What Is an Index? How Market Indexes Work

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An index is a number that tracks the combined value of a defined group of assets, such as stocks or bonds, so that the performance of a market or market segment can be followed with a single figure. The S&P 500, for example, condenses the prices of 500 large U.S. companies into one level that rises and falls as those companies’ share prices change. An index is a measurement, not a product. It is a hypothetical portfolio built from published rules, and you cannot buy it directly. Investors get exposure through funds and derivatives that are designed to follow it.

The term also covers economic, currency and lending benchmarks: any standardized number, built by a published method, that shows change over time.

How an Index Is Calculated

Every index has an owner, usually an index provider or exchange, and a methodology document. That document defines four things:

• Universe: which securities are eligible (for example, U.S.-listed large companies, investment-grade bonds or small-cap stocks).
• Selection rules: how constituents are chosen and when they are added or removed.
• Weighting: how much each constituent counts toward the index’s movement.
• Base value and divisor: the starting level the index is set to on its launch date, and the number used to keep the series consistent over time.

The index level is the weighted value of the constituents divided by a divisor. When an index launches, the divisor is chosen so the index starts at a round base, such as 100 or 1,000. After that, the divisor is adjusted whenever something changes that is not a market move, such as a stock split, a spinoff or a constituent replacement. Without that adjustment, a stock split would make the index fall even though no investor lost money.

The Three Main Weighting Methods

Weighting has the most influence on how an index behaves. The three standard approaches are:

1. Market-capitalization weighting. Each company counts in proportion to its market value (share price times shares outstanding). Larger companies move the index more. The S&P 500 uses this method, adjusted for float, meaning it counts only shares available for public trading rather than those held by insiders or governments.
2. Price weighting. Each company counts in proportion to its share price, regardless of company size. A $300 stock moves the index three times as much as a $100 stock for the same percentage change. The Dow Jones Industrial Average is the best-known price-weighted index.
3. Equal weighting. Every constituent gets the same weight at each rebalancing date, so the smallest company matters as much as the largest. This is sometimes called an unweighted index.

Worked Example: One Stock Rises 10%

The hypothetical index below has three stocks. The figures are invented to show the mechanics.

Assume Stock A rises 10% to $110 and the other two prices do not change.

• Cap-weighted version: total market value goes from $4.0 billion to $4.1 billion, a gain of 2.5%. Stock A held a quarter of the index, so a 10% move in A moves the index by a quarter of 10%.
• Price-weighted version: the sum of prices goes from $175 to $185, a gain of about 5.7%. Stock A has the highest price, so it dominates even though it is not the largest company.
• Equal-weighted version: each stock is one third of the index, so the gain is about 3.3%.

The same market event produces three different index returns. When two indexes covering similar companies diverge on a given day, weighting is often part of the explanation.

The divisor works like this in the price-weighted version. Suppose the index launched at 100, so the divisor was set at 1.75 ($175 ÷ 1.75 = 100). If Stock A later splits 2-for-1, its price halves to $50 and the sum of prices drops to $125. To keep the index at 100, the provider lowers the divisor to 1.25 ($125 ÷ 1.25 = 100).

Other Construction Choices

Providers also build indexes weighted by fundamentals such as revenue or dividends, indexes that cap any single constituent’s weight, and indexes that select stocks by factors such as value or low volatility. Bond indexes are usually weighted by the market value of debt outstanding, so the borrowers that have issued the most debt carry the most weight.

Two more choices affect what the number means:

• Price return vs. total return. A price return index reflects only price changes. A total return version assumes dividends or coupon payments are reinvested. Over long periods the gap between the two can be large for stock indexes, so comparing a fund’s total return with an index’s price return overstates the fund’s performance.
• Rebalancing and reconstitution. Rebalancing resets weights (essential for equal-weighted indexes, which drift as prices move). Reconstitution changes the membership list. Both happen on schedules set by the methodology, and funds tracking the index must trade around those dates.

Reading an Index Level

The absolute level of an index says little on its own. What matters is the percentage change. A hypothetical index launched at 1,000 and now at 6,500 is 6.5 times its starting value, but that says nothing about whether today was a good or bad day. A 65-point drop is a 1% decline at 6,500 and a 10% decline at 650.

Point moves also cannot be compared across indexes. A 300-point move in the Dow and a 300-point move in the S&P 500 are very different percentage changes because the two indexes sit at different levels and use different methods. For comparisons over months or years, percentage change over a matched period is the standard approach; year-over-year change is one common format.

