Index funds: the simple definition
An index fund is an investment fund that tracks a specific market index, like the S&P 500. Instead of trying to beat the market, it simply copies it by buying the same stocks in the same proportions as the index it follows.
How do index funds work?
Index funds follow a passive investment strategy. There is no manager trying to pick winners — the fund mechanically mirrors its benchmark:
- Choose an index. The fund selects an index to track, such as the S&P 500.
- Buy the stocks. It purchases all stocks in the index in the same proportions.
- Rebalance automatically. When the index changes its constituents, the fund adjusts.
- Distribute returns. Dividends and gains are passed through to investors.
Because nobody is being paid to research and trade, running costs are a fraction of those of an actively managed fund. That cost difference is the single most reliable predictor of long-run returns.
Types of index funds
Market index funds
Track entire stock markets.
- S&P 500 (500 largest US companies)
- Total Stock Market (entire US market)
- FTSE Developed Markets (global stocks)
Sector index funds
Focus on specific industries.
- Technology
- Healthcare
- Financials
International funds
Track foreign markets.
- European markets
- Emerging markets
- Asia-Pacific region
Bond index funds
Track bond markets.
- Government bonds
- Corporate bonds
- International bonds
Why choose index funds?
Low costs
Expense ratios are typically 0.03%–0.20%, against 0.50%–2.00% for active funds.
Instant diversification
One fund can hold hundreds or thousands of companies.
Simplicity
No need to research or monitor individual stocks.
Consistent performance
Matches the market, which historically beats most active funds after fees.
Transparency
The holdings are published, so you always know what you own.
Tax efficiency
Low turnover means fewer taxable events than actively traded funds.
Index funds vs individual stocks
| Aspect | Index funds | Individual stocks |
|---|---|---|
| Risk | Lower (diversified) | Higher (concentrated) |
| Research required | Minimal | Extensive |
| Time investment | Very low | High |
| Potential returns | Market average | Can be higher or lower |
| Cost to hold | 0.03%–0.20% per year | Trading costs only |
Concentration is the key difference. A single company can go to zero; an index of 500 companies cannot, because failing constituents are replaced.
Popular index funds to consider
VTI — Vanguard Total Stock Market ETF
Tracks the entire US stock market, roughly 4,000 companies.
Expense ratio: 0.03%Assets: $300B+
VOO — Vanguard S&P 500 ETF
Tracks the S&P 500, the 500 largest US companies.
Expense ratio: 0.03%Assets: $400B+
VXUS — Vanguard Total International Stock ETF
Tracks international developed and emerging markets.
Expense ratio: 0.08%Assets: $100B+
These are examples, not recommendations. Compare the full list in our index fund database, and check what capital gains tax applies where you live before committing.
How to get started
Open a brokerage account
Choose a reputable broker. The account opening process usually takes under 30 minutes.
Choose your index fund
A total market fund is the simplest starting point for maximum diversification.
Set up regular investments
Automate a fixed monthly amount so the decision is made once, not every month.
Stay patient
Index funds work best over long periods. Ten years is a reasonable minimum horizon.
What to do next
Understanding index funds is the easy part. The hard part is starting and then leaving it alone. Even small amounts invested regularly compound into meaningful sums over decades — you can model your own numbers with the calculator to see what a given contribution actually produces over your time horizon.