Educational content only. This article is for educational purposes only and does not constitute investment advice. Nothing here should be construed as a recommendation to purchase, hold, or dispose of any security. See our full compliance disclaimer.

Stock Market Indexes Explained: S&P 500, Dow Jones, and NASDAQ Composite

How indexes are built, what they measure, and how investors use them

Published 2026-04-19 · Back to Learning Hub

What Is a Stock Market Index?

A stock market index is a statistical measure that tracks the performance of a defined group of stocks. Rather than describing the price of a single security, an index aggregates price information across many securities into a single number, allowing investors, analysts, and policymakers to assess the general direction of a market or market segment at a glance.

The index level itself is an abstract number — it has no inherent dollar value. What matters is the change in that number over time: a 10% increase in an index level means the collective value of the included stocks rose by approximately 10% over the measurement period, subject to the specific weighting methodology.

Index construction requires answering three fundamental questions:

Which securities are included? An index may cover all stocks in a given market, stocks meeting specific size criteria, stocks in a particular sector, or stocks selected by a committee.
How are they weighted? The weighting methodology determines how much influence each constituent has on the overall index level. The most common approaches are market-capitalization weighting and price weighting, covered in detail below.
How and when is it rebalanced? Index providers periodically review and update the constituent list and the weightings to reflect market changes.

For a glossary of terms used in this article, visit the investing glossary. To explore individual US stocks included in major indexes, see the stocks directory.

The Big Three US Indexes

Three indexes dominate financial news coverage and serve as the primary benchmarks for the US equity market. Each was designed with a different purpose, covers a different set of companies, and uses a different weighting methodology.

S&P 500 (Standard & Poor's 500)

The S&P 500is widely regarded as the most representative benchmark of the US large-cap equity market. It was introduced in its current form in 1957 by Standard & Poor's (now part of S&P Global) and covers approximately 500 of the largest US-listed companies by float-adjusted market capitalization.

Weighting methodology:The S&P 500 uses float-adjusted market-capitalization weighting. Each company's weight in the index is proportional to its float-adjusted market cap — that is, the market value of shares that are publicly available for trading, excluding shares held by insiders, strategic investors, or governments. A company worth $3 trillion will have roughly ten times the index weight of a company worth $300 billion.

Constituent selection:Membership is not purely mechanical. The S&P Index Committee applies criteria including US domicile, a minimum float-adjusted market cap (updated periodically), positive as-reported earnings over the most recent four quarters, and adequate liquidity. The committee meets regularly and makes changes as needed; there is no fixed rebalancing schedule, though quarterly reviews are common.

Significance:Trillions of dollars in passive investment products — index funds and ETFs — track the S&P 500. It is also the most widely cited benchmark by actively managed US equity funds. For a deeper dive, see our article S&P 500 Explained.

Dow Jones Industrial Average (DJIA)

The Dow Jones Industrial Averageis the oldest surviving US stock market index, first published by Charles Dow and Edward Jones in 1896 with just 12 components. Today it tracks 30 large, publicly owned US companies selected to broadly represent the US economy. The word "industrial" is a historical artifact — the index now includes companies from finance, technology, healthcare, retail, and other sectors far beyond its manufacturing origins.

Weighting methodology: The DJIA is price-weighted. A stock's influence on the index is determined entirely by its share price, not its total market capitalization. A stock trading at $400 has four times the index weight of a stock trading at $100, regardless of the two companies' respective sizes. To account for stock splits and other corporate actions that change share prices without changing the underlying business value, the DJIA uses a continuously adjusted divisor (the "Dow divisor") rather than summing raw prices.

Limitations of price weighting: Critics note that price weighting is an economically arbitrary methodology. A company can affect the Dow more by having a high nominal share price than by being a larger or more economically important business. For this reason, most modern index construction favors market-cap weighting.

Significance:Despite its methodological limitations, the Dow remains a fixture of financial media coverage and serves as a quick-read indicator of large-cap US stock performance. Many investors track it alongside the S&P 500 for historical continuity.

NASDAQ Composite

The NASDAQ Compositetracks virtually all common stocks listed on the NASDAQ exchange — more than 3,000 companies at any given time. Unlike the S&P 500 or the Dow, it does not apply a selective screening process; inclusion is based primarily on listing venue. Because NASDAQ historically attracted technology, biotechnology, and growth-oriented companies, the Composite has a heavy weighting toward the technology sector.

