Key Takeaways
- An index fund is a mutual fund or ETF that tries to match the return of a market index, such as the S&P 500, instead of trying to beat it.
- ETFs trade on an exchange throughout the day at market prices. Index mutual funds are bought and sold once a day at the closing net asset value. For a long-term buyer, either works.
- The expense ratio is the yearly cost of owning the fund. In our hypothetical example, a 1.00% fee instead of 0.05% costs about $13,400 on a $10,000 investment over 30 years.
- An S&P 500 fund holds about 500 large U.S. companies, roughly 80% of the U.S. market's value. A total market fund adds thousands of mid-size and small companies. Both are reasonable core holdings.
- Index funds are not insured against losses and can fall sharply in a downturn. They are for money you will not need for at least several years.
Table of Contents
Index funds for beginners are usually the first recommendation you hear, and for good reason: one purchase gives you a slice of hundreds or thousands of companies, the costs can be very low, and there is no stock picking involved. But "buy an index fund" skips over the parts that actually trip beginners up. Which index? ETF or mutual fund? How much do fees matter? What do you click once the account is open?
This guide answers those questions in plain English, using the SEC's investor education materials as the reference point. We do not recommend specific funds. The goal is that you can look at any index fund and judge it yourself.
What Is an Index Fund?
The SEC's investor bulletin on index funds defines an index fund as a type of mutual fund or exchange-traded fund (ETF) that seeks to track the returns of a market index. A market index is just a list of securities chosen by a set of rules, plus a way of measuring how they perform together. The S&P 500, the Russell 2000 and total U.S. stock market indexes are common examples.
Instead of hiring analysts to pick the stocks they think will win, the fund simply buys what is in the index, in roughly the same proportions. If a company makes up 5% of the index, it makes up about 5% of the fund. This is called passive investing, as opposed to active management, where a manager tries to beat the market by buying and selling more often.
One distinction worth knowing: an index fund aims to match the index before fees. As the Investor.gov glossary puts it, the strategy is designed to achieve approximately the same return as the index before fees. Because the index itself has no costs, every index fund trails its index by at least its fees, plus a small amount of tracking error.
Why Beginners Use Index Funds
- Instant diversification. One fund spreads your money across hundreds or thousands of companies, so a single company's collapse is a small dent instead of a disaster.
- Low costs. Because a passive fund does not pay for stock research and trades less, it can often charge less. Investor.gov notes that passive management usually means less trading, lower realized capital gains and lower fees than actively managed funds.
- No forecasting required. You do not need to know which company or sector will do best. You own all of them.
- Simplicity. One or two funds can form a complete long-term stock portfolio, which makes it easier to keep investing steadily.
One caution from the SEC itself: not all index funds have lower costs than actively managed funds. "Index" describes the strategy, not the price. Always check the expense ratio.
ETF vs. Index Mutual Fund
The same index can be held through an ETF or a mutual fund, and many fund companies offer both versions. The SEC's bulletin on the characteristics of mutual funds and ETFs lays out the main difference: how and when you trade.
| Feature | Index ETF | Index Mutual Fund |
|---|---|---|
| How you trade | On a stock exchange, through a brokerage, any time the market is open | Directly with the fund (or through a brokerage), once per business day |
| Price you get | Market price at the moment you trade, which can be slightly above or below the fund's net asset value (NAV) | The NAV calculated at the end of the business day after you place the order |
| Minimum investment | Price of one share, or less at brokers that offer fractional shares | Set by the fund; some have none, others require a minimum initial purchase |
| Automatic investing | Depends on the brokerage | Widely available, by dollar amount |
| Typical fees | Expense ratio; 12b-1 fees typically do not apply | Expense ratio; some funds add 12b-1 or sales charges, so read the fee table |
For a beginner who buys and holds, the differences are small. If you like setting a fixed dollar amount to invest automatically each month, a mutual fund at the same company that runs it is often the easiest route. If you want to buy at any brokerage, or are investing in a taxable account, an ETF is often the more flexible choice. ETFs are frequently more tax-efficient in taxable accounts, though that depends on the specific fund. Inside an IRA, taxes on fund distributions do not matter year to year.
