Index funds are investment funds designed to match the performance of a specific market benchmark, like the S&P 500 or the total U.S. stock market. Instead of hiring a team of analysts to pick individual stocks, an index fund simply buys all (or a representative sample) of the securities in its target index, holds them in the same proportions, and delivers returns that closely mirror the market itself. They’ve become one of the most popular ways for everyday investors to build wealth, largely because they cost less and, over time, tend to outperform the majority of professionally managed alternatives.
How Index Funds Track the Market
Every index fund is tied to a specific benchmark. The S&P 500, for example, includes 500 of the largest publicly traded U.S. companies. A fund tracking that index owns shares in those same companies, weighted by each company’s market capitalization. That means the biggest companies (think Apple, Microsoft, Amazon) make up a larger slice of the fund than smaller ones.
The fund doesn’t just buy and forget. The underlying index is periodically rebalanced: companies that no longer meet the criteria (minimum market cap, sector representation, or other factors) get removed, and new qualifying companies take their place. When that happens, the fund adjusts its holdings to stay in sync. A committee reviews potential additions and removals, and the final changes flow through to every fund tracking that index. As an investor, you don’t need to do anything during this process. It happens automatically.
Most index funds use market-capitalization weighting, where bigger companies carry more influence on the fund’s returns. Some use equal weighting, giving every company the same share regardless of size, or revenue weighting. The weighting method affects how concentrated your investment is in the largest names.
Why They Cost So Much Less
The defining advantage of index funds is their low cost. Because the fund follows a preset formula rather than paying portfolio managers to research and trade individual stocks, operating expenses are dramatically lower. Actively managed mutual funds charge around 0.60% of your balance per year on average, according to Morningstar data. Many broad-market index funds charge 0.03% to 0.10%, and a few charge nothing at all. Fidelity, for instance, offers a large-cap index fund with a 0% expense ratio.
That gap sounds small in percentage terms, but it compounds significantly. On a $100,000 portfolio earning 7% annually, the difference between a 0.05% expense ratio and a 0.60% expense ratio adds up to tens of thousands of dollars over 25 years. Every dollar taken in fees is a dollar that isn’t compounding in your account.
How They Stack Up Against Active Funds
The case for index funds isn’t just about fees. It’s also about results. The SPIVA scorecard, which has tracked the performance of actively managed funds against their benchmarks for over 20 years, consistently finds that most active managers fall short. On average, more than 70% of actively managed large-cap funds lag the S&P 500 in any given period, and that figure has climbed as high as 85% in some years. The pattern holds across mid-cap and small-cap categories too.
The longer the time horizon, the worse the odds get for active management. Over 10- or 20-year stretches, the vast majority of actively managed funds in all categories underperform their benchmarks. There are always some managers who beat the market in a given year, but identifying them in advance, and finding ones who do it consistently, is extremely difficult. That’s the core argument for indexing: rather than trying to beat the market, you capture the market’s return at minimal cost.
What You Can Invest In
Index funds cover far more ground than just the S&P 500. You can build a diversified portfolio using index funds alone, spanning different asset classes, geographies, and sectors.
- U.S. stock market: Funds tracking the S&P 500, the total U.S. stock market, or narrower slices like mid-cap (S&P Mid-Cap 400) and small-cap (S&P SmallCap 600) indexes.
- International stocks: Funds following indexes like the FTSE All-World ex-US or MSCI EAFE, which cover developed and emerging markets outside the United States.
- Bonds: Funds tracking the U.S. Aggregate Bond index, short-term Treasury bonds, corporate bonds, or inflation-protected securities (TIPS).
- Sector-specific: Funds focused on technology, health care, consumer goods, or other industries.
- Global: Total world stock funds that combine U.S. and international holdings in a single fund.
A common starting approach is pairing a broad U.S. stock index fund with an international stock fund and a bond fund. The specific mix depends on your age, goals, and comfort with risk.
Index Funds vs. ETFs
You’ll encounter index funds in two forms: traditional mutual funds and exchange-traded funds (ETFs). Both can track the same index, and the underlying holdings may be identical. The differences are mechanical. Mutual fund shares are bought and sold once per day at the closing price. ETF shares trade throughout the day on a stock exchange, just like individual stocks. ETFs sometimes have slightly lower expense ratios and offer more flexibility for buying and selling, but mutual funds let you invest exact dollar amounts rather than buying whole shares. For long-term investors, either format works well.
How to Start Investing
Buying index funds is straightforward. You need an investment account, which can be a retirement account like a 401(k) or IRA, or a regular taxable brokerage account. Major brokerages like Vanguard, Fidelity, Schwab, and others all offer their own index funds alongside funds from competing providers.
Once your account is open and funded, you search for the fund by name or ticker symbol, choose how much to invest, and place the order. There’s no minimum investment for many index fund ETFs beyond the price of a single share (often between $30 and $500), and some brokerages allow fractional shares, letting you start with as little as $1. Traditional index mutual funds sometimes have minimums ranging from $0 to $3,000, depending on the provider.
If you already have a 401(k) through your employer, check the fund lineup. Most plans include at least one or two index fund options, typically an S&P 500 fund or a total market fund. These are often the lowest-cost choices available in the plan.
Who Offers Index Funds
The largest providers include Vanguard, Fidelity, Schwab, BlackRock (through its iShares ETF brand), and State Street. T. Rowe Price, DFA, and Nuveen also offer well-regarded options. Competition among these firms has driven costs down steadily over the past two decades, which benefits investors directly. When comparing funds that track the same index, the main differentiators are expense ratio, minimum investment, and whether the fund is structured as a mutual fund or ETF.
There’s no meaningful performance difference between two funds tracking the same index from different providers. A Vanguard S&P 500 fund and a Fidelity S&P 500 fund will deliver nearly identical returns. The expense ratio is the tiebreaker, and even there, the gaps among major providers are often just a few hundredths of a percent.