Weighting methodology: The NASDAQ Composite is market-capitalization weighted. Larger companies have proportionally more influence on the index level. The index includes both domestic US companies and international companies that are listed on the NASDAQ exchange.

NASDAQ-100 distinction: The NASDAQ Composite is frequently confused with the NASDAQ-100, a separate index that tracks the 100 largest non-financial companies listed on NASDAQ. The NASDAQ-100 is the basis for the widely traded QQQ ETF and excludes financial companies, making it even more technology-concentrated than the Composite.

Significance:Because of its technology weighting, the NASDAQ Composite tends to exhibit higher volatility than the S&P 500 and can diverge significantly during periods of sector-specific stress or euphoria (as was evident during both the dot-com bubble and its aftermath in 2000–2002, and during subsequent technology cycles).

Index Weighting Methodologies

The weighting methodology is one of the most consequential design choices in index construction. It determines which companies most influence the index's daily moves. The two methodologies used by the major US indexes are price weighting and market-cap weighting.

Price Weighting

The index level is calculated by summing the share prices of all constituent stocks and dividing by an adjusted divisor. A stock with a higher share price has more influence on the index level. The DJIA is the primary example of this approach.

Drawback:Share price alone does not reflect a company's economic size. Two companies with identical share prices have identical index influence even if one has ten times the market capitalization of the other. Stock splits, which reduce share price without changing underlying value, mechanically reduce a company's weight in a price-weighted index.

Market-Cap Weighting

Each constituent's weight is proportional to its market capitalization (or float-adjusted market cap). Larger companies have larger index weights, reflecting their greater share of the aggregate market. The S&P 500, NASDAQ Composite, and most modern indexes use this approach.

Consideration:Market-cap weighting means the index becomes increasingly concentrated in the largest companies as they grow. When a small number of very large companies account for a significant fraction of the index, the index's performance can be heavily influenced by a handful of stocks.

Other weighting approaches exist — including equal weighting (each stock has the same influence regardless of size) and fundamental weighting (based on financial metrics such as earnings or dividends) — but these are primarily associated with factor-based or "smart beta" strategies rather than mainstream market benchmarks.

Other Important US Indexes

Beyond the Big Three, several other US indexes are widely tracked by investors, economists, and fund managers.

Russell 2000

Maintained by FTSE Russell, the Russell 2000 tracks approximately 2,000 small-cap US companies — specifically, the 2,001st to the 4,000th largest US stocks by market cap (as of the most recent annual reconstitution). It is the most widely referenced small-cap US benchmark. Because small-cap companies tend to be more domestically focused and more sensitive to economic cycles, the Russell 2000 is often analyzed alongside the large-cap S&P 500 to gauge the breadth of US market performance. If large-caps are rising but small-caps are lagging, analysts may interpret this as a sign of narrow market leadership.

Wilshire 5000

The Wilshire 5000 Total Market Index was designed to represent the performance of the entire US equity market — all publicly traded US companies with available price data. Despite its name, it has included fewer than 5,000 stocks in recent years as the number of publicly listed US companies declined from its peak in the 1990s. It is sometimes called the "total market index" and is used by economists as a broad measure of aggregate US equity market value. Some total market index funds use the Wilshire 5000 or a similar all-market index as their benchmark.

VIX (CBOE Volatility Index)

The VIX, published by the Chicago Board Options Exchange (CBOE), is a measure of the stock market's expectation of 30-day forward volatility derived from the implied volatilities of S&P 500 index options. Unlike equity indexes, the VIX does not measure price levels or returns — it measures market-implied uncertainty. A reading below 15 is generally associated with low volatility and investor complacency; a reading above 30 is historically associated with elevated stress. The VIX peaked above 80 during the 2008–2009 financial crisis and again reached elevated levels during the March 2020 market dislocations.

For sector-level performance, see the sectors page, which tracks how each GICS sector of the S&P 500 is performing relative to the broader market.

Key International Indexes

Equity markets operate globally, and each major financial center has its own benchmark indexes. The following are among the most closely watched by international investors.