Expense Ratios: A Worked Fee Example
An expense ratio is the percentage of a fund's average net assets used each year to pay its operating expenses. You never see a bill; the cost is taken out of the fund's assets, which lowers your return. The SEC's bulletin on fund fees makes the key point: a fund with higher costs must perform better than a lower-cost fund just to give you the same return.
The yearly dollar cost is simple: on $10,000, a 0.05% expense ratio is $5 a year, while 1.00% is $100 a year. That gap looks small. Compounded over decades, it is not.
Example (hypothetical): You invest $10,000 once and leave it for 30 years. Both funds earn 6% a year before fees.
• Fund A, 0.05% expense ratio: net return 5.95% a year. After 30 years: about $56,600.
• Fund B, 1.00% expense ratio: net return 5.00% a year. After 30 years: about $43,200.
The difference is roughly $13,400, on a single $10,000 deposit, with both funds holding the same kind of investments. The 6% return is an illustration only, not a forecast, and real returns vary year to year.
With regular monthly contributions the gap gets bigger, because every new dollar pays the higher fee too. When you compare two funds that track the same index, the lower expense ratio is usually the deciding factor. FINRA's free Fund Analyzer tool, referenced in the SEC bulletin, lets you compare the cost of specific funds side by side.
Total Market vs. S&P 500
These are the two most common starting points, and beginners often agonize over the choice more than it deserves.
- S&P 500 index funds hold about 500 large U.S. companies. According to S&P Dow Jones Indices, the index covers approximately 80% of available U.S. market capitalization.
- Total U.S. stock market index funds hold those same large companies plus thousands of mid-size and small ones, covering nearly the entire investable U.S. market.
Both indexes are weighted by company size, so the large companies dominate either way. Because the S&P 500 makes up roughly four-fifths of the total market's value, the two tend to move closely together. A total market fund gives you a little more exposure to smaller companies, which can help or hurt in any given period. An S&P 500 fund is slightly more concentrated in the biggest names. Either is a reasonable core holding for a long-term investor. What matters more is picking one with a low expense ratio and sticking with it.
Neither fund includes international stocks or bonds. Many investors add a total international stock index fund for global diversification, and add bond funds as they get closer to needing the money. A target-date index fund packages all of this into one fund that gradually shifts toward bonds over time; check its expense ratio like any other fund.
What Index Funds Do Not Protect You From
Diversification protects you from one company failing. It does not protect you from the whole market falling. Broad stock indexes have had steep declines, and an index fund follows its index down as faithfully as it follows it up. The SEC also notes that an index fund has less flexibility than an active fund to react to falling prices, since it has to hold what is in the index.
Index funds are not bank deposits. The FDIC explicitly does not insure stocks or mutual funds, even when sold by a bank. At a SIPC-member brokerage, SIPC protects up to $500,000 (including $250,000 of cash) if the firm itself fails, but SIPC states plainly that it does not protect against a decline in the value of your investments.
That is why the usual order is: emergency fund first, high-interest debt paid down, then investing money you will not need for at least five years. If you are still building your safety net, start with our emergency fund guide.
How to Buy Your First Index Fund
- Choose the account. For retirement money, a Roth or traditional IRA (or your workplace 401(k)) gives you tax advantages. Our Roth IRA vs. traditional IRA guide explains which fits. For other long-term goals, a regular taxable brokerage account works.
- Open it at a low-cost brokerage or fund company. Look for no account minimum, no commissions on the funds you plan to buy, and access to low-cost index funds.
- Fund the account. Link your bank account and transfer the money. This step only moves cash into the account; it does not invest it.
