What an S&P 500 Index Fund Is and How It Works

An S&P 500 index fund holds shares in 500 large U.S. companies

An S&P 500 index fund is a fund that tracks the S&P 500, a list of 500 large American companies ranked by market value. When you own shares in an S&P 500 index fund, you own a small piece of all 500 companies at once. The fund automatically rebalances to match the index, so if Apple or Microsoft grows larger, your fund owns more of it; if a company shrinks or leaves the list, your fund owns less.

The S&P 500 is maintained by Standard & Poor's, a financial data company. It includes household names like Microsoft, Nvidia, Amazon, and Berkshire Hathaway, but also thousands of smaller large-cap companies you may never have heard of. The index is weighted by market value, meaning the biggest companies have the largest effect on the fund's performance.

You can buy S&P 500 index funds from most brokers — Vanguard, Fidelity, Schwab, and others all offer them. Some are structured as mutual funds, some as exchange-traded funds (ETFs). The difference is how you buy and sell them, not what they hold. Both track the same index and charge low fees because they simply copy the index rather than trying to beat it.

Key Takeaways

• An S&P 500 index fund owns shares in 500 large U.S. companies and automatically mirrors the index's composition and weightings.
• The fund's performance tracks the overall health of large American companies, not the performance of any single stock or manager's picks.
• S&P 500 index funds charge lower fees than actively managed funds because they simply copy the index rather than paying managers to pick stocks.
• You can buy S&P 500 index funds as mutual funds or ETFs through most brokers, and both types hold the same underlying companies.
• The S&P 500 is one of the most common building blocks in diversified portfolios because it spreads risk across 500 companies.

How the index is built and maintained

Standard & Poor's selects the 500 companies based on size, liquidity (how easily shares trade), and profitability. A company must be a U.S. corporation with a market value of at least several billion dollars to be considered. The index is weighted by market capitalization, meaning a company worth $3 trillion has far more influence on the index's movement than a company worth $50 billion.

The index changes over time as companies grow, shrink, or merge. When a company is added or removed, the fund automatically buys or sells shares to stay aligned with the index. This happens a few times per year, not constantly. Because the index is so large and well-established, changes are usually predictable and don't cause dramatic shifts in the fund's holdings.

Why investors choose S&P 500 index funds

The S&P 500 is broad enough to reduce risk through diversification — you own 500 companies across different industries — but focused enough to be a core holding. It includes technology, healthcare, finance, energy, consumer goods, and industrials, so a downturn in one sector is usually offset by strength in another.

S&P 500 index funds also charge very low fees. Vanguard's S&P 500 ETF (VOO) charges 0.03% per year, meaning you pay $3 annually on a $10,000 investment. Fidelity's mutual fund version (FXAIX) charges 0.015%. These fees are far lower than actively managed funds, which often charge 0.5% to 1% or more because they employ managers to research and pick stocks.

Many investors use an S&P 500 index fund as the foundation of a portfolio and add other funds to round it out — perhaps a bond fund for stability, an international fund for exposure outside the U.S., or a small-cap fund for companies smaller than those in the S&P 500. Others simply hold an S&P 500 fund and nothing else, betting that owning 500 of the largest U.S. companies is diversification enough.

The difference between mutual funds and ETFs

Both mutual fund and ETF versions of the S&P 500 index hold the same 500 companies and track the same index. The main difference is how you buy and sell them. A mutual fund is priced once per day after the market closes; you place an order during the day, but the price you pay is set at 4 p.m. Eastern time. An ETF trades throughout the day like a stock, so you can buy or sell at any time the market is open and see the price change in real time.

For most investors, this difference matters very little. If you are buying and holding for years, the timing of your purchase or sale on a given day is unlikely to affect your long-term returns. ETFs have a slight edge for tax efficiency in taxable accounts because of how they are structured, but the difference is small. Mutual funds are often simpler for automatic contributions — many employers and retirement plans default to mutual funds.