Index	Country / Region	Constituents	Weighting
FTSE 100	United Kingdom	100 largest London Stock Exchange companies	Market-cap weighted
Nikkei 225	Japan	225 large Tokyo Stock Exchange companies	Price-weighted
DAX 40	Germany	40 largest Frankfurt Stock Exchange companies	Market-cap weighted (free float)
MSCI World	23 developed markets	~1,400 large and mid-cap stocks	Market-cap weighted
MSCI Emerging Markets	24 emerging markets	~1,200 large and mid-cap stocks	Market-cap weighted

The FTSE 100(pronounced "Footsie") is maintained by the FTSE Group and updated quarterly. It is heavily weighted toward international revenue earners — many of its constituents generate most of their revenues outside the UK — so it does not always reflect UK domestic economic conditions. The Nikkei 225 uses the same price-weighted methodology as the DJIA and is often criticized for the same methodological reasons. The DAX is notable for being a total return index in its headline calculation, meaning dividends are assumed to be reinvested — a difference from the price-return methodology of many other country indexes.

How Indexes Are Used: Benchmarking, ETFs, and Futures

Indexes serve multiple functions across the financial ecosystem.

Benchmarking

The most fundamental use of an index is as a benchmark— a performance standard against which an investment portfolio or fund is evaluated. A US large-cap equity fund is typically benchmarked against the S&P 500; a small-cap fund against the Russell 2000. If a fund returns 8% in a year when its benchmark returns 10%, the fund is said to have underperformed by 2 percentage points, or to have had negative "alpha." Benchmark comparison is the standard methodology used by the investment management industry, regulators, and individual investors to assess fund manager performance.

Index Funds and ETFs

An index fund is a mutual fund or ETF designed to replicate the performance of a specific index by holding the same securities in the same proportions. Because index funds do not require active stock selection, they typically carry lower expense ratios than actively managed funds. The growth of passive investing — primarily through index funds and ETFs — has been one of the most significant structural shifts in the US investment industry over the past three decades. Trillions of dollars in investor assets are now allocated through passive index-tracking vehicles.

Futures and Derivatives

Major equity indexes are the underlying reference for a large and liquid market in index futures and options. S&P 500 futures (traded as E-mini and Micro E-mini contracts on the CME) allow institutional and sophisticated retail participants to gain or hedge equity exposure without holding the underlying stocks. Index futures also trade around the clock, providing a real-time indicator of sentiment before regular equity market hours. Financial media frequently cite futures levels ("S&P 500 futures are up 0.3%") as a proxy for anticipated market direction.

Index Investing Explained

Index investing — also called passive investing — refers to the strategy of building a portfolio designed to match the composition and performance of a market index rather than attempting to select individual securities that will outperform the market.

The intellectual foundation for index investing comes from the Efficient Market Hypothesis (EMH), associated with economist Eugene Fama, which posits that publicly available information is already reflected in stock prices. If markets are efficient, consistently beating the market through stock selection becomes extremely difficult, and the costs of attempting to do so (management fees, transaction costs, and tax drag) become a significant drag on net returns.

Empirical research, including long-running studies by S&P Global through its SPIVA (S&P Indices Versus Active) reports, has consistently found that the majority of actively managed US equity funds underperform their benchmark indexes over long measurement periods after accounting for fees. This finding has driven the large-scale shift toward passive investing observed in the US market since the 1990s.

For investors considering compound growth over time, the CAGR calculator can help illustrate how different rates of return compound over various time horizons.

It is important to note that index investing does not eliminate investment risk — an investor holding an S&P 500 index fund experiences the full drawdown of the US large-cap market during downturns. Index investing eliminates stock-specific (idiosyncratic) risk through diversification but retains broad market (systematic) risk.

Total Return vs. Price Return

Most published index levels are price return versions — they reflect only capital appreciation or depreciation of constituent stocks. Dividends paid by those stocks are excluded from the calculation. This is the version most commonly quoted in financial media.

A total return version of the same index assumes that all dividends paid by constituent companies are reinvested back into the index on the ex-dividend date. Over long time periods, the difference between total return and price return can be substantial.

