Guides · 6 Oct 2026 · 4 min read
What is an index fund? How it works and why it’s so popular
An index fund buys every company in a market index, so you own a slice of the whole market at a very low cost. Here's how they work, what they cost and where they fall short.
The short answer
An index fund is an investment fund that holds all (or a representative sample) of the securities in a market index, such as the S&P 500, so its returns track that index. Because it doesn't pay managers to pick stocks, it usually charges very low fees and offers broad diversification in a single purchase.
Key takeaways
- An index fund copies a market index instead of trying to beat it.
- One purchase spreads your money across hundreds or thousands of companies.
- Very low fees are the main advantage, and fees compound over decades.
- Index funds still fall when the market falls; they remove stock-picking risk, not market risk.
If you’ve read any beginner investing advice, you’ve probably been told to “just buy an index fund.” So what is an index fund, and why do so many investors, from first-timers to famous billionaires, recommend them? Here is a plain-English explanation.
What is an index fund, in simple terms?
A market index is a list of companies that represents part of the market. The S&P 500 tracks about 500 large US companies. The MSCI World and FTSE All-World track thousands of companies across developed or global markets. Other indices cover bonds, small companies or single countries.
An index fund is a fund that buys the companies in an index in the same proportions, so it rises and falls in line with that index. It doesn’t try to beat the market. It aims to be the market, minus a small fee.
When you buy one unit of an S&P 500 index fund, you indirectly own a tiny slice of every company in it, from the largest tech giants to the smallest names on the list.
How does an index fund work?
- The fund follows a rulebook. The index provider decides which companies are in and how much each weighs, usually by size (market capitalization).
- The fund copies it. When companies join or leave the index, the fund buys or sells them. Nobody is guessing which stocks will do well.
- You get the index return, minus costs. Dividends from the companies are either paid to you or reinvested, depending on the fund version you choose (“income” vs “accumulating”).
Because the strategy is mechanical, running the fund is cheap, and that is the key to its appeal.
Why are index funds so popular?
Low fees
Actively managed funds pay analysts and portfolio managers to pick stocks. Their annual fees (the expense ratio) are often in the range of 0.5–1% or more. Broad index funds frequently charge well under 0.2%, and some charge less than 0.1%.
That gap sounds tiny. It isn’t. On $100,000 invested for 30 years, a 7% annual return grows to about $761,000. If a 1% fee cuts the return to 6%, you end up with about $574,000. The difference, roughly $187,000, goes to fees rather than to you.
Most professionals don’t beat the index
S&P Dow Jones Indices publishes a long-running scorecard called SPIVA. It has consistently found that the large majority of actively managed US large-cap funds trail the S&P 500 over 10- and 15-year periods once fees are included. Beating the market is possible, but identifying in advance who will do it is very hard.
Instant diversification
One purchase spreads your money across hundreds or thousands of companies. If one collapses, the damage to your portfolio is small. Diversification is one of the few ways to reduce risk without giving up expected return.
Simplicity
There is little to research and little to monitor. That makes it easier to do the thing that matters most: keep investing consistently for years.
What are the downsides of index funds?
Index funds aren’t perfect, and it’s worth knowing their limits.
- They fall with the market. An index fund removes the risk of picking a bad company, not the risk of the whole market dropping. Stock indices have fallen 30–50% in major crashes.
- Concentration can creep in. Indices weighted by company size give the largest companies the biggest weight. At times, a handful of tech giants have made up a large share of the S&P 500.
- No downside protection. A fund manager can choose to hold cash in a crisis; an index fund stays fully invested.
- Average by design. You will never beat the market with an index fund. For most people that’s a feature, not a bug.
Index fund vs actively managed fund
| Index fund | Active fund | |
|---|---|---|
| Goal | Match the index | Beat the index |
| Typical annual fee | Low (often under 0.2%) | Higher (often 0.5–1%+) |
| Who picks the stocks | The index rules | A manager or team |
| Long-term record | Matches the market minus a small fee | Most trail their index after fees |
| Transparency | Holdings mirror the index | Holdings change at the manager’s discretion |
How do you choose an index fund?
- Pick the market you want. A global index gives the broadest diversification; a single-country index (such as the S&P 500) is more concentrated.
- Compare fees. Between two funds tracking the same index, the cheaper one usually wins over time.
- Check tracking difference. How closely has the fund matched its index after costs? Smaller gaps are better.
- Choose the format. Index funds come as mutual funds or ETFs. Our comparison of ETF vs mutual fund explains the practical differences.
- Decide income vs accumulation. Accumulating versions reinvest dividends automatically, which suits long-term growth.
How do index funds fit into a portfolio?
Many investors use one or two broad index funds as the core of their portfolio: for example a global stock index fund for growth and a bond index fund for stability. The balance between them depends on your timeline and comfort with ups and downs. Our guide to stocks vs bonds covers how to think about that split, and how to start investing puts it all into a step-by-step plan.
Adding a fixed amount every month, known as dollar-cost averaging, pairs naturally with index funds and lets compound interest do its work.
The bottom line
An index fund is a low-cost way to own a whole market in one purchase. It won’t make you rich overnight and it will fall when markets fall, but for long-term investors its combination of low fees, broad diversification and simplicity is hard to beat.
This guide is general information, not personal financial advice. The value of investments can fall as well as rise.
Frequently asked questions
Are index funds safe?
They are diversified, which removes the risk of one company sinking your portfolio, but they still rise and fall with the market. A stock index fund can drop 30% or more in a bad year. They suit money you can leave invested for many years.
What is the difference between an index fund and an ETF?
Index fund describes the strategy (tracking an index); ETF describes the structure (a fund that trades on a stock exchange like a share). Many ETFs are index funds, and many index funds are mutual funds rather than ETFs.
How do index funds make money?
You earn from the companies inside the fund: their share prices rising over time and the dividends they pay. The fund passes these returns on to you minus its small annual fee.
Can you lose money in an index fund?
Yes. If the index falls, the fund falls too. Over long periods broad indices have historically recovered from downturns, but past performance does not guarantee future results.
Sources: Investor.gov (U.S. SEC) — Mutual funds and ETFs, S&P Dow Jones Indices — SPIVA research, FINRA — Investing basics
