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The S&P 500 is a benchmark for large-cap U.S. stocks; the Nasdaq-100 tracks the largest non-financial companies listed on Nasdaq, including domestic and international firms. The difference is not simply “500 stocks versus 100” or “broad market versus tech”: each index applies its own eligibility and weighting rules, so each represents a different slice of the equity market.
What is the S&P 500?
The S&P 500 is a large-cap U.S. equity index maintained by S&P Dow Jones Indices (S&P DJI). The provider describes it as comprising 500 leading companies and covering approximately 80% of available U.S. market capitalization. That 80% is a scope description, not a fixed proportion that applies exactly every day. S&P DJI’s index page reported 503 constituent securities as of August 31, 2026. A company can have more than one eligible share class, so the number of securities need not match the number of companies.
The index is intended to gauge the large-cap segment of the U.S. equity market, rather than include every publicly traded U.S. company. S&P DJI describes it as “widely regarded as the best single gauge of large-cap U.S. equities.” Its rules determine which companies qualify and how the index is maintained.
What is the Nasdaq-100?
The Nasdaq-100 is an index of 100 of the largest domestic and international non-financial companies listed on The Nasdaq Stock Market, selected based on market capitalization under the index’s rules. The exchange listing requirement is central: a company does not qualify merely because it is large or based in the United States. Financial companies are excluded.
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Nasdaq’s index overview describes the benchmark in those terms. Its methodology effective May 1, 2026 sets out the applicable selection, weighting and maintenance rules; rules can change, so the methodology date matters when assessing eligibility.
How do the S&P 500 and Nasdaq-100 differ?
| Feature | S&P 500 | Nasdaq-100 |
|---|---|---|
| What it represents | Large-cap segment of the U.S. equity market | Largest Nasdaq-listed non-financial companies, domestic and international |
| Eligibility lens | Large-cap U.S. companies that meet the index’s rules | Nasdaq listing and non-financial status, followed by size-based selection under the methodology |
| Weighting | Float-adjusted market capitalization | Modified market capitalization |
| What the exposure suggests | Broad large-cap U.S. equity exposure, still affected by the largest constituents | A narrower listing- and sector-screened slice of the market |
Both use market capitalization in their weighting approach, so larger eligible companies generally have more influence. The methods are not identical: the S&P 500 uses float-adjusted market-cap weighting, while the Nasdaq-100 uses modified market-cap weighting. As S&P DJI explains, an index methodology is a set of rules governing an index’s creation, calculation and maintenance. Different rules can produce different index results.
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Is the Nasdaq-100 just a technology index?
No. It often has substantial exposure to technology companies, but “technology index” is not its defining rule. Its defining screen is Nasdaq listing plus non-financial status, with selection based on company size and the methodology. The index also includes companies from other industry groups. It is therefore more precise to call it a Nasdaq-listed, non-financial-company benchmark than a pure technology index.
How concentrated is the S&P 500?
Although the S&P 500 covers a large share of available U.S. market capitalization, its market-cap weighting means the largest companies can account for a meaningful portion of its value. S&P DJI reported that the top ten constituents represented 37.8% of the index and the largest constituent represented 8.1% as of August 31, 2026. Those are dated provider figures, not permanent weights; concentration changes as prices and index membership change. The cited figures describe the S&P 500 only and are not a like-for-like concentration comparison with the Nasdaq-100.
Can you buy an index directly?
No. An index is a calculation used as a benchmark, not a security you can purchase. Investors can get index-linked exposure through products such as exchange-traded funds or mutual funds that track or use an index as a benchmark. The product, not the index itself, is what an investor buys.
Funds that follow the same index can still differ in fees, tracking behavior, structure and availability through a particular investment account. Check the fund’s current documents and expenses rather than assuming that an index name alone determines what you will pay or how closely a fund will follow its benchmark. S&P DJI’s index mathematics methodology and its discussion of index methodology explain why index rules matter to the results products may seek to track.
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Which index is the better fit?
Neither is automatically better; the relevant question is which market exposure you intend to hold. The S&P 500 is designed as a large-cap U.S. market gauge. The Nasdaq-100 is a narrower benchmark constrained by Nasdaq listing and non-financial status. Before choosing a fund linked to either, consider how that exposure fits with your other investments and compare the specific fund’s costs, tracking and account availability.
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