The S&P 500 is the most widely followed gauge of the US stock market, tracking about 500 of the largest publicly traded American companies. When people say the market was up or down, they often mean this index. This guide explains what it contains, how it is constructed, and why it matters.

What it tracks

The S&P 500 measures the performance of roughly 500 large US companies spanning every major sector, from technology to healthcare to energy. Together these firms represent about 80% of the total value of the US stock market, so the index is a strong proxy for the whole. It is maintained by a committee at S&P Dow Jones Indices, which selects the members. To qualify, companies must meet standards for size, liquidity, and profitability.

How it is weighted

The index is weighted by float-adjusted market capitalization, meaning each company's influence is proportional to the market value of its publicly tradable shares. Larger companies therefore carry far more weight than smaller ones, and the biggest few can drive a large share of daily moves. This is why the index can rise even when many of its members fall, if the giants gain. Weighting by market value keeps the index reflective of where investor money actually sits.

Why it is the default benchmark

Because it is broad, transparent, and long-running, the S&P 500 is the standard yardstick against which fund managers and portfolios are measured. Beating the S&P 500 consistently is famously difficult, which is a central argument for low-cost index investing. Countless index funds and ETFs simply aim to replicate it. Its long history also makes it a reference point for expected long-run stock returns.

What it is not

The S&P 500 covers only large US companies, so it excludes small-caps, most mid-caps, and all international stocks. It is not the entire market, despite often being treated as such, and it is concentrated in its largest members. Relying on it alone leaves out diversification into smaller and foreign companies. Many investors pair it with broader or international funds for fuller coverage.

If the S&P 500 rises 1% on a given day, it means the combined, cap-weighted value of its roughly 500 members rose about 1%. A handful of the largest companies can account for much of that move because of their outsized weight in the index.

Key takeaways

  • The S&P 500 tracks about 500 large US companies, roughly 80% of US market value.
  • A committee selects members based on size, liquidity, and profitability.
  • It is weighted by float-adjusted market cap, so the biggest firms dominate.
  • It is the default benchmark most funds are measured against.
  • It excludes small-caps and international stocks, so it is not the whole market.

Common mistakes

FAQ

Are there exactly 500 companies in the S&P 500?

It targets around 500, but the exact count varies slightly because some companies have multiple share classes included.

Can I invest in the S&P 500 directly?

Not the index itself, but many low-cost index funds and ETFs are built to track it closely.