Index Funds Explained: How They Work and How to Start
Quick answer: An index fund buys a small piece of every company in a list, like the 500 companies in the S&P 500, so you own many stocks at once for a low yearly fee. To start, open a retirement or brokerage account, pick one broad low-cost index fund, and set up a small automatic monthly deposit. Its value goes up and down with the market, so you can lose money.↗ Share on X
An index fund is a single investment that buys a small piece of every company in a list, called an index. For example, an S&P 500 index fund owns shares of the 500 large U.S. companies in the S&P 500. You don't pick stocks; the fund simply copies the list. That makes index funds cheap, simple, and spread out across many companies, which is why they are a popular first investment. To start, you open a brokerage or retirement account, choose a broad, low-cost index fund, and set up an automatic monthly deposit, even a small one.
How does an index fund work, step by step?
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1. A company like S&P Dow Jones Indices or FTSE Russell creates and maintains a list of companies (the index).
2. A fund company (for example Vanguard, Fidelity, Schwab, or iShares) creates a fund that copies that list.
3. You buy shares of the fund. Your money is mixed with money from many other investors.
4. The fund uses that money to buy all the stocks or bonds on the list.
5. When the index changes, the fund changes too. You don't have to do anything.
Because nobody is paid to research and pick stocks, the fund's yearly cost is usually low. That yearly cost is called the expense ratio. Many broad index funds charge less than 0.10% a year, which is $10 or less per year for every $10,000 invested. Actively managed funds, where a manager picks stocks, often charge much more.
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What kinds of index funds can you choose from?
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This content is informational and is not investment advice or financial consulting.
Not all index funds are the same. They follow different lists. Here are the most common types:
| Type of index fund | What it owns | Common examples of indexes |
|---|---|---|
| Large U.S. companies | About 500 of the biggest U.S. companies | S&P 500 |
| Total U.S. stock market | Thousands of U.S. companies, big and small | CRSP US Total Market, Wilshire 5000 |
| International stocks | Companies outside the U.S. | MSCI EAFE, FTSE Global All Cap ex US |
| Total world stock | U.S. and international companies together | FTSE Global All Cap, MSCI ACWI |
| U.S. bonds | Loans to the U.S. government and companies | Bloomberg U.S. Aggregate Bond |
| Target-date funds | A mix of stock and bond index funds that gets safer as a chosen year gets closer | Usually labeled with a year, like 2055 |
A "broad" fund owns many companies across the whole market. A "narrow" fund owns one industry, like technology or energy. For beginners, broad funds are simpler and less risky than narrow ones, because one industry having a bad year hurts you less.
Index fund vs ETF: what is the difference?
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Retirement Savings Gap Before 40: The 4-Step Math to Check →You will see both names. They can follow the same index. The difference is mostly in how you buy them.
| Index mutual fund | Index ETF (exchange-traded fund) | |
|---|---|---|
| How you buy it | Directly from the fund company or a broker | On the stock market, like a stock |
| When the price is set | Once a day, after the market closes | All day long, while the market is open |
| Minimum to start | Some require $1,000 to $3,000; some have no minimum | The price of one share, or less if your broker sells fractional shares |
| Automatic monthly deposits | Usually easy to set up | Depends on the broker |
For a beginner who plans to invest a fixed amount every month and leave it alone, either one works. Pick the one your account makes easiest.
What are the real benefits, and what are the risks?
Benefits:
- Diversification. One purchase gives you hundreds or thousands of companies. If one company fails, it is a small part of your money.
- Low cost. Low expense ratios mean more of your money stays invested.
- Simple. You don't need to research companies or watch the news every day.
- Easy to automate. You can invest the same amount every payday without thinking about it.
Risks (read these carefully):
- The value goes up and down. Index funds follow the market. When the market drops, your fund drops too. Stock index funds have fallen sharply during past crashes and taken time to recover.
- No promise of profit. Past results do not tell you what will happen next. You can lose money, especially over short periods.
- You get the average, not the best. An index fund will never beat the market it copies. It aims to match it, minus its small fee.
