What is an index fund and how does it work?

An index fund is a type of mutual fund or exchange-traded fund (ETF) that holds a portfolio of stocks or bonds designed to match the performance of a specific financial market index, such as the S&P 500. Instead of trying to pick winning stocks, an index fund simply aims to mirror the index's returns. This approach is called passive investing because the fund manager does not make frequent buy or sell decisions. For a beginner, index funds offer a low-cost way to own a slice of the entire market without having to research individual companies.

What exactly is a market index?

A market index is a list of stocks or bonds that represents a particular segment of the market. For example, the S&P 500 tracks the performance of about 500 of the largest U.S. companies. The Dow Jones Industrial Average follows 30 major companies. Indexes are not funds themselves; they are just measurements. An index fund takes that list and buys the same securities in the same proportions, so the fund's value rises and falls with the index. This is why index funds are often described as diversified investments: they spread your money across many companies and industries at once.

How an index fund works step by step

When you put money into an index fund, the fund provider pools your cash with other investors' money. The fund then buys the stocks or bonds that make up the target index. For a fund tracking the S&P 500, it will hold shares of all 500 companies, with larger companies making up a bigger portion of the portfolio. This is called market-cap weighting. The fund does not need a team of analysts to decide what to buy; it simply follows the index.

Indexes change over time. Companies may be added or removed, and their market values shift. The fund automatically adjusts its holdings to stay aligned with the index. Because these changes are infrequent, index funds have low turnover, which means fewer trading costs and lower expense ratios compared to actively managed funds. An actively managed fund tries to beat the market by picking stocks, which requires more research and trading, and those costs are passed on to investors.

Key terms to know

• Index fund: A fund that mirrors a market index.
• Mutual fund: A pooled investment vehicle that buys a diversified portfolio.
• ETF: An exchange-traded fund, which trades on stock exchanges like individual stocks but also tracks an index.
• Expense ratio: The annual fee charged by the fund, expressed as a percentage of your investment.
• Diversification: Spreading investments across many assets to reduce risk.
• Passive management: A hands-off approach that follows an index rather than trying to beat it.

A worked example: Priya's S&P 500 index fund

Let's say Priya invests $10,000 in an S&P 500 index fund with an expense ratio of 0.10%. Over one year, the S&P 500 index returns 8%. The fund's performance will closely match that, minus the fee. So the fund's gross return is 8% of $10,000, which is $800. The fee is 0.10% of $10,000, which is $10. Priya's net gain is $790, giving her an ending balance of $10,790.

Now compare that to an actively managed fund with a 1.0% expense ratio that also returns 8% before fees. The fee would be $100, leaving Priya with $10,700. Over many years, that difference compounds. The power of compound interest means lower fees can significantly boost long-term returns.

The risks of index funds

Index funds are not risk-free. They rise and fall with the market. If the overall stock market drops, an S&P 500 index fund will drop too. There is no protection against market downturns. Also, index funds are tied to the specific index they track. A fund tracking only U.S. large-cap stocks will miss out on gains in international markets or small companies. Diversification within an index reduces company-specific risk, but it does not eliminate market risk. Investing always carries the risk of losing money, and past performance does not guarantee future results.

What people get wrong about index funds

• Mistake 1: Thinking an index fund is the same as an index. An index is just a list; an index fund is a product you can buy. You cannot invest directly in the S&P 500, but you can buy a fund that tracks it.
• Mistake 2: Assuming all index funds are identical. Two funds tracking the same index can have different expense ratios, tracking errors, and tax efficiencies. Always compare fees and performance history.
• Mistake 3: Believing index funds always outperform active funds. While many active funds fail to beat their benchmark over long periods, some do. Index funds aim to match the market, not beat it. In a down market, they lose value just like the index.
• Mistake 4: Ignoring the impact of fees. Even a 0.5% difference in expense ratio can cost thousands of dollars over decades. For example, a 1% fee on a $100,000 portfolio over 30 years can reduce your ending balance by over $30,000 compared to a 0.1% fee, assuming a 7% annual return.

Putting it together

Index funds offer a simple, low-cost way to invest in the market. They work by mirroring a specific index, providing broad diversification, and keeping fees low through passive management. While they carry market risk, they are a popular choice for long-term investors. For more on how compounding can affect your investments, see our compound interest calculator. And if you're saving for retirement, learn about how compound interest works in retirement accounts.

For further reading, the SEC's Investor.gov explains mutual funds and ETFs. The Federal Reserve publishes data on interest rates, and the IRS sets retirement contribution limits.