How Index Funds Work and Why Investors Use Them
What an index fund is
An index fund is a mutual fund or ETF that holds the same stocks (or bonds) as a published market index, in the same proportions. When you buy shares of an index fund, you own a slice of all those holdings at once.
The most common example is a fund that tracks the S&P 500, which holds 500 large U.S. companies. If you buy the fund, you own a tiny piece of all 500 companies. The fund manager does not pick which stocks to include — the index itself defines that. The manager's job is simply to hold what the index holds and keep costs low.
Index funds exist because most actively managed funds — where a manager picks individual stocks — charge higher fees and often underperform the index they are measured against. An index fund gives you the market's return minus a small fee, rather than betting on a manager to beat the market.
Key Takeaways
- An index fund automatically holds all the stocks in a published index like the S&P 500, so you own the market rather than betting on individual stock picks.
- Index funds charge lower fees than actively managed funds because the manager simply copies an index instead of researching and selecting stocks.
- You can buy index funds as mutual funds (which trade once per day) or as ETFs (which trade throughout the day like stocks).
- Index funds come in many varieties — some track U.S. stocks, some track international stocks, some track bonds, and some track real estate or other assets.
How index funds differ from actively managed funds
An actively managed fund pays a manager to research companies, decide which ones to buy and sell, and try to beat the index. That research costs money, so the fund charges higher fees — often 0.5% to 1% or more of your investment per year. Even with that cost, most actively managed funds do not beat their index over long periods. Some do, but picking which ones will succeed in the future is difficult.
An index fund simply buys and holds the stocks in the index. The manager's only job is to track the index accurately and keep costs down. Fees are typically 0.03% to 0.20% per year. Because the fund is not trying to beat the market, it cannot underperform by much — you get close to the market's return, minus the small fee.
Over 20 or 30 years, that fee difference compounds. A 0.10% fee costs far less than a 0.75% fee, and the math works in the index fund's favor even if the actively managed fund occasionally picks winners.
Index funds as mutual funds versus ETFs
Index funds come in two legal structures: mutual funds and ETFs (exchange-traded funds). Both hold the same index, but they trade differently.
A mutual fund trades once per day, after the market closes. You place an order during the day, but you do not know the exact price until the close. You pay no commission to buy or sell (at most brokers), and you can set up automatic monthly investments easily. Mutual funds work well for long-term investors who are not trading in and out.
An ETF trades throughout the day like a stock. You see the price in real time and can buy or sell whenever the market is open. You may pay a small commission (though many brokers now offer commission-free ETF trades). ETFs are more flexible if you want to adjust your holdings quickly, but that flexibility costs a bit more in trading friction.
For most long-term investors, the choice between a mutual fund and an ETF version of the same index does not matter much. Pick whichever fits your broker and your habits.
Types of index funds and what they track
Index funds exist for nearly every market segment. The most common track U.S. stocks — the S&P 500 (large companies), the Russell 2000 (small companies), or the entire U.S. stock market. Others track international stocks in developed countries or emerging markets. Bond index funds track government bonds, corporate bonds, or a mix.
Specialty index funds track real estate (REITs), commodities, or specific sectors like technology or healthcare. Some track the entire world stock market in one fund. The index you choose depends on what part of the market you want to own and how much of your portfolio you want to allocate to it.
A common beginner approach is to buy one or two broad index funds — such as a U.S. stock index and an international stock index — and hold them for decades. That simplicity is part of why index funds appeal to many investors.
How costs affect your returns over time
Index funds charge an annual fee called an expense ratio, expressed as a percentage of your investment. A fund with a 0.10% expense ratio costs $10 per year on a $10,000 investment. A 0.75% expense ratio costs $75 on the same amount.
That difference seems small, but it compounds. Over 30 years, assuming 7% annual returns, $10,000 grows to roughly $76,000 in a 0.10% fee fund and roughly $55,000 in a 0.75% fee fund — a difference of $21,000 on the same starting amount. The lower-cost fund wins because you keep more of your gains.
When comparing index funds that track the same index, the one with the lowest expense ratio is usually the best choice. Different brokers and fund companies offer different versions of the same index, so it pays to compare.
Why investors choose index funds
Index funds appeal to investors for several reasons. First, they are simple — you own the market, not a bet on a manager's skill. Second, they are cheap — lower fees mean more of your money stays invested and compounds. Third, they are predictable — you know roughly what you will get (the index return minus the fee), with no surprises from poor stock picks.
Index funds also reduce the temptation to trade too much. Because you own the whole market, there is no reason to chase individual stocks or panic-sell when one sector drops. That discipline tends to improve returns over time.
For investors who do not have time or interest in researching individual companies, index funds offer a straightforward way to build wealth through the stock market.
Getting started with index funds
To buy an index fund, you need a brokerage account — an account with a company like Fidelity, Vanguard, Charles Schwab, or many others. Open the account, fund it with money, and search for the index fund you want. Most brokers let you search by index name (such as "S&P 500") and show you the available options, their fees, and their performance.
Choose the fund with the lowest expense ratio for the index you want, place your order, and the shares are yours. You can hold them for years without doing anything, or you can add to them regularly through automatic investments. Selling works the same way — place an order and the cash goes back to your account.
Many brokers also offer target-date funds, which are collections of index funds chosen for a specific retirement year. These funds automatically shift from stocks to bonds as you approach retirement, so they require even less decision-making.
Frequently Asked Questions
Can I lose money in an index fund?
Yes. If the index drops, your fund drops with it. Index funds own stocks, and stocks go up and down. Over long periods (10+ years), stock markets have historically recovered from drops, but there is no may provide. If you need the money soon, index funds may not be right for you.
Do I have to pick just one index fund?
No. Many investors own several index funds to spread their money across different parts of the market — for example, a U.S. stock index, an international stock index, and a bond index. This mix is called asset allocation, and it depends on your age, goals, and comfort with risk.
What is the difference between an index fund and an index ETF tracking the same index?
They hold the same stocks in the same proportions, so their returns are nearly identical. The main difference is how they trade: mutual funds trade once daily, ETFs trade throughout the day. Fees are usually similar. For most long-term investors, the choice does not matter much.
Can I use index funds in a retirement account?
Yes. Index funds work in IRAs, 401(k)s, and other retirement accounts. In fact, they are popular in retirement accounts because their low fees and simplicity make them ideal for long-term holding. Many 401(k) plans offer index fund options.
What if the index I want to track is not available at my broker?
Most major brokers offer index funds for the most common indexes. If your broker does not have the exact fund you want, you can usually find a similar one tracking the same index, or switch to a broker that does offer it. Vanguard, Fidelity, and Schwab all have broad selections.