- Pick the fund. Decide on the index first (S&P 500, total U.S. market, or a target-date fund), then compare the funds that track it. Check the expense ratio, any minimum investment and whether it is an ETF or mutual fund.
- Place the order. For a mutual fund, enter a dollar amount and it buys at that day's closing NAV. For an ETF, enter a number of shares or a dollar amount (if fractional shares are offered). A limit order, which sets the highest price you will pay, avoids surprises when prices are moving.
- Automate future contributions. Set a recurring transfer and, where available, an automatic investment on the same schedule. Regular investing matters more than timing.
If you are working with a small starting amount, our guide to investing your first $1,000 walks through this process with a specific example.
Common Beginner Mistakes
Leaving the money in cash
Transferring money into an IRA or brokerage account is not the same as investing it. If you skip the "buy" step, the money sits in a cash position. Check your holdings after your first deposit.
Ignoring the expense ratio
Two funds tracking the same index can charge very different fees. As the example above shows, the cheaper one usually wins over long periods.
Owning several funds that hold the same thing
An S&P 500 fund, a total market fund and a large-cap growth fund overlap heavily. Owning all three adds complexity without much extra diversification.
Selling in a downturn
The most expensive index fund mistake is behavioral: selling after a big drop and missing the recovery. Decide in advance that you will keep contributing through declines, and only invest money you will not need soon.
Confusing "index" with "safe" or "simple"
Some funds track narrow, custom-built or leveraged indexes. The SEC warns that leveraged and inverse ETFs are generally designed for short holding periods and can behave very differently from what a long-term investor expects. A beginner's core holding should be a broad, plain index.
Chasing last year's winner
Switching to whichever index fund or sector did best recently often means buying high. Pick a broad index, keep costs low, and let time do the work.
Frequently Asked Questions
Are index funds good for beginners?
For most beginners, yes. A broad index fund gives instant diversification across hundreds or thousands of companies, often at a very low cost, and requires no stock picking. They still rise and fall with the market, so they suit money you will not need for at least several years.
Should I buy an index ETF or an index mutual fund?
For a long-term investor, either is fine. ETFs trade throughout the day at market prices and can be bought at almost any brokerage. Index mutual funds trade once a day at the closing net asset value and make automatic dollar-amount investing easy. Compare expense ratios and choose whichever fits how you plan to invest.
What is a good expense ratio for an index fund?
Lower is better, and broad index funds are among the cheapest funds available. When two funds track the same index, the one with the lower expense ratio usually comes out ahead over time. In our hypothetical example, a 1.00% fee instead of 0.05% cost about $13,400 on $10,000 over 30 years.
Is an S&P 500 index fund the same as a total market fund?
No, but they are similar. An S&P 500 fund holds about 500 large U.S. companies, which make up roughly 80% of the market's value. A total market fund also holds thousands of mid-size and small companies. Because the large companies dominate both, their returns tend to be close.
Can you lose money in an index fund?
Yes. An index fund falls when its index falls, and broad stock indexes have had large declines. Index funds are not FDIC insured. Broad U.S. stock indexes have recovered from past declines over time, but past recoveries are no guarantee, which is why you should only invest money you will not need for several years.
How much money do I need to start investing in index funds?
Often very little. Many brokerages have no account minimums and offer fractional shares of ETFs, so you can start with a small amount. Some index mutual funds have a minimum initial investment, so check before you buy.
Conclusion
An index fund is the simplest way for a beginner to own a diversified slice of the stock market: you match the market instead of trying to beat it, and you keep more of the return by paying less in fees. Whether you choose an ETF or a mutual fund, an S&P 500 or total market index, matters less than three things: keeping the expense ratio low, investing regularly, and not selling when the market drops.
Before you buy, make sure the basics are in place with an emergency fund, choose the right account with our Roth vs. traditional IRA guide, and then follow the step-by-step walkthrough in how to invest your first $1,000.