how index funds actually work

In short

A friend of mine once asked why I trust a fund that simply copies a list of companies. I told her the copying is the whole point. If you are holding a few dollars and wondering where to put them, an index fund is one plain way to own a slice of many companies at once. It follows a published list instead of paying someone to pick winners. You pay a small yearly fee, and the fund shows that fee as an expense ratio. Check that number before anything else, because a fee of 0.05 percent and a fee of 1 percent grow far apart over the years. Read the fund's prospectus, the official document that lists its costs and rules. Then decide for yourself, in your own time.

The whole of it

What it is

I once watched my grandfather sort apples at a county fair, and he never tried to guess which single apple was best. He bought the whole crate and let the crate do the work. An index fund is a bit like that crate. It is a pooled fund that holds the same stocks or bonds as a published list, called an index. The S&P 500 is a well known example, a list of 500 large American companies kept by S&P Dow Jones Indices.

If you have ever wondered who decides what goes on the list, the answer is a committee at the company that publishes it. The fund does not argue with the committee. It follows. When a company joins the list, the fund buys it. When one drops off, the fund sells it.

Plain and simple. That is the idea.

An index fund comes in two common shapes. A mutual fund is priced once a day, after the market closes. An exchange traded fund, or ETF, trades all day like a single stock. Both can track the same index. The shape changes how you buy it, not what is inside.

How it works

A neighbor of mine runs a small hardware store, and she keeps her shelves stocked in the same proportions as what her customers buy. The fund does something close to that. Most big index funds weight each company by its size, which the industry calls market capitalization. That is the share price times the number of shares. A bigger company gets a bigger slice of the fund.

Say a tiny index holds only two companies. Company A is worth $90 billion and Company B is worth $10 billion. Together they come to $100 billion. A is $90 billion out of $100 billion, so it fills 90 percent of the fund. B fills the other 10 percent. When A grows, its slice grows too, without anyone placing a trade.

You might ask where the money goes. When you put $1,000 into the fund, it joins a pool with every other investor. The fund buys small pieces of every company on the list, in those same proportions. You own shares of the fund, and the fund owns the companies.

The fund also collects dividends, the cash payments some companies make to their owners. It either passes them along to you or buys more shares with them, depending on the choice you make. Either way, the fund pays its own bills out of the assets it holds, which is how the expense ratio gets collected. You never get a separate bill. The cost simply comes out of the fund's value a little each day.

The fund buys and sells rarely, since it only changes when the list changes. That quiet pace tends to keep trading costs down. It can also mean fewer taxable events in a regular account, though the details depend on the fund and on the tax rules where you live.

The numbers, and where to find yours

I have found that people feel better once they know where the facts sit. Every fund in the United States has to file a prospectus with the Securities and Exchange Commission, and you can look up any fund on the SEC's EDGAR database. Near the front sits a fee table. Find the line called total annual fund operating expenses. That is the expense ratio, shown as a percent of what you hold.

The fund company's own page for each fund lists the same figure, along with the index it follows and the date the numbers were last updated. The prospectus also shows the minimum amount to start, which varies a lot from one fund to the next. Some funds ask for $3,000 to begin. Some ask for nothing at all.

For retirement accounts, the yearly limits on how much a person can contribute are set by law and change over time. The current figure for an IRA is $7,500 for 2026 (source, checked 10/4/2026), and the Internal Revenue Service publishes it. Check the IRS page rather than trusting an old number from memory.

A worked example

Let me tell you about a woman named Marisol, who is 30 years old and has $10,000 to set aside for retirement. She finds two funds that follow nearly the same index. One charges 0.05 percent a year. The other charges 1 percent a year. She wonders if the gap matters, so she works it out with a pencil.

For this example, we will say both funds earn 7 percent a year before fees, and she adds nothing more. That 7 percent is only a round figure for the arithmetic. It is not a promise of anything.

The first fund keeps 7 percent minus 0.05 percent. That is 6.95 percent a year. The second keeps 7 percent minus 1 percent, which is 6 percent a year.

Now she lets the money grow for 30 years. The first fund multiplies by 1.0695 for each year. After 30 years, $10,000 times 1.0695 raised to the 30th power comes to about $76,100. The second fund multiplies by 1.06 for each year. After 30 years, $10,000 times 1.06 raised to the 30th power comes to about $57,400.

The difference is $76,100 minus $57,400, which is $18,700. That is real money, and it came from a fee that looked small on the page.

Marisol did not need a degree to see it. She needed a pencil and ten minutes. You can do the same with your own figures. Change the starting amount, the years, and the two fees, and watch what the gap does.

Where it goes wrong

I would be a poor friend if I only told you the bright side. A fund that follows the market goes down when the market goes down. If the index falls 30 percent, so does the fund, because it holds what the index holds. Nothing in the design protects you from that. It only promises to follow.

Some index funds follow narrow lists. A fund that tracks one industry, or one small country, holds far fewer companies than one that tracks a broad list. Fewer companies means less spreading of the risk, even though the word index sits right in the name. Always read what the fund actually holds.

Fees can hide too. A low expense ratio is only one cost. Some funds charge a sales fee when the investor buys, called a load. The prospectus must list it, but a person has to look. An ETF can also carry a small gap between its buying price and selling price, which is a cost to the trader.

Tracking is not perfect. A fund can lag its index by a hair, called tracking error, from fees and timing. Usually the gap is small. Still, it exists.

Watch the habits, too. A person who sells in a panic after a drop locks in the loss. That mistake belongs to the person, not the fund, and it is a common one. Slow and steady has served many people well, but nothing here is advice to buy or sell any fund.

Questions to answer before you leave this page

What is the expense ratio of the fund you are looking at, and where in the prospectus did you find it? Which index does the fund follow, and do you know what that list holds? If the fund fell 30 percent next year, would you stay calm or lose sleep, and what does that tell you about the amount you can set aside? Is there a sales fee, a minimum to start, or a gap in trading price that adds to the cost? Have you worked the numbers with your own starting amount and your own years, the way Marisol did? And have you checked the IRS page for the current contribution limit before you fund an account?