What Is a Market Index? How Stock Indexes Work

Search Fintrev

Search our guides, reviews and glossary.

A market index is a hypothetical portfolio of securities, built and maintained by an index provider under published rules, whose value is reported as a single number that tracks the performance of a market or segment of it. A stock index such as the S&P 500 does not trade by itself. It is a calculation whose value rises and falls with the combined prices of the stocks (or bonds, currencies or commodities) it is designed to represent.

How a Market Index Works

Every index starts with a methodology document. It defines three things:

1. Eligibility and selection. Which securities qualify, such as by exchange listing, country, size, sector or bond maturity, and whether a committee applying published criteria (as with the S&P 500 and the Dow) or a mechanical rule, such as ranking every eligible company by market cap at a set date, chooses the final list.
2. Weighting. How much influence each component has on the index value.
3. Maintenance. How often the list is reviewed (reconstitution), how weights are reset (rebalancing), and how corporate actions such as stock splits, spinoffs and mergers are handled so they do not distort the number. Components that no longer qualify, for example after shrinking, being acquired or delisting, are dropped at the next review or immediately when needed.

The provider then calculates the index value from component prices, often continuously during trading hours for major stock indexes. Because the rules are published, anyone can see what an index includes and why a company was added or removed.

Calculating a market index

Most indexes are a weighted average of their components. The weighting choice changes the result considerably, as this example shows.

Hypothetical example: three-stock index

Price-weighted. Add the share prices and divide by a divisor. With a starting divisor of 3: (200 + 50 + 20) / 3 = 90.

• If Stock A rises 10% to $220, the index becomes 290 / 3 = 96.67, a gain of about 7.4%.
• If Stock B rises 10% instead, to $55, the index becomes 275 / 3 = 91.67, a gain of about 1.9%.

The highest-priced stock dominates, even though it is the smallest company by market value.

Market-cap-weighted. Divide the current total market cap by the total at a base date, then multiply by a chosen base value such as 100. Here the starting total is $10 billion, so the index starts at 100.

• If Stock A rises 10%, its cap becomes $2.2 billion, the total becomes $10.2 billion and the index reads 102, a 2% gain.
• If Stock B rises 10%, its cap becomes $5.5 billion, the total becomes $10.5 billion and the index reads 105, a 5% gain.

Now the largest company carries the most weight. Many cap-weighted indexes, the S&P 500 among them, count only float-adjusted shares, meaning those available to public investors, rather than all shares outstanding.

Equal-weighted. Each component gets the same weight at each rebalance, so a 10% move in any of the three stocks moves the index by about 3.3% until weights drift apart.

The divisor. A price-weighted index needs a divisor that changes over time. Suppose Stock A splits 2-for-1 and its price falls to $100. Nothing about the companies’ value has changed, but the sum of prices drops to 170. To keep the index at 90, the provider resets the divisor to 170 / 90 ≈ 1.889. Divisors also adjust when components are swapped, which is why the divisor of a long-running price-weighted index bears no resemblance to the count of its stocks.

Interpreting a market index

The level of an index, whether 90 or 40,000, mostly reflects its base value and history, so comparing levels across indexes says nothing useful. What matters is percentage change over a period: a day, a month or a year-over-year comparison. A 500-point move means something very different on an index at 5,000 than on one at 40,000.

Check also what kind of return is being quoted. A price return index reflects price changes only. A total return version of the same index assumes dividends are reinvested. Over long periods the gap between the two can be substantial, so comparing a fund’s total return against a price-only index flatters the fund.

Types of Market Indexes

Indexes are grouped by what they cover:

• Broad market: large portions of a country’s stock market, such as the S&P 500 or the Russell 1000.
• Size-based: mid-cap and small-cap segments, such as the S&P MidCap 400, the Russell Midcap, the Russell 2000 and the S&P SmallCap 600.
• Sector and theme: a single industry or theme, such as the Nasdaq-100 Technology Sector Index or indexes built around clean energy, blockchain or robotics.
• International and regional: the FTSE 100 for large companies listed in London, the price-weighted Nikkei 225 for Japan, and indexes for Europe or emerging markets.
• Fixed income: bond markets, sliced by issuer type, credit quality or maturity, such as U.S. and global aggregate bond indexes.
• Other assets: currencies and commodities. The U.S. Dollar Index, for example, measures the dollar against a basket of foreign currencies.

Major Stock Indexes

In the United States, three indexes are quoted most often:

• Dow Jones Industrial Average: 30 large U.S. companies, price-weighted. It is the oldest and most frequently cited U.S. index.
• S&P 500: about 500 leading U.S. companies chosen by a committee and weighted by float-adjusted market cap. It covers a much larger cross-section of the economy than the Dow.
• Nasdaq Composite: nearly all common stocks listed on the Nasdaq exchange, cap-weighted and heavily tilted toward technology.

How Indexes Are Used

Benchmarks. A fund’s prospectus names a benchmark index, and performance reports compare the fund against it. That comparison is the basic test of an active manager: a large-cap U.S. fund that trails the S&P 500 after fees has delivered less than a passive alternative. Benchmarks can also factor into how fund managers are evaluated and paid.

Index funds and ETFs. An individual cannot buy an index directly, and buying hundreds of components separately is costly and impractical. Index funds and ETFs solve this by replicating the index, holding all or most of its constituents in roughly the same weights and trading to match the new list when components change. They still charge an expense ratio, but it is generally much lower than fees for actively managed funds.

Market reading. An index shows the direction of a whole segment without examining each stock. Analysts also apply valuation measures to entire indexes, such as an index-level P/E ratio built from aggregate earnings.

Allocation. Investors use segment indexes to judge which parts of the market have led or lagged and to set their exposure to size, sector or region.

Limitations of a Market Index

• Weighting distorts the picture. A price-weighted index lets a high-priced stock outweigh a larger company. A cap-weighted index can become concentrated in a handful of the largest companies, so a “broad” index may move mostly on a few names.
• Selection is a choice. Committee-based indexes involve judgment, and even rule-based ones reflect decisions about eligibility. Removing companies that shrink and adding growing ones also means the current list overstates how a fixed basket would have performed.
• Coverage gaps. The Dow’s 30 stocks or a single-country index can diverge sharply from the wider market.
• Index returns are not investor returns. An index carries no fees, taxes or trading costs. A fund tracking it lags slightly, and the difference is known as tracking error.

Index vs. Related Terms

• Index vs. average: the Dow is still called an “average,” but because of its divisor it is no longer a simple average of its stock prices.
• Indexes vs. indices: both are accepted plurals. “Indices” is more common in trading and outside the U.S.

The Bottom Line

Anyone comparing funds, reading market news or choosing index funds and ETFs needs to know how an index is built and weighted. Judge an index by its percentage change rather than its level, and check whether a quoted return includes reinvested dividends. Since an index carries no fees, taxes or trading costs, expect a fund that tracks it to lag slightly.