The S&P 500 is the most widely followed measure of the US stock market. When news reports say "US stocks rose today", they usually mean this index. It tracks about 500 of the largest US companies, and trillions of dollars in index funds follow it. This article explains what the S&P 500 is, how companies get in, why a handful of giants move it so much, and how it differs from the Dow and the Nasdaq-100, with figures from PositoAI's data.
What the S&P 500 is
The S&P 500 is a stock market index: a single number that sums up the share prices of about 500 large companies listed in the US. It was launched in 1957 and is maintained by S&P Dow Jones Indices, whose committee decides which companies are in it. It covers roughly four-fifths of the value of all listed US companies, so it is often used as a stand-in for "the US market".
It has slightly more than 500 share lines, because a few companies, such as Alphabet, have two classes of shares in the index.
How companies get in
A company is not added just because it is large. The committee looks at several conditions:
Based in the US and listed on a US exchange.
Large enough: a minimum market value, which the committee raises from time to time as markets grow.
Profitable: positive total profit over the latest four quarters, and in the latest quarter.
Widely traded: enough shares available to the public and traded regularly.
The index is reviewed every quarter, and changes can also happen at other times, for example after a merger. A company leaves when it no longer fits, is bought, or is replaced by a more suitable one.
Why the biggest companies count the most
The S&P 500 is weighted by market value: a company's share price moves the index in proportion to its size. A 1% rise in a company worth $5 trillion moves the index far more than a 1% rise in one worth $20 billion. This means a few very large companies can lift or drag the whole index.
Example on PositoAI: the S&P 500's biggest companies
On 20 Oct 2026, the S&P 500 companies on PositoAI were worth about $69 trillion in total. The largest were Nvidia (about $5.2 trillion), Apple ($5.0 trillion), Alphabet ($4.2 trillion), Microsoft ($4.0 trillion) and Amazon ($2.8 trillion), followed by Meta, Broadcom, Tesla, Eli Lilly and Micron.
The seven companies often called the "Magnificent 7" made up about 35% of the whole index's value, so their share prices had a large effect on its daily moves. (As on 20 Oct 2026; values change daily.)
Sectors inside the index
The index spreads across all major industries, but not evenly. Because its weights follow market value, sectors with very large companies take a bigger share than their number of companies alone would suggest.
Example on PositoAI: S&P 500 by sector
Technology made up about 38% of the index's value from about 85 companies. Communication services (which includes Alphabet and Meta) was about 11% from fewer than 20 companies. Financial services, consumer cyclical (which includes Amazon and Tesla) and healthcare were each about 10%. Utilities, real estate and basic materials, with about 80 companies between them, together made up only about 5%.
Of about 490 member companies on PositoAI, 85 had a P/E below 15, about 220 between 15 and 30, and 163 at 30 or above, while 25 had no meaningful P/E because of a loss. (As on 20 Oct 2026.)
S&P 500, Dow and Nasdaq-100
The Dow Jones Industrial Average (Dow 30) holds just 30 large companies and is weighted by share price, not market value, so a company with a high share price counts more regardless of its size.
The Nasdaq-100 holds the 100 largest non-financial companies listed on the Nasdaq exchange, so it leans heavily towards technology.
The S&P 500 is broader than both, which is why it is the most common benchmark for US large companies.
How people invest in it
An index cannot be bought directly. Instead, index funds and exchange-traded funds (ETFs) hold the same companies in the same proportions, so their value moves with the index. Several such funds are also available to Indian investors through international mutual funds, within limits set by regulators.
Common misconceptions
"The S&P 500 is the 500 biggest US companies." Size matters, but profit, trading and other conditions also decide membership.
"An index fund spreads money evenly." It follows market value, so the largest companies take a large share.
"A rising index means most stocks rose." A few giants can lift the index even when many smaller members fall.
Frequently asked questions
Who decides changes to the S&P 500?
A committee at S&P Dow Jones Indices, following published rules.
How often does it change?
It is reviewed every quarter, and changes can happen at other times too.
Is the S&P 500 the same as the US economy?
No. Its companies earn a large part of their sales abroad, and it leaves out small companies and private businesses.
What is India's closest equivalent?
Broad Indian indexes such as the Nifty 50 and Sensex play a similar role, though with far fewer companies.
How to use this on PositoAI
The S&P 500 page lists its member companies with their Positivity Score and key figures, alongside pages for the Dow 30 and Nasdaq-100. Each company's PosiFunda page shows its fundamentals, and the Top 10 lists include S&P 500 versions. PosiStep 14 of PosiCourse, PositoAI's free stock market course, explains the key ratios.
Suggested reading
Who Are the Magnificent 7? · What Is the SEC and a 10-K? · What is an Index? · What is an ETF? · What is Market Capitalization?
Summary
The S&P 500 tracks about 500 large US companies chosen by a committee for size, profit and trading, and is reviewed every quarter. Because it is weighted by market value, a few giants carry great weight: on 20 Oct 2026 the Magnificent 7 made up about 35% of it, and technology about 38%. It is broader than the Dow 30 and Nasdaq-100, and people invest in it through index funds and ETFs.