apple, Microsoft, Amazon.com, Alphabet (Google) and Facebook is $6,899,217.77 million, while the total market value of top 100 U.S. corporations is $21,130,618,13 million.

For full historical data, check the historical datasets provided by Siblis Research.

Market Caps of The Largest US Companies (USD, M)

Purchase the U.S. Equities Researcher Dataset by Siblis Research and analyze the historical market values of almost ten thousand public U.S. companies for the past 40 years. The dataset provides historical market caps, revenues, earnings & adjusted share prices (total returns) of 9,527 individual public stocks, including delisted companies. Examine a sample dataset from here.

Outstanding Shares Used to Calculate Market Capitalization

The outstanding common shares are taken from the SEC quarterly (10-Q/K) reports of the companies. This number is not fixed but keeps fluctuating when public firms are issuing more, buying back and sometimes splitting their shares. For majority of the companies, one of the quarters will end on 31st Dec. But some companies are using a fiscal year that differs from a calendar year and they might not have a quarter that ends on December. For these firms, the outstanding shares are taken from a quarter that ends on October or November.

Different Stock Classes & Market Capitalization

Many companies have different types of common stock (note: preferred stock means a security that is a mixture of debt and common stock and is not considered as a separate share class). The most common reason for multiple share classes is to restrict the voting power of some share classes. This is especially common with high-tech companies whose founders want to enjoy the benefits of going public but still maintain full control of their companies. A good example of this is Alphabet (Google) that has two publicly traded share lines, A (GOOGL) and C (GOOG), and also B-class that is not publicly traded. A has one, B has ten and C has no votes. These shares with more votes are usually traded at slightly higher price, although it is very hard to define what is the exact monetary value of having more voting power.

When market cap is calculated, all of the common share classes need to be taken account. The common practice is to combine the number of outstanding shares of all classes and use the price of the share class with the largest trading volume. Alternative method is to calculate the market cap of all the share lines separately and then summing up the value.
For the companies with multiple share lines, the different classes are combined as one figure on the table below.

Note about Alphabet/Google: Before the year 2014, the B-class shares are combined with A-class and the price of A-line is used as the price. From 2014 onwards, B-class is no more combined with A-class and these shares are not taken account when calculating the market value for the company.

Market Cap & Market Value of a Firm

Market capitalization of a company means the market value of all of the company’s shares during a certain point of time. The market cap can be seen as a public opinion of what is the monetary value of a public corporation. It is commonly used to rank companies by their sizes. Calculating the market cap is not always straight forward. A company can have multiple share classes trading with different prices and also some share lines that are not publicly traded but hold by e.g. the founders of a company. There are also other metrics used to give a value for a corporation. Enterprise value takes account also the outstanding debt and preferred stock.