This blog post is the first in a four-part series. It explores the four major stock indices, which are the benchmarks that shape how we talk about “the market.”
Before diving into the S&P 500 specifically, it’s worth taking a step back to understand what a stock index really is and why it matters. From how they’re constructed to what they actually represent, indices form the foundation of nearly every market conversation — and every investor’s frame of reference.
If you’re new to investing and want a refresher on stocks, check out this primer before continuing.
What is a Stock Index?
A stock index is a measure of the prices of a preselected group of stocks. It is effectively a weighted average, with each stock in the index weighted according to a preset method. The price of each stock in the index is multiplied by its weight. The value of the index is the sum of each product (stock price x weight).
For example, suppose that we have three stocks. Stock A has a price of $35 per share, stock B has a price of $25 per share, and stock C has a price of $40 per share. We can construct a price-weighted index. First, calculate the weight of each stock. Then, multiply each stock’s price by its weight. Finally, take the sum of each product.
The sum of the prices is $100, so the weight of each stock is 0.35, 0.25, and 0.40, respectively. The value of the price-weighted index is thus:
(0.35 x $35) + (0.25 x $25) + (0.40 x $40) = $34.5
The problem with a price-weighted index is that stock prices do not give an indication of the relative size of the index components. But there are, of course, other ways to construct stock indices. One popular way of constructing an index is to use market capitalization as the weighting methodology.
Market Capitalization Weighting Explained
Market capitalization (market cap) refers to the aggregate value of all a company’s outstanding stock shares. This is simply calculated as the stock price times the number of shares outstanding.
To see how a market capitalization index is constructed, let’s go back to the above example. Suppose that stock A has 1 million shares outstanding, stock B has 5 million shares outstanding, and stock C has 700 thousand shares outstanding. The market caps of the three stocks are:
- Market cap stock A: 1 million shares x $35 = $35 million
- Market cap stock B: 5 million shares x $25 = $125 million
- Market cap stock C: 700 thousand shares x $40 = $28 million
The total market cap for the three stocks is $188 million, and the weight of each stock is 0.1862, 0.6649, 0.1489, respectively. The value of the market cap weighted index is:
(0.1862 x $35) + (0.6649 x $25) + (0.1489 x $40) = $29.096
The Takeaway: that different weighting methodologies can lead to very different values.
The S&P 500: America’s Most-Watched Index
Among the various indices reported in the financial press, the S&P 500 is the most widely recognized. For many investors and market commentators, the S&P 500 is “the market.” The S&P is also viewed by many people as a barometer of the health of the broader economy, an assertion that, in my opinion, has become less true in recent years.
The S&P 500 measures the performance of 500 U.S. large capitalization companies. (On a slight technical note: although the S&P 500 index has 500 companies, it can have slightly more than 500 stocks. This is primarily due to companies having more than one share class. For example, Alphabet has two share classes in the index. Some companies issue more than one share class – usually to allow company insiders disproportionate voting rights.)
The S&P 500 debuted in its current form in 1957. It came to prominence in the 1970s when Vanguard launched its first index fund, as investors could then gain exposure to the index without having to individually purchase shares in 500 companies.
The S&P 500 weighs the components in the index by float-adjusted market capitalization. This means that market capitalization is adjusted for market float, which are the shares available to the investing public.
ONE CONSEQUENCE OF THE WAY THE S&P 500 IS CONSTRUCTED IS THAT THE LARGER A STOCK'S FLOAT-ADJUSTED MARKET CAPITALIZATION, THE GREATER THE IMPACT THE STOCK WILL HAVE ON THE INDEX LEVEL.
Because technology companies have become the largest companies in the U.S. economy, the S&P 500 may not represent the wide diversification it once did. The index represents 11 sectors, but as of October 31, 2025, 38.1% of the index was in one sector – information technology.
The concentration is not just among sectors, but also in individual stocks. As of the time of this writing, Nvidia, a company with a nearly $5 trillion market capitalization, represents close to 8% of the S&P 500. The top 10 stocks in the index represent almost 40% of the total index value. I talked more about this growing dominance of the top 10 companies — and what it means for investors trying to track “the market” in Episode 14 of the Fundamentals Unfiltered podcast.
Does the S&P 500 Still Reflect the Broader Economy?
The S&P 500 will continue to be an important measure of the stock market. And both institutions and individual investors will continue to invest in the various investment products which track the index. But whether the S&P 500 represents an accurate barometer of the broader U.S. economy is another question.
Summary
Stock indices simplify complex markets — but their construction matters more than most realize. The S&P 500 remains the most influential benchmark in finance, yet its growing concentration in large technology companies raises important questions about diversification and what “the market” truly represents. Understanding how these indices are built is essential for any investor who wants to think beyond the headlines.
Key Takeaways
- A stock index tracks a group of stocks based on a weighting method.
- Price-weighted indices emphasize stock price, not company size.
- Market-cap-weighted indices give larger companies more influence.
- The S&P 500 uses float-adjusted market cap weighting.
- Technology now dominates the S&P 500, driving most of its movement.
- The index may no longer mirror the true breadth of the U.S. economy.
