An index fund is a collection of stocks or bonds that mirrors a specific market list

An index fund is a fund that holds the same stocks or bonds as a published market index. An index is simply a list — for example, the S&P 500 is a list of 500 large U.S. companies. An index fund that tracks the S&P 500 buys and holds those same 500 companies in the same proportions. When you own shares of that fund, you own a tiny piece of all 500 companies at once.

The fund manager does not pick which stocks to include or try to beat the market. Instead, they follow the index mechanically. If the index adds a company, the fund adds it. If a company leaves the index, the fund sells it. This is called passive management, and it is the defining feature of an index fund.

Index funds exist for nearly every market segment: U.S. large companies, U.S. small companies, international stocks, bonds, real estate, and combinations of these. You can buy index funds through a brokerage account, a retirement account like a 401(k) or IRA, or sometimes directly from the fund company.

Key Takeaways

  • An index fund holds the exact same stocks or bonds as a published index, in the same proportions, rather than trying to beat the market.
  • The fund manager follows the index mechanically and does not pick individual securities or time market moves.
  • Index funds charge lower fees than actively managed funds because there is less work involved in running them.
  • You can own an index fund through a brokerage, a retirement account, or directly from the fund company, and you can buy and sell shares any trading day.

How an index fund differs from an actively managed fund

An actively managed fund pays a manager to research companies, decide which ones to buy and sell, and try to outperform the market. That research and trading cost money. The fund charges higher fees — often 0.5% to 2% of your money per year — to pay for it.

An index fund does none of that. The manager simply buys the stocks on the index list and holds them. Fees are much lower, typically 0.03% to 0.20% per year. Over decades, that difference in fees compounds. A fund charging 1.5% per year will grow significantly less than an identical fund charging 0.10% per year, even if both hold the same stocks.

Actively managed funds sometimes beat their index in a given year, but most do not beat it over 10 or 20 years. The index fund, by definition, matches the index — it does not try to beat it and does not claim to.

What you actually own when you buy an index fund

When you buy shares of an S&P 500 index fund, you own a fractional stake in all 500 companies in that index. If the fund holds 1,000 shares of Apple and you own 0.001% of the fund, you own 0.001 shares of Apple (plus 0.001 shares of Microsoft, Coca-Cola, and 497 others).

You do not own the fund company itself. You own the fund — a pool of money that holds the stocks. The fund company (Vanguard, Fidelity, BlackRock, or another firm) manages the pool and keeps it aligned with the index. You receive dividends if the companies in the index pay them, and you can sell your shares anytime the market is open.

The index itself is not a fund and is not something you can buy directly. It is a list maintained by an index provider (S&P Dow Jones Indices, MSCI, Russell, or others). Many different fund companies offer funds that track the same index, so you can choose based on fees, account type, or other features.

Why fees matter more in index funds than in active funds

Because an index fund's only job is to match the index, the fund's performance depends almost entirely on its fees. A cheaper index fund will outperform a more expensive one tracking the same index, all else equal. This makes fee comparison straightforward: lower is better.

With an actively managed fund, a higher fee might be worth it if the manager's skill makes up for it. With an index fund, there is no skill to pay for — the manager is following a list. Paying 0.50% per year instead of 0.05% means giving up 0.45% of your returns every year for no added benefit.

Over 30 years, that 0.45% difference compounds into a substantial loss. If you invested $10,000 in two identical index funds with the same holdings but different fees, the cheaper one would have thousands of dollars more at the end.

How index funds are bought and sold

You buy index fund shares through a brokerage account (online or at a bank), a retirement account (401(k), IRA, or similar), or sometimes directly from the fund company. You place an order during market hours, and the trade settles in one or two business days. You can sell anytime the market is open.

Index funds are priced once per day, after the market closes. If you place an order to buy or sell during the day, you get that day's closing price, not the price when you placed the order. This is different from individual stocks, which trade throughout the day at changing prices.

Some brokerages charge a commission to buy or sell a fund; many do not. Some fund companies let you buy their funds directly with no commission. Check your brokerage's fee schedule before you trade.

Common index fund types and what they track

The most common index funds track U.S. stock indexes. The S&P 500 index fund holds 500 large U.S. companies. Total U.S. stock market index funds hold thousands of companies of all sizes. Small-cap index funds hold smaller companies. Each tracks a different published index.

International index funds track stocks outside the U.S. — developed markets like Europe and Japan, or emerging markets like Brazil and India. Bond index funds track government or corporate bonds. Some funds combine stocks and bonds in a single fund, tracking a blended index.

Real estate index funds track real estate investment trusts (REITs). Sector index funds track specific industries like technology, healthcare, or energy. The index provider publishes the rules for each index, and any fund company can offer a fund that tracks it.

Frequently Asked Questions

Do I get voting rights if I own an index fund?

Yes, but the fund company usually votes on your behalf. When companies in the index hold shareholder votes, the fund company casts votes based on its proxy voting policy. You can see how most fund companies vote on their websites, but you cannot override their votes for individual holdings.

What happens if a company leaves the index?

The fund sells that company's stock and buys the company that replaces it on the index. This happens automatically and is part of the fund's normal operation. You do not have to do anything — the fund manager handles it.

Can I lose money in an index fund?

Yes. If the stocks or bonds in the index fall in value, your fund falls in value too. Index funds do not protect you from market downturns. They simply give you broad exposure to the market at low cost.

Is an index fund the same thing as an ETF?

No, but many ETFs are index funds. An ETF is a type of fund that trades like a stock throughout the day. An index fund is a fund that tracks an index. Some index funds are ETFs; some are mutual funds that price once per day. The two terms describe different things.

How often does the index change?

It depends on the index. The S&P 500 changes a few times per year when companies are added or removed. Some indexes change more often; others change rarely. The index provider publishes the rules for when and how changes happen.