How Indexes Are Used

Benchmarks for Performance

The most common use is as a yardstick. A U.S. large-cap stock fund typically compares its return with the S&P 500; a core bond fund, with the Bloomberg US Aggregate Bond Index. If a fund returned 8% while its benchmark returned 10%, the investor would have done better simply owning the market segment. A benchmark must match what the portfolio holds: measuring a small-cap fund against a large-cap index says little about the manager’s skill.

Passive Investing and Index Funds

Indexing is a passive strategy. An index fund holds the constituents of an index (or a representative sample) in their index weights, aiming to match the index’s return rather than beat it. Index funds come as mutual funds and exchange-traded funds (ETFs); the Vanguard S&P 500 ETF (VOO) and the SPDR S&P 500 ETF Trust (SPY) both track the S&P 500.

With no research team selecting securities and little trading beyond index changes, index funds generally charge lower fees than actively managed funds. S&P Dow Jones Indices’ SPIVA reports have found that most active managers trail their benchmark over long periods. Fees and trading costs still leave an index fund’s return slightly below the index; the gap is called tracking difference.

Trading Index Derivatives

Traders also take positions on indexes through futures, options and, in some markets, contracts for difference (CFDs) whose value follows an index such as the S&P 500, Nasdaq 100, FTSE 100 or Nikkei 225. These products let a trader go long or short on a whole market in one position, often with leverage, which magnifies both gains and losses.

Indexes Outside the Stock Market

• Economic indexes measure prices or activity. The Consumer Price Index tracks the cost of a basket of goods and services and is the basis for reported U.S. inflation rates.
• Currency indexes such as the U.S. Dollar Index (DXY) track one currency against a weighted basket of others.
• Adjustable-rate loans set their interest rate as an index plus a margin. Many U.S. adjustable-rate mortgages now use the Secured Overnight Financing Rate (SOFR). As a hypothetical example, with SOFR at 3% and a 2% margin, the rate would be 5%, resetting at each adjustment subject to any caps.
• Indexed annuities credit returns based on an index but typically cap them, and many track the price return only, so dividends are excluded.

Major Stock and Bond Indexes

In the United States, the most widely followed stock indexes are:

• S&P 500: about 500 large U.S. companies, float-adjusted market-cap weighted, covering roughly 80% of available U.S. equity market value. It is the most common U.S. stock benchmark.
• Dow Jones Industrial Average: 30 large U.S. companies, price-weighted. It is the oldest major U.S. average but the narrowest.
• Nasdaq Composite: all Nasdaq-listed stocks, heavily weighted to technology. The Nasdaq 100 covers the 100 largest non-financial companies listed there.
• Russell 2000: the usual benchmark for small-cap U.S. stocks.
• Wilshire 5000 Total Market Index: a broad measure of the whole U.S. stock market.

Internationally, the FTSE 100 tracks large UK-listed companies, the Nikkei 225 is a price-weighted measure of Japanese stocks, and the MSCI EAFE Index covers developed markets outside the U.S. and Canada.

For bonds, the Bloomberg US Aggregate Bond Index tracks the U.S. investment-grade bond market, including Treasuries, corporate bonds and mortgage-backed securities. Emerging Markets Bond Indexes track government debt issued by developing economies.

Limitations of Indexes

• Concentration. In a cap-weighted index, a handful of the largest companies can account for a large share of the return, so an index with hundreds of members can be less diversified than its member count suggests.
• Selection effects. Companies that shrink or fail are removed and growing companies are added, so a long-term index history describes a changing group, not the same companies throughout.
• Not an investable return. The index return excludes fees, taxes and trading costs.
• Methodology differences. Two indexes with similar names can use different rules, so “U.S. small caps” or “global stocks” can mean different things depending on the provider.

Index, Index Fund and Related Terms

Index vs. index fund. The index is the rulebook and the number it produces. The index fund is the investable product that buys the securities to follow it. A fund can be closed, change benchmarks or charge fees; the index itself has no assets.

Index vs. sector or screen. A sector index follows a defined industry by fixed rules. A custom list of stocks filtered by your own criteria, for example with a stock screener, is a watchlist, not an index, unless it is maintained under consistent published rules.

Indexes vs. indices. Both plurals are correct and mean the same thing. “Indexes” is more common in U.S. financial writing, while “indices” is widely used in British English and among traders and brokers, especially for index CFD and futures markets.

Indexing vs. indexed. “Indexing” in investing means the passive strategy of tracking an index. “Indexed” describes something whose value is linked to an index, such as an indexed annuity, an inflation-indexed bond or a pension benefit adjusted by a price index.