Popular S&P 500 mutual funds include Vanguard 500 Index Fund (VFIAX), Fidelity Spartan 500 Index Fund (FXAIX), and Schwab U.S. Large-Cap ETF (SCHX). Popular ETFs include Vanguard S&P 500 ETF (VOO), iShares Core S&P 500 ETF (IVV), and Fidelity S&P 500 ETF (FXAIX). All of these track the same index and charge fees under 0.04% per year.

What you earn from an S&P 500 index fund

You earn money from an S&P 500 index fund in two ways: dividends and capital appreciation. Dividends are payments that companies in the index make to shareholders, usually quarterly. The fund collects these dividends and passes them to you. You can reinvest them automatically to buy more shares, or take them as cash. Capital appreciation is the increase in the fund's share price as the companies in the index grow in value.

Your total return is the combination of dividends and price appreciation. Historically, the S&P 500 has returned roughly 10% per year on average over long periods, though this varies widely year to year. Some years it rises 20% or more; other years it falls 10% or more. Past performance does not predict future results, and returns depend on when you buy and sell.

Risks and limitations

An S&P 500 index fund is not risk-free. If the U.S. stock market declines, the fund declines with it. During the 2008 financial crisis, the S&P 500 fell roughly 37%. During the 2020 pandemic shock, it fell about 34% before recovering. If you need the money in the next few years, a sharp decline could force you to sell at a loss.

An S&P 500 fund also gives you no exposure to international companies, bonds, or other asset types. If you want a truly diversified portfolio, you may need to combine it with other funds. Additionally, because the index is weighted by market value, the largest companies have the most influence. In some years, a handful of mega-cap tech stocks drive most of the index's gains, leaving smaller companies in the index behind.

Finally, an S&P 500 index fund will never beat the market — by definition, it matches the market. If you are hoping to outperform, you would need to pick individual stocks or use an actively managed fund, both of which carry higher costs and higher risk of underperformance.

How to buy an S&P 500 index fund

You can buy an S&P 500 index fund through any major brokerage: Vanguard, Fidelity, Schwab, E*TRADE, Interactive Brokers, or others. Open an account, deposit money, and search for the fund by its ticker symbol (VOO, IVV, FXAIX, VFIAX, or others). You can buy as little as one share, and most brokers charge no commission.

If you have a 401(k) through your employer, your plan likely offers at least one S&P 500 index fund option. Check your plan's investment menu. If you have an IRA, you can buy any S&P 500 fund available through your IRA provider. If you are saving outside a retirement account, use a regular taxable brokerage account.

Many investors set up automatic monthly contributions to an S&P 500 fund, buying a fixed dollar amount each month regardless of the price. This approach, called dollar-cost averaging, removes the pressure to time the market perfectly and builds discipline into saving.

Frequently Asked Questions

Is an S&P 500 index fund the same as owning the S&P 500?

Yes, functionally. When you own shares in an S&P 500 index fund, you own a proportional piece of all 500 companies in the index. You do not own them directly — the fund holds the shares — but your returns track the index's performance almost exactly, minus a tiny fee.

Can I lose money in an S&P 500 index fund?

Yes. If the stock market declines, the fund declines with it. The S&P 500 has fallen 20% or more several times in the past 50 years. If you need the money soon, a market downturn could force you to sell at a loss. Over very long periods (20+ years), the index has always recovered and reached new highs, but short-term losses are real.

Why would I buy an S&P 500 fund instead of picking individual stocks?

An S&P 500 fund spreads your risk across 500 companies, so a single bad pick does not sink your portfolio. Individual stock picking requires research, time, and skill; most individual investors underperform the index. An S&P 500 fund also charges far lower fees than paying an advisor to pick stocks for you.

Do I need to do anything after I buy an S&P 500 index fund?

No. The fund automatically rebalances to track the index, collects and distributes dividends, and adjusts when companies are added or removed from the index. You can simply hold it and check your balance periodically. Many investors buy and hold for decades without touching it.

What is the difference between an S&P 500 fund and a total market index fund?

An S&P 500 fund holds 500 large companies. A total market index fund holds roughly 3,500 companies of all sizes — large, mid-cap, and small-cap. A total market fund is slightly more diversified but also includes smaller, riskier companies. Both are low-cost, passive approaches; the choice depends on whether you want exposure to smaller companies.