Index Funds for Beginners

An index fund is a mutual fund or exchange-traded fund (ETF) that aims to match the performance of a market index, such as the S&P 500, rather than trying to beat it. Instead of paying a manager to pick investments, you own a slice of everything in the index. That makes index funds simple, broadly diversified and often inexpensive, which is why they’re a common starting point for new investors.

They’re not risk-free, though. Below, you’ll learn how index funds work, what to compare when choosing one, how much fees can matter over decades, and the risks to understand before you invest.

What is a market index?

A market index is a measuring stick for a group of investments. It tracks the performance of a “basket” of stocks or bonds meant to represent part of the market. You can’t invest in an index directly, but as the SEC’s investor education office explains, index funds provide an indirect way to invest in one.

Some common types of indexes:

Most stock indexes are weighted by market capitalization, meaning bigger companies make up a bigger share of the index. That’s worth knowing: in a cap-weighted index, a handful of very large companies can drive a sizable part of the results.

How index funds work

An index fund buys the securities in its target index, either all of them or a representative sample, and adjusts its holdings when the index changes. Because it isn’t trying to outsmart the market, it trades less often than a typical actively managed fund.

That passive approach has a few practical advantages:

• Lower costs. No team of analysts picking stocks generally means lower management expenses.
• Built-in diversification. One fund can spread your money across hundreds or thousands of companies, so a single company’s trouble has a limited effect.
• Transparency. You know what you own: whatever is in the index.
• Less trading. Lower turnover can mean lower trading costs and, in taxable accounts, fewer capital gains distributions.

The SEC also cautions that not all index funds have lower costs than actively managed funds, so you still need to check.

Broad-market vs. S&P 500 index funds

Two of the most common choices for U.S. stocks are funds tracking the S&P 500 and funds tracking the total U.S. stock market.

• S&P 500 funds hold large U.S. companies. Because those companies make up most of the U.S. market’s total value, these funds often perform similarly to total-market funds.
• Total-market funds add mid-size and small companies. That gives you broader diversification, though small companies are a modest share of a cap-weighted index.

Neither is “right.” The bigger decisions are usually how much you put in stocks vs. bonds, whether you add international exposure, and keeping costs low.

Why expense ratios matter so much

Every fund charges an annual fee called the expense ratio, expressed as a percentage of your investment. It’s taken out of the fund’s assets automatically, so you never see a bill, but it reduces your return.

In dollar terms, the fee seems tiny. On a $10,000 investment:

• A 0.05% expense ratio costs about $5 a year.
• A 0.75% expense ratio costs about $75 a year.

Over decades, though, the difference compounds. The SEC notes that fees that seem small can have a major impact on your portfolio over time.

A worked example: two funds, same index

Suppose you invest $400 a month for 30 years, a total of $144,000 in contributions. Imagine two funds tracking the same index with a hypothetical 7% average annual return before fees. Returns are never guaranteed, and real markets don’t deliver a steady 7%; this example simply isolates the effect of cost.

To keep it simple, we subtract each fund’s expense ratio from the return and compound monthly:

Both funds hold the same investments, but Fund B’s higher fee leaves you with about $61,633 less after 30 years. You’d have paid that difference for nothing extra.

You can test your own contribution, timeline and rate in the compound interest calculator. To see why the gap widens so much in later years, read our guide on how compound interest works.

Where to find a fund’s costs and details

Before you buy, look up the fund’s prospectus or fact sheet and check:

1. Expense ratio. Lower is generally better for funds tracking the same index.
2. The index it tracks. Make sure it’s the market segment you actually want.
3. Tracking difference. How closely the fund’s returns have matched its index over time.
4. Minimum investment and any trading costs or account fees.
5. Fund type. Mutual fund or ETF, which affects how you buy and sell it.

The risks of index funds

The SEC’s bulletin is clear that index funds involve risk like any investment. Key ones include:

• Market risk. An index fund falls when its index falls. A stock index fund can lose a large share of its value in a downturn, and recoveries can take years.
• Lack of flexibility. The fund keeps holding what’s in the index even when prices are dropping.
• Tracking error. The fund may not match its index exactly.
• Underperformance. After fees and trading costs, a fund will typically trail its index slightly.
• Concentration. In a cap-weighted index, a few large companies or one sector can dominate.

Because of these risks, money you’ll need in the next few years, like your emergency fund or a home down payment, generally belongs somewhere more stable, such as a savings account.

How beginners commonly get started

1. Build your foundation first. Have an emergency fund and a plan for high-interest debt; paying off a card charging over 20% is a guaranteed saving, while market returns are not.
2. Pick an account. Index funds are available in workplace retirement plans, IRAs and regular taxable brokerage accounts. Tax-advantaged accounts can help your money grow more efficiently; our guide to Roth vs. traditional IRAs explains the options.
3. Choose a simple mix. Decide how much goes to stocks and bonds based on your timeline and how you’d feel during a big drop.
4. Automate contributions. Investing a fixed amount on a schedule builds the habit and means you buy at a range of prices.
5. Leave it alone. Selling in a panic during a downturn locks in losses. Review your mix about once a year rather than reacting to headlines.

The bottom line

Index funds give you a low-cost, diversified way to own a broad slice of the market without picking individual investments. The key things to compare are the index a fund tracks and its expense ratio, because, as the example shows, a fee difference of 0.70 percentage points could cost you over $60,000 across 30 years of steady investing. Remember that index funds still rise and fall with the market, so use them for long-term goals and keep short-term money somewhere safer.

This content is for educational purposes only and isn't financial advice. Calculator results are estimates based on the numbers you enter. For decisions about your situation, consider talking with a qualified professional. Read our full disclaimer.