What is the S&P 500 Index and What Companies Does It Include?

Simply put, the S&P 500 Index tracks the performance of 500 large-cap US companies and is widely regarded as a benchmark for the US large-cap stock market and even the overall US economy^[1]. It is compiled by S&P Dow Jones Indices and is weighted by free-float market cap, meaning the larger the company, the higher its weight in the index^[2]. Think of it as a 'basket of stocks' containing the 500 largest US companies, but each company has a different 'weight': large-cap companies (like Apple) have a bigger share in the basket, while smaller ones have a smaller share. This weighting means that price movements of big companies affect the index far more than those of small companies.

Many people assume the S&P 500 is simply the top 500 companies by market cap, but that's not the case. The index's constituents are not automatically generated by market cap ranking; instead, the Index Committee of S&P Dow Jones Indices reviews and decides based on publicly disclosed criteria such as market cap thresholds (reviewed quarterly), positive GAAP earnings, liquidity, public float, and listing on US exchanges, with the committee having some discretion in the process^[3]. The specific rules are detailed in the official methodology document, including total and free-float market cap thresholds, and positive GAAP net income for the most recent quarter and the sum of the last four quarters^[4]. In other words, it's not just 'big enough to get in'; you also need to meet 'hard criteria' like profitability and liquidity, and the committee has the final say—it's a bit like a 'talent show' rather than 'automatic admission by score.'

How Can Ordinary Investors Buy S&P 500 Index Funds or ETFs?

First, investors cannot directly 'buy' the S&P 500 index itself; they can only invest indirectly through index funds or ETFs that track the index^[8]. Index funds use a passive investment strategy, aiming to achieve roughly the same returns as the index they track (before fees)^[8]. Think of an index fund as a 'pre-packaged basket': the fund company buys the stocks according to the index's composition and weights, and when you buy shares of the fund, you indirectly own a piece of those stocks.

You can buy S&P 500 index funds/ETFs in taxable brokerage accounts, IRAs, or employer-sponsored retirement accounts like 401(k)/403(b)^[13]. It's recommended to max out tax-advantaged accounts first and prioritize products with lower expense ratios^[13]. In practice, the steps are roughly: open a brokerage account (taxable or retirement); deposit funds; search for the ticker of your target fund (like VOO, IVV, or SPY, which we'll discuss later); decide on the amount or number of shares and place the order; if you plan to hold long-term, consider setting up automatic periodic investments (dollar-cost averaging) rather than investing a lump sum all at once. If you're not clear on the difference between ETFs and traditional mutual funds, you can first check out What is an ETF and how is it different from stocks and mutual funds—in short, ETFs can be bought and sold on the exchange like stocks at any time during trading hours, while traditional index funds (open-end mutual funds) are typically bought or redeemed at the net asset value (NAV) calculated once after market close each day. For long-term dollar-cost averaging, both work, but ETFs have lower minimums and more trading flexibility.

What's the Difference Between the S&P 500, the Dow Jones, and the Nasdaq?

The Dow Jones Industrial Average includes only 30 stocks and is price-weighted; the Nasdaq Composite includes over 2,500 stocks with a higher weight in tech and greater volatility; the S&P 500, with 500 stocks, market-cap weighting, and broader industry coverage, is considered more representative of the overall US large-cap market^[12]. The Dow is price-weighted, meaning that stocks with higher prices (like those costing several hundred dollars per share) have a greater impact on the index, regardless of their market cap; the Nasdaq is more tech-heavy, so when tech stocks surge or plunge, the Nasdaq moves more dramatically.

So, if you want to invest in US large-cap stocks, the S&P 500 is usually a more comprehensive benchmark. It covers 11 sectors (according to the GICS—Global Industry Classification Standard), including information technology, financials, healthcare, etc., unlike the Nasdaq, which is overly concentrated in tech, or the Dow, which has only 30 stocks.

What Is the Historical Average Annual Return of the S&P 500?

Since the S&P 500 officially expanded to 500 stocks in 1957, the annualized return including dividend reinvestment has been roughly around 10%, but this is a long-term average; single-year fluctuations can be extreme, and past performance does not guarantee future results^[11]. This 10% is a 'long-term average,' not a guarantee of 10% every year, but a 'geometric average' over decades. For example, one year might be up 30%, the next down 20%, but the long-term average is close to 10%.

This means that investing in an S&P 500 index fund over the long term could theoretically yield returns close to this figure, but short-term losses can be significant—for instance, during the 2008 financial crisis, the S&P 500 fell about 37%. So, if you plan to invest in the S&P 500, be mentally prepared for 'possible short-term losses' and use money you won't need for a long time (e.g., 10+ years).

What Are the Differences Between VOO, SPY, and IVV, and Which Should You Choose?

Among mainstream S&P 500 ETFs, Vanguard's VOO and BlackRock's iShares IVV both have expense ratios of 0.03% per year, making them among the most cost-effective options for long-term dollar-cost averaging^[10]. State Street's SPDR S&P 500 ETF (SPY, launched in 1993, one of the world's first ETFs) uses a unit investment trust (UIT) structure with an expense ratio of 0.0945%, offers extremely high liquidity, and is often used by traders for short-term trading, but its long-term holding cost is higher than VOO/IVV^[11]. The expense ratio is the annual management fee deducted from the fund's assets; although it looks like just a fraction of a percent, the difference compounds significantly over the long term. For example, on a $100,000 investment, a 0.03% fee costs only $30 per year, while 0.0945% costs $94.50—over decades, the difference can amount to thousands of dollars.

For long-term investors, VOO and IVV have lower fees; for short-term traders, SPY offers better liquidity. Which one you choose depends on your investment goals and trading habits. If you're a long-term dollar-cost averager, VOO or IVV is more cost-effective; if you trade frequently, SPY may have tighter bid-ask spreads, but its long-term holding cost is higher.

How Often Does the S&P 500 Rebalance, and How Are Companies Added or Removed?

The S&P 500 rebalances quarterly on the third Friday of March, June, September, and December after the market close, adjusting the constituent list and weights accordingly^[5]. Rebalancing is like regularly 'tidying up the basket': removing companies that no longer meet the criteria, adding new ones that qualify, and adjusting weights to reflect the latest market cap changes.

As mentioned earlier, inclusion criteria include market cap thresholds, positive earnings, liquidity, etc., and are decided by the Index Committee^[3]—even if a company has a large market cap, it may not be included if it fails profitability or liquidity requirements, or it may be removed during rebalancing. This is why the S&P 500 list isn't fully automatic but is dynamically adjusted by the committee based on rules.

Is It Safe to Buy S&P 500 Index Funds? Can You Lose Money?

All investments carry risk, and S&P 500 index funds are no exception. Although the S&P 500 has historically returned about 10% annually over the long term, short-term volatility can be significant, even resulting in annual losses^[11]. For instance, it lost about 37% in 2008 and also dropped quickly in early 2020 during the pandemic. So, you might see losses for a year or two after buying—that's normal market fluctuation.

Additionally, although the S&P 500 includes 500 companies, it is not fully diversified. As of June 2026, seven tech giants—Apple, Microsoft, Alphabet (Google's parent), Amazon, Nvidia, Meta, and Tesla—together account for about 33.8% of the S&P 500's total market cap, indicating that the index's returns are highly dependent on a few leading stocks^[7]. In other words, when you buy an S&P 500 fund, more than a third of your position is effectively a bet on these seven tech stocks—if they perform poorly, the index will be noticeably dragged down.