What an index fund does

An index fund is a collection of stocks or bonds that mirrors a specific market index — a list of companies or securities grouped by a rule. The fund buys and holds the same investments in the same proportions as the index it tracks, so when the index moves up or down, your fund moves with it. You own a small piece of every holding in the fund, which means you own a small piece of many companies at once instead of betting on a few.

The most common index funds track the S&P 500, which is 500 large U.S. companies chosen and weighted by market value. Other funds track the total U.S. stock market, international stocks, bonds, or combinations of all three. Because the fund straightforward buys what the index contains and holds it, the costs are usually much lower than funds where a manager picks stocks by hand.

Key Takeaways

  • Index funds hold all or most of the securities in a published market index, so your returns match the index's performance minus a small fee.
  • The fund rebalances when the index changes — selling companies that drop out and buying companies that enter — but you do not have to do anything.
  • You pay an expense ratio, a yearly percentage fee that covers the fund's operating costs, which is typically 0.03% to 0.20% for index funds.
  • Index funds are bought and sold through a brokerage account, and you can set up automatic monthly contributions if your brokerage offers it.
  • Your returns depend entirely on how the index performs; index funds do not try to beat the market, only match it.

How the fund buys and holds securities

When you buy shares of an index fund, your money goes into a pool with other investors' money. The fund manager uses that pool to purchase the securities that make up the index. If the index has 500 companies, the fund buys shares in all 500, usually in the exact proportion they represent in the index. A company that makes up 2% of the index's total value will make up roughly 2% of the fund's holdings.

The fund holds these securities continuously. It does not try to time the market or sell when prices rise. Instead, it sits with the holdings and collects any dividends the companies pay. Those dividends are either reinvested into more shares of the fund or paid out to you, depending on the fund type and your account settings.

What happens when the index changes

Market indexes are not static. Companies are added and removed based on the index's rules. When the S&P 500 adds a new company, the index fund must buy shares in that company. When a company is removed, the fund sells those shares. This process is called rebalancing, and it happens automatically — you do not have to do anything.

Rebalancing also occurs when the market value of holdings shifts. If one company's stock price rises sharply, it may represent a larger percentage of the index than it did before. The fund will sell some of that company's shares and buy more of others to restore the correct proportions. Again, this is handled by the fund manager, not by you.

The costs you pay

Index funds charge an expense ratio, a yearly fee expressed as a percentage of your investment. For example, a fund with a 0.10% expense ratio costs $10 per year for every $10,000 you have invested. This fee covers the fund's operating costs — staff, trading, record-keeping, and regulatory compliance. The expense ratio is deducted automatically from the fund's returns, so you never write a check for it.

Index funds typically charge between 0.03% and 0.20% per year, though some charge less and some charge more. The difference matters over time. On a $50,000 investment, a 0.05% fund costs $25 per year, while a 0.50% fund costs $250 per year. Over decades, that difference compounds. You can find the expense ratio in the fund's prospectus or on your brokerage's website.

How your returns work

Your return from an index fund comes from two sources: the change in the fund's share price and any dividends paid out. If you buy 100 shares at $50 each and the fund's value rises to $55 per share, your shares are now worth $5,500 — a $500 gain. If the fund pays a dividend of $0.50 per share, you receive $50, which you can take as cash or reinvest into more shares.

Your total return is the sum of these gains and dividends, minus the expense ratio. Because index funds aim to match their index, not beat it, your returns will be slightly lower than the index itself — the difference is the expense ratio. If the S&P 500 rises 10% in a year and your S&P 500 index fund charges 0.10%, your fund will return approximately 9.90%.

Where and how to buy index funds

Index funds are bought and sold through a brokerage account — an account with a company like Fidelity, Vanguard, Charles Schwab, or many others. You open an account, link a bank account or transfer money in, and then search for the index fund you want by its ticker symbol or name. Once you find it, you place an order to buy a specific number of shares or a specific dollar amount.

Most brokerages let you set up automatic monthly contributions, which means money transfers from your bank account to your brokerage on a schedule you choose, and is automatically invested in the funds you select. This is called dollar-cost averaging and removes the pressure of trying to time the market. You can also make one-time purchases whenever you have money to invest.

Index funds versus actively managed funds

An actively managed fund employs a manager or team to research companies and decide which to buy and sell, trying to outperform the market. These funds charge higher expense ratios — often 0.50% to 1.50% or more — because they pay for research and trading. Index funds, by contrast, straightforward hold what the index contains and trade only when the index changes, keeping costs low.

Over long periods, most actively managed funds do not beat their index after fees are paid. This is why many investors choose index funds: lower costs, predictable returns that match the market, and no need to monitor a manager's performance. Index funds work best for investors who want broad market exposure without frequent trading or high fees.

Frequently Asked Questions

Do I own actual shares of the companies in an index fund?

Yes, you own a fractional share of each company in the index. If the fund holds 500 companies and you own one share of the fund, you own 1/500th of a share in each company (simplified — the actual math depends on the fund's structure). You do not own them directly; the fund holds them on your behalf. You receive voting rights and dividends proportional to your ownership.

What if the index fund I want is not available at my brokerage?

Most major brokerages carry index funds from multiple providers, but not all funds are available everywhere. You can search your brokerage's fund list by index name or ticker symbol. If a specific fund is not available, your brokerage likely offers a similar fund tracking the same index. For example, if you cannot find the Vanguard S&P 500 fund, Fidelity or Schwab will have their own S&P 500 index fund with comparable costs.

Can I lose money in an index fund?

Yes. If the index declines, the fund's value declines with it. During market downturns, index funds can lose 20%, 30%, or more of their value. However, index funds are designed for long-term holding. Historically, the stock market has recovered from every downturn and reached new highs over periods of 10 years or more. Selling during a downturn locks in losses; holding through it has historically led to gains.

How often should I check my index fund balance?

You can check it as often as you like, but frequent checking often leads to emotional decisions. Most investors benefit from checking quarterly or annually. If you have set up automatic monthly contributions, you can straightforward let them run without monitoring. The fund's value will fluctuate with the market, and that is normal and expected.

What is the difference between an index fund and an ETF?

An exchange-traded fund (ETF) is a type of index fund that trades like a stock — you can buy and sell it throughout the day at changing prices. A traditional index mutual fund is priced once per day after the market closes. Both track an index and have low costs, but ETFs offer more flexibility in timing your trades. For most investors holding long-term, the difference is small.