How Index Funds Work Without Jargon

An index fund is a mutual fund or exchange-traded fund (ETF) that tries to match the returns of a market index instead of trying to beat it. The index is only a published set of rules for choosing and weighting securities; the fund copies those rules, so its holdings and results are meant to follow the index rather than a manager's judgment.

Key takeaways

An index is a rulebook that produces a list of securities and weights. You cannot buy an index directly; you buy a fund that copies it.
A fund copies its index by holding every security in it or only a sample, and the SEC notes that sampling can make performance less likely to match the index.
Two costs matter most: the yearly expense ratio, and for ETFs the gap between the buy and sell price (the spread) each time you trade. Brokerage commissions or purchase and sale fees can apply on top.
Tracking difference is how far the fund finished from its index; tracking error is how bumpy that gap was along the way.
Index funds carry the same risks as the securities in the index, so they do not cushion a fall and they inherit the index's concentration.

What an index is: a rulebook

The SEC's Investor Bulletin on index funds describes a market index as a measure of the performance of a "basket" of securities, such as stocks or bonds, meant to represent a sector of a stock market or of an economy. It names the S&P 500, the Russell 2000 and the Wilshire 5000 Total Market Index as examples, and points out that you cannot invest in an index itself.

Many indexes decide how much each security counts using market capitalization, the share price times the number of shares outstanding. In a market-cap-weighted index, bigger companies take up a bigger share. Some indexes instead weight by share price, per the same bulletin. The best-known example is the Dow Jones Industrial Average, which the Investor.gov glossary describes as an index of 30 "blue chip" stocks of U.S. companies.

Illustrative example: a three-company index

Take a made-up index that holds three companies, weighted by market cap. Company A is worth $600 billion, B is worth $300 billion and C is worth $100 billion, for $1,000 billion in total.

Company	Market cap	Weight in index	Share of a $10,000 fund holding
A	$600 billion	60%	$6,000
B	$300 billion	30%	$3,000
C	$100 billion	10%	$1,000

No one chose these weights by opinion. The rule (“hold these three, weight by size”) produced them, and a fund copying the rule would end up with exactly this split. How real indexes add, remove and reweight members is set by each index provider's published methodology.

How a fund copies an index

According to the SEC bulletin, index funds take one of two approaches: some invest in all of the securities in the market index, while others invest in only a sample of them. Those approaches are usually called full replication and sampling.

	Full replication	Sampling
What the fund holds	Every security in the index, at about the index weights	A subset chosen to resemble the index
Main appeal	Holdings mirror the index closely	Avoids buying every small or hard-to-trade security
Main risk	Higher trading cost for very large or illiquid indexes	The SEC says performance may be less likely to match the index

The table's last column follows the bulletin's wording on tracking error: a fund that holds only a sampling of the index's securities may be less likely to match its performance. The first two rows describe the general logic of each approach rather than a rule; a fund's prospectus states which method it uses.

A real prospectus shows what sampling looks like. The summary prospectus for Vanguard Total Bond Market Index Fund (April 28, 2026), used here only to illustrate the method, says the fund "invests by sampling" its bond index, meaning it holds a selection of securities that, taken together, resembles the full index on key risk factors and other characteristics. The same document warns that this sample may vary from the full index and that the fund's performance may not match it, a risk it calls tracking error risk.

The bulletin adds that some index funds also use derivatives, such as options or futures, to help meet their objective. It also warns that newer funds tracking custom-built indexes (often called smart beta or quantitative funds) may not work the way this article describes, so those need their own reading.

What you pay to own one

Two costs do most of the work, and they arrive in different ways.

The expense ratio. The SEC defines it as the percentage of a fund's average net assets used each year to pay operating expenses, and says you will find the figure in the fund's prospectus fee table (Investor.gov glossary). It is taken from fund assets, so you never get a bill. On $10,000, a 0.05% ratio is $5 a year and a 0.50% ratio is $50 a year.

The trading spread (ETFs). ETF shares trade on an exchange with two prices, a bid and a higher ask, and the difference is the spread. The SEC's ETF bulletin gives an example: shares quoted at $59.50 bid and $60.00 ask have a 50-cent spread, and buying 200 shares at the ask and selling them straight back at the bid loses $100. That is 0.83% of the $60 price, paid on a round trip. The bulletin adds that more liquid ETFs with higher trading volume typically have tighter spreads, and that an ETF's website shows its median bid-ask spread. Mutual fund shares are not traded on an exchange in the same way: unlike an ETF, a mutual fund sells its shares directly to investors and redeems them directly, so there is no bid-ask spread of this kind.

Smaller charges on top. The same ETF bulletin notes that you may pay brokerage commissions when you buy and sell ETF shares, and that an ETF's market price can sit above or below the value of its holdings (a premium or discount). On the mutual fund side, the SEC's index fund bulletin suggests asking what fees you can expect to pay for buying, owning and selling the fund. A prospectus fee table, like the one in the bond fund prospectus above, lists "shareholder fees" paid directly from your investment separately from the annual operating expenses, and that prospectus adds that brokerage commissions are not included in the table.

The SEC also cautions that not every index fund costs less than an actively managed one, so the number needs checking fund by fund.

Tracking difference and error

Because a fund pays its costs and may not hold the index perfectly, it rarely matches the index exactly. The SEC's index fund bulletin lists fees and expenses, trading costs and tracking error as reasons a fund can underperform its index. Two terms describe the gap, and they answer different questions.

