The S&P 500 tracks the share prices of 500 of the largest U.S. public companies, market-cap weighted. It covers about 80% of U.S. stock-market value and is the default benchmark for "the market".
The 500 companies are not treated equally: each one's weight is proportional to its total market value, so the biggest names move the index far more than the smallest. That is why the S&P 500 can rise on a day when most of its members actually fell — the gap market breadth is designed to reveal.
Not the index itself, but you can buy a low-cost index fund or ETF that holds all 500 companies in the same proportions.
Because it is cap-weighted: the largest companies move the index far more than the smallest, so a few big winners can lift it.
No — the Dow tracks 30 price-weighted companies; the S&P 500 tracks 500 companies weighted by market value, a much broader gauge.
Browse all S&P 500 tickers to see this metric applied to individual companies.
Educational research only — not investment advice.