An index fund is a mutual fund or ETF that tries to track a market index instead of trying to pick winning investments. The goal is to match the performance of a chosen index, not beat it.
An index is a predefined basket of investments that measures part of a market. The S&P 500, for example, tracks 500 large U.S. companies. You can’t invest directly in an index, but an index fund gives you exposure to it by holding the same assets in the same proportions.
Examples Of Indexes
Common indexes include:
- S&P 500: tracks 500 large U.S. companies across most industries
- Russell 2000: tracks about 2,000 smaller U.S. companies (“small-cap” stocks)
- Total U.S. stock market indexes: try to cover the entire U.S. stock market, including small, mid, and large companies
- International stock indexes: cover companies in other countries (like developed markets in Europe and Japan, or emerging markets like Brazil and India)
- Bond market indexes: track baskets of government or corporate bonds
Different indexes track different parts of the market. An S&P 500 index fund isn’t the same as a total world stock index fund. If you invest in both thinking they’re the same, you may end up with more overlap than you expect, the S&P 500 companies appear in both.
Why People Like Index Funds
Index funds are popular because they can offer:
- Diversification: a single fund can hold hundreds or thousands of investments
- Lower costs: no team of analysts trying to beat the market, so fees tend to be much lower
- Simple strategy: buy the whole market, don’t try to outsmart it
- Broad market exposure: you capture the overall market’s return, not just a few picks
- Less trading than many active funds: lower turnover generally means fewer taxable events in a brokerage account
Lower cost matters more than it sounds. A fund with a 1% annual fee costs significantly more over 30 years than one charging 0.05%, even if their pre-fee returns are identical. The math here ties directly to How Compound Interest Works, small differences in costs compound just like returns do, but working against you.
Index Funds vs Actively Managed Funds
| Index Fund | Actively Managed Fund | |
|---|---|---|
| Goal | Match the index | Beat a benchmark |
| Management | Passive, follow the index rules | Active, managers make picks |
| Expense ratio | Usually very low (0.03%–0.20%) | Usually higher (0.50%–1.5%+) |
| Performance vs market | Generally matches market, minus fees | Often lags market after fees over time |
| Transparency | Holdings are predictable and published | Holdings change frequently |
| Taxes in taxable accounts | Generally more efficient | Higher turnover can create tax bills |
Research consistently shows that most actively managed funds underperform their benchmark index after fees over the long run. It’s not that the managers are bad, costs are just hard to overcome.
Index Funds Still Have Risk
An index fund can lose money. If the market it tracks falls, the fund falls too. A broad stock index fund might drop 30–40% in a serious market downturn. That’s normal volatility over long periods, but it matters a lot if you need the money soon.
An index fund also may not be as diversified as it sounds if the underlying index is narrow. A fund tracking one sector is very different from a fund tracking thousands of companies. A “technology index fund” is still a concentrated bet on one industry. Understanding what investment risk really means can help you pick the right type of index fund for your goals.
Mutual Fund Or ETF?
Index funds can be mutual funds or ETFs. The index strategy is separate from the fund structure.
When comparing index funds, look at:
- The index tracked: make sure you know what market or segment it covers
- Expense ratio: lower is almost always better for index funds
- Holdings: confirm the fund actually holds what you expect
- Tax considerations: ETF index funds are generally more tax-efficient in taxable accounts
- Minimum investment: some mutual fund versions require a minimum initial investment; ETF versions often don’t
- Brokerage fees: some platforms offer commission-free trading for certain funds
For a side-by-side look at ETFs and mutual funds, see What Is An ETF? and What Is A Mutual Fund?.
Frequently Asked Questions
Q: Can index funds lose money?
Yes. An index fund tracks a market, so if that market falls, the fund falls too. During the 2008 financial crisis, broad U.S. stock index funds dropped roughly 50%. They eventually recovered, but it took years. The key is that index funds tend to recover over long time horizons, which is why your investing time horizon matters so much.
Q: Are index funds better than actively managed funds?
For most people over long periods, yes, mainly because of lower costs. After fees, the majority of actively managed funds underperform their benchmark index. Some investors use a mix of both, which is fine. The bigger issue is making sure whatever you own fits your goals and timeline.
Q: What’s the difference between an index fund and an ETF?
An index fund is a strategy; an ETF is a structure. Most ETFs happen to be index funds, but not all. And many index funds are structured as mutual funds. The two terms are related but not interchangeable. See What Is An ETF? for a full breakdown.
Q: How do I buy an index fund?
You need a brokerage account, a 401k, or an IRA. Once you have an account, you can search for the fund by name or ticker symbol and purchase shares. See What Is A Brokerage Account? if you’re starting from scratch.
Learn More
- Investor.gov: Index Funds
- SEC.gov: Mutual Funds and ETFs, A Guide for Investors