S&P 500
The S&P 500 is the benchmark most people mean by "the market." It holds around 500 of the largest US companies across every sector and is the yardstick most funds are measured against.
How the S&P 500 is built
A committee selects large, profitable US companies; each is weighted by its float-adjusted market cap, so the biggest companies drive most of the index’s moves.
Methodology: official index-provider documentation ↗
What to check before using the S&P 500 as a benchmark
- It is a large-cap benchmark, not the entire US stock market; smaller companies are outside its remit.
- Float-adjusted market-cap weighting gives its largest companies the greatest influence on returns.
- The provider reviews constituents and rebalances on its published schedule, so membership is not permanent.
Biggest companies in the S&P 500
via SPY · as of July 2026Top holdings shown via the tracking ETF SPY (membership only, no weights). Source: stockanalysis.com.
How to invest in the S&P 500
You can’t buy an index directly — you buy a fund that tracks it. The most common way to follow the S&P 500 is the SPY ETF. Because it’s market-cap weighted, remember the largest holdings drive most of the moves — if you already own those names, you may be more concentrated than the broad label suggests.
Other major indexes
Frequently asked about the S&P 500
What does the S&P 500 measure?
The S&P 500 measures the large-cap segment of the US equity market through roughly 500 selected companies. It is widely used as a benchmark, but it does not represent every publicly traded US company.
Why do the largest S&P 500 companies matter so much?
The S&P 500 uses float-adjusted market-cap weighting. A company with a larger publicly tradable market value has more influence on index returns than a smaller constituent.
Can you invest directly in the S&P 500?
No. An index is a measurement rule, not a security. Investors who want similar exposure typically use a fund designed to track it, such as SPY.
Educational overview of an index and its membership — not investment advice, and not a recommendation to buy or sell any index fund or company. Holdings are a periodic snapshot and change over time.