Index funds are one of the most reliable ways to build long-term wealth, and they require almost no expertise to use correctly. They outperform most actively managed mutual funds over time, charge a fraction of the fees, and require almost no ongoing maintenance. If you have not started investing yet, or if you are wondering whether index funds belong in your portfolio, this guide covers everything you need to know.
What Is an Index Fund?
An index fund is a type of investment fund — either a mutual fund or an exchange-traded fund (ETF) — designed to track the performance of a specific market index. The S&P 500 index, for example, tracks the 500 largest publicly traded companies in the United States. An S&P 500 index fund holds those same 500 companies in the same proportions, so when the index goes up, the fund goes up by roughly the same amount.
The key difference between an index fund and an actively managed fund is that no one is trying to pick stocks. The fund simply mirrors the index. That sounds boring, but it is actually a massive advantage: lower costs, lower taxes, and returns that match the market rather than trailing it.
Why Index Funds Beat Most Active Managers
The evidence here is not subtle. According to S&P Dow Jones Indices, roughly 80% to 90% of actively managed large-cap funds underperform the S&P 500 over a 10-year period. The reason is straightforward: active managers charge higher fees, and those fees drag down returns. Even skilled managers have to overcome a 1% to 2% annual expense ratio just to break even with the index.
Index funds, by contrast, often charge expense ratios of 0.03% to 0.20% per year. On a $100,000 portfolio, the difference between a 1.5% expense ratio and a 0.03% expense ratio is roughly $1,470 per year — money that stays in your account and compounds over time.
Types of Index Funds to Know
Total Market Index Funds
A total market index fund holds every publicly traded stock in the United States — thousands of companies across every sector and size. Vanguard Total Stock Market ETF (VTI) and Fidelity ZERO Total Market Index Fund are two popular examples. This is the broadest possible exposure to U.S. equities in a single fund.
S&P 500 Index Funds
An S&P 500 index fund tracks the 500 largest U.S. companies, which together represent about 80% of the total U.S. stock market by value. Vanguard S&P 500 ETF (VOO), iShares Core S&P 500 ETF (IVV), and SPDR S&P 500 ETF Trust (SPY) are the three largest. If you invest in just one fund for the rest of your life, an S&P 500 index fund is a defensible choice.
International Index Funds
International index funds give you exposure to stocks outside the United States. Vanguard Total International Stock ETF (VXUS) covers thousands of stocks in developed and emerging markets. Adding international exposure can reduce the risk that any single country’s economic problems drag down your entire portfolio.
Bond Index Funds
Bond index funds hold a broad basket of government or corporate bonds. They tend to be less volatile than stock funds and can provide stability during stock market downturns. Vanguard Total Bond Market ETF (BND) is one of the most widely held.
How to Start Investing in Index Funds
Step 1: Open an Account
You need a brokerage account to buy index funds. For most people, the best starting point is a tax-advantaged account:
- 401(k): If your employer offers a 401(k) with matching contributions, start here. The match is free money — contribute at least enough to get the full match before investing anywhere else.
- IRA or Roth IRA: Once you have captured the employer match, a Roth IRA is often the next best option. Contributions are made with after-tax dollars, but all growth and withdrawals in retirement are tax-free. In 2026, you can contribute up to $7,000 per year ($8,000 if you are 50 or older).
- Taxable brokerage account: If you have maxed out your tax-advantaged accounts or need access to your money before retirement, a taxable brokerage account at Fidelity, Charles Schwab, or Vanguard works well.
Step 2: Choose Your Funds
You do not need more than two or three funds to build a complete portfolio. A simple starting point:
- 80% to 90% in a total market or S&P 500 index fund
- 10% to 20% in an international stock index fund
- An optional allocation to a bond index fund if you are within 10 to 15 years of retirement
Simpler is usually better. The “three-fund portfolio” — U.S. stocks, international stocks, bonds — is one of the most recommended approaches in personal finance.
Step 3: Set Up Automatic Contributions
The best habit you can build is automating your investments. Set up a recurring transfer each month — even $100 or $200 — so you buy consistently regardless of whether the market is up or down. This strategy, called dollar-cost averaging, removes the temptation to time the market and keeps you investing through volatility.
Step 4: Leave It Alone
Index fund investing is a long-term strategy. The biggest mistake new investors make is checking their accounts too often and panicking during market downturns. Markets decline regularly — by 10% or more at least once a year on average — but they have recovered every time in history over a long enough time horizon. Stay invested, keep contributing, and resist the urge to sell.
What to Look for When Comparing Index Funds
- Expense ratio: Lower is better. For major index funds, anything above 0.20% is high.
- Tracking error: How closely does the fund follow its target index? Most major ETFs track very closely.
- Fund size: Larger funds tend to have tighter bid-ask spreads if you are buying ETFs, and are less likely to be closed or merged.
- Tax efficiency: ETF index funds tend to be more tax-efficient than mutual fund versions in taxable accounts because of how they handle redemptions.
Common Index Fund Questions
Can I lose money in an index fund? Yes. Index funds go up and down with the market. If the S&P 500 drops 30%, your S&P 500 index fund drops roughly 30% too. The key is that markets have historically recovered and grown over long periods. Short-term losses are a normal part of long-term investing.
How much do I need to start? Many ETF index funds can be purchased for the price of a single share, and some brokerages offer fractional shares so you can start with as little as $1. Fidelity and Schwab offer zero-minimum index mutual funds as well.
Is now a good time to invest? The research consistently shows that investing now — regardless of market conditions — beats waiting for the “right” time. Time in the market beats timing the market.
Bottom Line
Index funds are not exciting, and that is the point. They offer broad diversification, rock-bottom costs, and returns that match the market — which turns out to be better than what most professional fund managers deliver. Open a tax-advantaged account, pick one or two low-cost index funds, set up automatic contributions, and let compound growth do the work over decades. That is the whole strategy.