Tracking difference: fund return minus index return over a period. It says how much you gained or lost relative to the index.
Tracking error: how much that gap varies from period to period, usually measured as the standard deviation of the gaps. It says how consistent the copying is.

The SEC bulletins cited here do not define the two terms precisely. The clearest regulatory definitions come from the European Securities and Markets Authority, whose guidelines on ETFs and other UCITS issues (ESMA/2014/937) define annual tracking difference as the difference between the annual return of an index-tracking fund and that of its index, and tracking error as the volatility of that difference. US fund documents often use "tracking error" more loosely, for any failure to match the index, as the bond fund prospectus above does, so check how a fund's own documents use the words.

Illustrative example: two funds, one index

Suppose an index returns 10.00% in a year. Fund X returns 9.93%, so its tracking difference is 9.93 − 10.00 = −0.07 percentage points. If its expense ratio is 0.05%, the fees explain roughly 0.05 of that gap and about 0.02 comes from other frictions such as trading costs or holding a little cash. (Fees are taken through the year from average assets, so the split is approximate.)

Fund Y returns 9.90%, a difference of −0.10 points. The month-by-month gaps (fund return minus index return, in percentage points) show the other side:

Fund	Twelve monthly gaps	Sum of gaps	Standard deviation (monthly)	Annualized
X	−0.01, 0.00, −0.02, +0.01, −0.01, −0.01, 0.00, −0.02, +0.01, −0.01, 0.00, −0.01	−0.07	0.010	0.035
Y	−0.20, +0.15, −0.30, +0.25, −0.10, +0.20, −0.25, +0.10, −0.15, +0.30, −0.20, +0.10	−0.10	0.21	0.74

annualized tracking error = monthly standard deviation of gaps x square root of 12

The two funds ended the year only 0.03 points apart, yet Fund Y's monthly gaps swung about 21 times more than Fund X's (0.74 versus 0.035 annualized). A reader looking at tracking difference alone would see two similar funds; tracking error shows that Y followed its index far less tightly. The monthly gaps are simplified, so their sum approximates rather than exactly equals the yearly figure.

What an index fund does not do

The SEC's bulletin is direct: an index fund is subject to the same general risks as the securities in the index it tracks, and it may have less flexibility than a non-index fund to react to price declines in those securities. Two consequences follow.

No downside protection. If the index falls 20%, a fund that copies it well falls by about the same amount, plus its costs. Copying the index is the product; avoiding its losses is not.
Concentration follows the index. The weights in the three-company example show how it works: a cap-weighted index puts 60% in the largest name because the rule says so. A fund holding thousands of securities can still have a large share of its value in a handful of them, or in one sector, if the index is built that way. The fund's top-holdings list shows this directly.

An index fund is also not automatically better or worse than an actively managed fund. It is a different trade: a rule instead of a manager, usually with a different cost profile, which the SEC says should be checked for each fund.

Reading a fund fact sheet

A fund's core facts are spread across its fact sheet, website page, prospectus and most recent shareholder report, and they are not all equally detailed. The SEC advises reading the prospectus and latest shareholder report before investing and lists questions worth asking, including what fees and expenses you will pay and how the makeup of the fund's index is determined. Turned into a fact sheet routine, six questions cover most of it.

What is the ticker or fund name, and is it the share class I mean? One fund can come in several share classes, including an ETF class, with different expense ratios, so match the symbol exactly.
Which index does it follow? Look up that index's rules: what it holds, how it weights and whether it is a traditional or a custom-built index.
What is the expense ratio? The prospectus fee table is the authoritative place for it.
How many holdings does it have, and what are the biggest? A count far below the index's count suggests sampling. The top-ten list shows concentration.
How far did it finish from the index, and how steadily? Compare fund and index returns over several periods to see tracking difference, and look at how the gap moved year to year.
For an ETF, what is the median bid-ask spread? The SEC says it is published on the ETF's website.

Frequently asked questions

Is an index fund the same thing as an ETF?

No. The SEC describes an index fund as a type of mutual fund or ETF that seeks to track a market index, so an ETF can be an index fund but not every ETF is one. The difference is the wrapper: mutual fund shares are bought from the fund company, while ETF shares trade on an exchange throughout the day at market prices.

Do index funds only track stock indexes?

No. The SEC says an index measures a basket of securities, such as stocks or bonds, and bond index funds exist: the bond fund prospectus cited above describes a fund that tracks a broad index of US investment-grade taxable bonds, including government, corporate and mortgage-backed bonds. Which securities a fund holds depends entirely on its index.

Can I buy the index itself?

No. According to the SEC, you cannot invest directly in a market index, which is why index funds exist: they give indirect exposure by copying the index's holdings.

Do index funds use derivatives?

Some do. The SEC notes that some index funds use options or futures to help achieve their objective. The fund's prospectus describes whether it does, and the risk section covers what that adds.

Where can I see exactly what a fund holds?

Funds disclose their portfolio holdings in shareholder reports and regulatory filings, available from the fund's website and on the SEC's EDGAR database. The SEC also says most ETFs post their portfolio holdings on their websites daily.

A short checklist before you buy

Open the prospectus or summary page and write down the index name, expense ratio and share class.
Read the index provider's description of its rules, so you know what the fund will hold when markets change.
Compare the fund's and index's returns over at least three separate years to see tracking difference, and note how much the gap moved.
Check the top holdings and the sector split to see how concentrated the fund is today.
If it is an ETF, find the median bid-ask spread on its website and decide how often you would trade.
Model the effect of a given expense ratio on your own contributions with the Investment Growth calculator, treating results as illustrations rather than forecasts.