S&P 500 Index Funds, Explained for Beginners
An S&P 500 fund lets you own a slice of about 500 large US companies in one purchase. Here is how the index works and what to check before you buy.
Key takeaways
- The S&P 500 tracks about 500 large US companies chosen by a committee at S&P Dow Jones Indices.
- It is weighted by market value, so the biggest companies have the most influence.
- Low fees matter: the difference between 0.03% and 1% a year adds up to thousands of dollars over time.
- You can buy it as an ETF or a mutual fund. Both hold the same stocks.
What the S&P 500 is
The S&P 500 is a stock market index that represents large US companies across all major sectors. A committee adds and removes companies using rules on size, trading volume and profitability. Because it covers roughly 80% of the value of the US stock market, it is the most common yardstick for "how the market did."
The index is weighted by float-adjusted market capitalization. A company worth $3 trillion moves the index far more than a company worth $30 billion. That means the largest technology companies currently carry a large share of the index, which is worth knowing: owning the S&P 500 is diversified, but not equally spread.
How it compares with the Dow and the Nasdaq-100
| Index | Companies | Weighting |
|---|---|---|
| S&P 500 | ~500 large US firms | Market value |
| Dow Jones Industrial Average | 30 | Share price |
| Nasdaq-100 | 100 largest non-financial Nasdaq firms | Modified market value |
ETF or mutual fund?
An ETF trades on the stock exchange all day like a share, and many brokers let you buy fractional shares. A mutual fund is bought directly from the fund company at one price after the market closes, often with a minimum first investment. For most people in a taxable brokerage account, ETFs are slightly more tax-efficient. Inside a 401(k), you simply pick whatever S&P 500 option the plan offers.
Why the expense ratio matters
The expense ratio is the yearly fee, taken as a percentage of your balance. On a $10,000 investment, 0.03% costs $3 a year and 1% costs $100 a year. That gap compounds. Several large S&P 500 funds charge well under 0.10%. If two funds track the same index, the cheaper one is usually the better choice.
Try the numbers yourself in our compound interest calculator: subtract the fee from your expected return and compare the results.
Dividends
Many S&P 500 companies pay dividends. Index funds collect them and pass them to you, usually every quarter. Choose automatic reinvestment if you want them to buy more shares and compound.
Risks to understand
- It falls with the market. The index has had declines of more than 30% several times. Money you need within a few years may belong somewhere steadier.
- Concentration. A handful of giant companies drive a big share of returns.
- US only. It does not include international stocks or bonds.
Dollar-cost averaging
Investing a fixed amount on a schedule, such as every paycheck, means you buy more shares when prices are low and fewer when they are high. It does not guarantee a profit, but it removes the pressure of guessing the right day to buy, and it is how most 401(k) savers already invest.
This article is general education, not personalized financial advice. Read our editorial policy and disclaimer.
