Free course · Module 1
Index Funds, ETFs and the S&P 500
Understand what an index fund owns, how ETFs differ from mutual funds and why the expense ratio deserves a closer look.
Written by the StockEducation Editorial Team · Reviewed by Associate Professor Charles Lo, CPA, PhD · Updated 16 Oct 2026 · How we check lessons
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Idea 1 of 4
Track a basket, understand the weights.
An index is a benchmark. An index fund is an investment that tries to follow it. You cannot buy the S&P 500 index directly. Funds may hold all its stocks or a representative sample. Their returns can differ from the benchmark because of costs and tracking differences. Source 1.
The S&P 500 weights stocks by float-adjusted market value: price multiplied by the shares available to investors. Bigger eligible holdings get more weight, rather than every company getting an equal slice. Source 2.
If A rises 10% and B and C do not move, this toy index rises 6% before costs.
How does a company enter the S&P 500?
Does owning many stocks remove the risk?
Name the index, then check what gets the largest weights.
Idea 2 of 4
The strategy and the wrapper are different.
“Index” describes the strategy; “ETF” describes the fund structure. Both mutual funds and ETFs can use an index strategy or active management. An ETF label alone does not tell you the holdings or the risks. Source 3.
Mutual fund orders use the next calculated net asset value (NAV), usually at the business day’s end. ETF shares trade during market hours at market prices, which may differ from NAV.
Next calculated NAV
Submit an order; pricing usually follows the day’s close.
Market price
Trade on an exchange while the market is open.
Check the exact fund, share class and account before comparing costs.
What about minimums, spreads and taxes?
Compare the fund’s strategy separately from how its shares trade.
Idea 3 of 4
Small percentages are real dollars.
The expense ratio is an annual operating-cost percentage. In the prospectus fee table, look for Total Annual Fund Operating Expenses and the exact share class. Funds pay operating costs from their assets; other charges may be separate. Source 4.
For a fixed hypothetical average balance of $10,000, 0.05% is about $5 a year and 0.64% is about $64: a $59 difference. These are practice rates, not a current industry average or a product recommendation.
$10,000 × (0.64% − 0.05%) = $59.
Is the lowest expense ratio the whole comparison?
What does the evidence on active funds show?
Compare costs in dollars as well as percentages.
Idea 4 of 4
Compare before you choose.
Change one assumption at a time. Use this local exercise to compare two hypothetical annual expense ratios on the same fixed average balance. It estimates operating costs only; it does not forecast returns or choose a fund for you.
What does a fee gap cost in dollars?
Hypothetical · one yearFund B’s annual operating cost is $59 higher at this average balance.
Calculation: fixed average balance × annual expense ratio ÷ 100. Rounded to cents where needed. No returns, growth, contributions, trading costs, taxes or fee waivers are modeled. Real costs change with assets and the fund’s terms.
Try a real comparison without buying anything
Write down the benchmark, holdings and total costs before choosing a fund.
Quick check
Three quick checks. Feedback straight away.
What primarily determines a stock’s weight in the S&P 500?
Choose an answer to see the explanation.
Which fund type can retail investors trade on an exchange during market hours?
Choose an answer to see the explanation.
On a fixed $10,000 average balance, what does a 0.05% annual expense ratio represent?
Choose an answer to see the explanation.
Review your takeaway card and the learning path
- Read the benchmark. An index fund follows a strategy; it is not a guarantee.
- Check the weights. Many holdings can still contain concentrated exposure.
- Separate the wrapper. ETF and mutual fund trading mechanics differ.
- Count the cost. Compare the exact fund’s operating expenses and other charges.
Reading markers and these practice checks stay on this device. They do not mark the LMS lesson complete.
Sources and assumptions
- SEC: Index Funds. Tracking methods, market risks and costs.
- S&P U.S. Indices Methodology, July 2026. Eligibility, committee selection and float-adjusted weighting; thresholds can change.
- SEC: Characteristics of Mutual Funds and ETFs, April 2025. Structures, NAV, market trading and US tax-account context.
- SEC: Mutual Fund and ETF Fees and Expenses, July 2025. Expense-ratio definition, prospectus fee table and additional costs.
- SPIVA U.S. Mid-Year 2026. Report 1a: All Large-Cap funds, 20 years ending 30 June 2026; historical category comparison and methodology.
Sources checked 16 October 2026. Company weights and fee examples are hypothetical. The SPIVA statistic is dated, not a forecast. This lesson uses US fund and account examples; local rules differ. No fund recommendation, transaction or LMS completion is produced by the practice controls.
StockEducation provides general education, not personal financial, tax or legal advice. Nothing here recommends a specific security. Investing involves risk, including loss of money you invest.
