In investing, an index is a standardized measurement that condenses the performance of a group of securities into a single number. When someone asks what indices are in investing, the short answer is that they are formulas: a specified list of stocks, bonds, or other assets, combined by a specified weighting method, producing one figure that moves up or down as the underlying components move. The S&P 500 tracks 500 large U.S. companies and covers roughly 80% of available domestic stock market capitalization.1S&P Global. S&P 500 That one number is what news anchors quote when they say the market was up or down on the day.
What an Index Is For
An index does two jobs. First, it summarizes. Instead of tracking thousands of individual securities, you look at one figure that represents the weighted average performance of everything inside it. Second, it benchmarks. A fund that returned 9% sounds good until you learn its benchmark returned 12%. That comparison separates skill from luck and gives investors a baseline for what a passive strategy would have delivered.
One point worth internalizing early: you cannot buy an index directly. An index is a mathematical construct, not a product. When people say they “invest in the S&P 500,” they mean they bought a fund built to replicate the index’s performance. Related, but not the same thing.
How Indices Are Weighted
The weighting method decides which components drive the number. Two indices with identical members but different weighting schemes can produce meaningfully different returns over the same period. Three approaches dominate.
Price-Weighted
A price-weighted index adds up the share prices of its components and divides by a special number called a divisor. A stock trading at $300 has roughly ten times the influence of a stock trading at $30, no matter how large or small the underlying companies actually are. The Dow Jones Industrial Average works this way.2S&P Dow Jones Indices. Dow Jones Industrial Average So does Japan’s Nikkei 225.3Nikkei Inc. Nikkei Stock Average Index Guidebook
Market-Capitalization-Weighted
Market-cap weighting assigns influence based on a company’s total market value: current share price multiplied by shares outstanding.4S&P Global. Methodology Matters – Section: Weighting A $2 trillion company moves the index far more than a $20 billion one. The S&P 500 uses a variation called float-adjusted market-cap weighting, which counts only shares available for public trading rather than all shares in existence.5S&P Global. S&P U.S. Indices Methodology This is the most common method globally. The trade-off is concentration risk: when a handful of mega-cap firms grow rapidly, they can dominate the index to the point where the other components barely register.
Equal-Weighted
An equal-weighted index gives every component the same importance. The smallest company has as much say as the largest.6Deutsche Börse Group. Whitepaper: Understanding Equal-Weighted Investment Strategies These indices rebalance on a schedule, usually quarterly, resetting each stock back to the same target weight.7S&P Global. Equal-Weight Indexing: One-Stop Shopping for Size and Style Between rebalances, winners drift higher and get trimmed back at the next reset. More diversified, but you’re regularly trimming winners and adding to laggards.
Price Return Versus Total Return
Most index levels quoted in headlines are price return figures, which track only changes in stock prices. Dividends are ignored. A total return version of the same index reinvests dividends back into the calculation, producing a higher number over time that more accurately reflects what an investor actually earned.
The gap matters more than most people realize. Dividends historically account for a meaningful share of long-run stock returns. When you compare a fund’s performance to “the S&P 500,” check whether the comparison uses the price return or total return version. Comparing a dividend-reinvesting fund against a price-only benchmark makes the fund look better than it actually performed against a true passive strategy.
Indices You’ll Hear Quoted
The Dow Jones Industrial Average is a price-weighted index of 30 large U.S. blue-chip companies, covering all industries except transportation and utilities.2S&P Dow Jones Indices. Dow Jones Industrial Average Launched in 1896, it remains the most frequently quoted benchmark on cable news despite being far narrower than modern alternatives.
The S&P 500 tracks 500 large publicly traded companies and covers approximately 80% of available U.S. market capitalization, which is why professionals use it as the default proxy for the domestic stock market.1S&P Global. S&P 500
The Nasdaq Composite includes over 3,000 securities listed on the Nasdaq exchange and skews heavily toward technology and growth companies. Being broad and tech-heavy, it tends to be more volatile than the S&P 500 and is often watched as a proxy for the technology sector.
Outside the U.S., the FTSE 100 covers the 100 most highly capitalized companies on the London Stock Exchange, weighted by market capitalization.8LSEG. FTSE UK Index Series The Nikkei 225 plays a similar role for Japan, though it uses price weighting.3Nikkei Inc. Nikkei Stock Average Index Guidebook
Indices Beyond Stocks
Equity indices get the most attention, but indices exist for almost every asset class. Fixed-income indices track bond performance across maturities and credit qualities; the Bloomberg U.S. Aggregate Bond Index is the standard benchmark for domestic investment-grade bonds. Commodity indices track prices of physical goods like gold, oil, and agricultural products, and they respond to supply shocks, geopolitics, and weather rather than corporate earnings. Sector indices isolate one industry from the broader market, so you can compare, say, technology against healthcare without the noise of everything else.
How Indices Change Over Time
An index is not a fixed list. Providers periodically add and remove components through a process called reconstitution and adjust weightings through rebalancing. These updates usually happen quarterly or semi-annually.
During reconstitution, the provider reviews whether each current member still meets eligibility criteria such as minimum market capitalization, trading volume, or profitability. Companies that no longer qualify are replaced. The S&P 500 uses a committee that selects replacements based on published criteria rather than a purely mechanical formula. When a stock is added to a major index, demand for its shares tends to spike as index funds buy in, which is one of the more practical consequences of reconstitution for individual investors.
How You Actually Invest in an Index
Since you can’t buy an index directly, two vehicles do the job of replicating one.
Index Mutual Funds
An index mutual fund holds the same securities in the same proportions as its target benchmark. The goal is to match the index’s return as closely as possible; the difference between fund and index performance is called tracking error. These funds are regulated under the Investment Company Act of 1940.9Legal Information Institute. 17 CFR Part 270 – Rules and Regulations, Investment Company Act of 1940 They price once per day at market close, so any order you place executes at that closing figure.
Exchange-Traded Funds
ETFs pursue the same goal but trade on exchanges throughout the business day, like individual stocks.10Nasdaq. ETFs Under the Hood: The Primary and Secondary Markets You can buy or sell an ETF at 10:30 in the morning at the market’s current price, whereas a mutual fund order placed at the same time would execute at that afternoon’s close. ETF shares trade at market-determined prices that can occasionally differ slightly from the fund’s actual net asset value.11FINRA. Memorandum to Address NASD Rules on Secondary Trading in Shares of Exchange-Traded Funds Registered Under the Investment Company Act of 1940
Why Index Investing Has Taken Over
The argument for indexing comes down to two numbers. The average expense ratio for an index equity mutual fund runs around 0.05%, compared with roughly 0.64% for an actively managed equity fund. That gap compounds. On a $500,000 portfolio over 30 years, the fee difference alone can cost six figures in lost growth.
Performance data reinforces the fee point. According to the SPIVA Global Scorecard published by S&P Dow Jones Indices, approximately 90% of active equity fund managers underperformed their benchmark index over a 10-year horizon. That’s not a one-off year; the pattern has held for as long as S&P has been tracking it. Higher fees and lower odds of beating the benchmark together explain why index funds and ETFs have attracted trillions of dollars over the past two decades. For most people saving for retirement, matching the market cheaply beats trying to outperform it expensively.