Why the Distinction Matters for Investors

S&P 500 index funds and ETFs collect dividends and either distribute them to shareholders or reinvest them (in accumulation share classes). Their performance reflects total return behavior.
When comparing a fund's performance against a benchmark, using the total return index is the appropriate apples-to-apples comparison — a fund that collects and reinvests dividends should be compared against the total return index, not the price return index.
The S&P 500 total return index has historically produced higher long-run returns than the price return index by a meaningful margin due to dividend compounding, though the dividend yield of the index has declined considerably from its historical averages.

Index Reconstitution and Rebalancing

Indexes are not static. They are periodically reviewed and updated to reflect changes in the underlying market. Two related processes govern this: reconstitution and rebalancing.

Reconstitution

Reconstitutionrefers to changes in the list of constituent securities — additions and deletions. For the S&P 500, reconstitution occurs on an as-needed basis, with the Index Committee meeting to review candidates when a vacancy arises (due to a merger, delisting, or failure to meet eligibility criteria) or when a particularly compelling candidate warrants a review.

The Russell indexes operate differently: the Russell 2000 and Russell 1000 undergo a comprehensive annual reconstitution each June, in which all US stocks are re-ranked by market cap and index memberships are reset. This scheduled reconstitution is one of the most active single trading periods in US equity markets, as hundreds of stocks move in and out of the indexes simultaneously, driving significant buying and selling by index tracking funds.

Rebalancing

Rebalancing refers to adjustments in the weights assigned to existing constituents. For market-cap-weighted indexes that use float-adjusted weighting, the weights of individual stocks change daily as prices fluctuate — no periodic rebalancing is needed in the traditional sense. However, some indexes impose weighting caps (for example, capping any single stock at 25% of the index) and must rebalance periodically to enforce those caps.

Index effect:When a stock is announced as a new addition to a major index, it frequently experiences a price increase in the days between the announcement and the effective date, as index funds purchase shares in anticipation of needing to hold the stock. This phenomenon is sometimes called the "index effect" and has been documented extensively in academic finance literature.

Frequently Asked Questions

What is the difference between the S&P 500 and the Dow Jones Industrial Average?

The S&P 500 tracks 500 large US companies and weights them by float-adjusted market capitalization, so larger companies have a proportionally larger influence on the index. The Dow Jones Industrial Average tracks only 30 large US companies and is price-weighted, meaning a stock's influence is determined by its share price rather than the company's total market value. As a result, a high-priced stock with a relatively small market cap can have an outsized effect on the Dow, while the S&P 500 is considered a broader and more representative measure of the US large-cap equity market.

Can individual investors invest directly in an index?

No. An index is a mathematical benchmark, not a financial product. Investors cannot purchase the S&P 500 index itself. Instead, they can gain exposure to the index's performance through financial products that track it — primarily index mutual funds and exchange-traded funds (ETFs). These products hold the securities in the index (or a representative sample) and aim to replicate the index's returns before fees. The fund's expense ratio and tracking error determine how closely the fund's returns mirror the index.

What is the VIX and how is it related to stock market indexes?

The VIX (CBOE Volatility Index) measures the market's expectation of 30-day volatility in the S&P 500, derived from the prices of S&P 500 options contracts. It is sometimes called the 'fear gauge' because it tends to spike during periods of market stress. Unlike equity indexes, the VIX reflects implied (forward-looking) volatility rather than the level or performance of underlying stocks. Investors can gain VIX exposure through futures and options products, though these instruments are complex and behave differently from equity index products.

What happens during index reconstitution?

Index reconstitution is the periodic process by which the index committee reviews and updates the index's constituent stocks. Companies that no longer meet the index criteria (due to declining market cap, delistings, mergers, or other factors) are removed and replaced with qualifying candidates. For indexes like the S&P 500, reconstitution events can move individual stock prices significantly — stocks added to a major index often see buying pressure as index funds must purchase shares to maintain their tracking, while removed stocks may experience selling pressure for the same reason.

What is the difference between total return and price return for an index?

A price return index measures only the capital appreciation (or depreciation) of its constituent stocks — it does not account for dividends. A total return index assumes that dividends paid by index constituents are reinvested back into the index. Over long periods, the difference between total return and price return can be substantial, because reinvested dividends compound over time. When evaluating the performance of an ETF or mutual fund against a benchmark, it is important to confirm whether you are comparing against the price return or total return version of the index, as the choice significantly affects the comparison.