- Narrow funds can swing more. A fund that owns only one industry or one country can rise or fall much more than a broad fund.
Because of these risks, money you will need in the next few years (for rent, a car, or an emergency) usually belongs in a savings account, not in stock index funds.
How do you start investing in index funds?
Follow these steps in order.
Step 1: Cover the basics first. Before investing, pay off high-interest credit card debt and keep an emergency fund in a savings account. Investing while carrying debt at 20% interest rarely makes sense.
Step 2: Check your workplace plan. If your job offers a 401(k) or 403(b) with a match (your employer adds money when you contribute), contributing at least enough to get the full match is often the first place to start. Look at the plan's fund list for words like "Index," "S&P 500," "Total Market," or "Target Date."
Step 3: Choose the account type. If you don't have a workplace plan, or you want to invest more, you can open:
- an IRA (individual retirement account), Roth or traditional, which has tax benefits but limits on how much you can put in each year and rules about taking money out;
- a regular brokerage account, which has no special tax benefits but lets you take money out anytime.
The yearly IRA limits change, so check the current number on IRS.gov.
Step 4: Pick one broad, low-cost fund. Compare these three things on the fund's page:
1. What it tracks. A total market, S&P 500, total world, or target-date fund is a common beginner choice.
2. Expense ratio. Lower is better. Compare similar funds side by side.
3. Minimum investment. Make sure you can meet it.
Step 5: Set up automatic deposits. Choose an amount you can keep up every month, even $25 or $50. Investing the same amount on a schedule is called dollar-cost averaging. You buy more shares when prices are low and fewer when prices are high, and you avoid trying to guess the right moment.
Step 6: Leave it alone and check once or twice a year. Watching the balance every day makes it tempting to sell during a drop, which is when many people lock in losses.
What mistakes do beginners make with index funds?
- Buying too many funds that own the same thing. An S&P 500 fund and a total U.S. market fund overlap a lot. Owning both doesn't add much diversification.
- Chasing last year's winner. A narrow fund that jumped last year can fall just as fast.
- Ignoring fees. Two funds can track the same index with very different expense ratios. Over decades, that difference adds up.
- Selling in a panic. Market drops are normal. Selling after a big drop turns a temporary loss on paper into a real one.
- Forgetting about taxes. In a regular brokerage account, selling at a profit can create a tax bill. Retirement accounts have different rules.
When should you talk to a professional?
Index funds are simple, but your situation may not be. Consider talking to a fee-only fiduciary financial planner (someone who is legally required to act in your best interest and is paid by you, not by commissions) if you:
- are close to retirement and need to decide how much risk to take;
- received a large sum, like an inheritance or a settlement;
- have a complicated tax situation or own a business;
- feel unsure whether to pay down debt or invest.
For tax questions, a certified public accountant (CPA) or enrolled agent can help.
Your next step
Log in to your workplace retirement plan or open the website of a large brokerage. Find one broad index fund, either a total market, S&P 500, or target-date fund, and write down three things: what index it tracks, its expense ratio, and its minimum investment. Compare it with one similar fund from another company. Once you know which one is cheaper and fits your account, set up an automatic monthly deposit for an amount you can keep paying even in a tight month.
This article is general education, not personal investment advice.
FAQ
Can you lose money in an index fund?
Yes. An index fund follows the market, so when the market falls, the fund falls too. Losses are more likely over short periods, which is why money you need soon usually belongs in savings.
What is the difference between an index fund and an ETF?
Both can track the same index. An index mutual fund is priced once a day and bought from the fund company or a broker; an ETF trades on the stock market all day like a stock.
How much money do I need to start investing in index funds?
It depends on the fund. Some mutual funds ask for $1,000 to $3,000, others have no minimum, and many brokers sell fractional ETF shares, so you can often start with $25 to $50.
What is an expense ratio?
It is the yearly fee a fund charges, shown as a percentage of your money. An expense ratio of 0.10% costs $10 a year for every $10,000 invested.
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Educational content, not personalized financial advice. Sources cited where applicable.