What is the S&P 500?
Updated on August 24, 2026 · by Alex
The S&P 500 is an index that tracks the price of the 500 largest listed companies in the United States, weighted by market value. When someone says “the market was up 1%”, they are almost always talking about this index.
It is a basket. Instead of tracking one share price, it tracks 500 at once and boils them down to a single number. The companies are picked by a committee at S&P Dow Jones Indices using published criteria: they have to be listed in the United States, clear a minimum size, trade enough, and have posted profits for several quarters.
The part almost nobody has straight is that it is weighted by market capitalisation. It is not the average of 500 equally weighted companies: each one counts for what it is worth. A three trillion dollar company moves the index hundreds of times more than a twenty billion dollar one, even though both are in it.
In practice, the top ten positions — today dominated by big tech — account for roughly a third of every move. Saying “the S&P was up” sometimes means “the same seven names were up”.
It is the yardstick everything else is measured against. When a fund says it returned 14% for the year, the immediate question is what the S&P 500 did over the same year: if the index did 18%, that fund destroyed value. It is the industry's default benchmark.
It is also the most bought asset in the world by proxy: the ETFs that track it take in automatic contributions from millions of retirement plans every month. That constant flow buys all 500 without looking at price, which supports the index and concentrates even more weight in the companies that are already big.
Believing that owning the S&P 500 means you are diversified. You are by number of companies, not by actual risk. With a third of the index in a handful of tech names tied to the same cycle — artificial intelligence, semiconductors, digital advertising — a correction there drags the whole index down no matter how many banks, drugmakers and utilities sit inside it.
The second mistake is treating it as “the US economy”. These are 500 listed, large companies: the mid-sized firm is not there, the private one is not there, and a good share of their revenue comes from outside the country. The S&P can rise in a year when the American economy does badly, and it has, more than once.
Rule of thumb: when you read “the market”, ask yourself whether the person means the index or the ten companies that move it. It is usually the latter.
