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S&P 500 index facts
Almost everyone owns the S&P 500 in some form and almost nobody knows how it's put together. It isn't the 500 largest US companies. It doesn't hold exactly 500 things. And membership isn't decided by a formula, it's decided by a committee of people applying rules and then using judgement.
This page is the mechanics. The constituent count, the total market value, how the weights are worked out, who decides what goes in, and the rules people assume exist but don't. I built it because index construction sounds like a technical footnote right up until it explains why two honest sources hand you different numbers for the same thing on the same day. There's a section on the constituent count, one on float adjustment, and one on the eligibility rules, which are stricter and more discretionary than most people assume.
Three sources: the S&P factsheet for the headline figures, Slickcharts for individual weights, and Damodaran's dataset for one date. Each is linked at the bottom with the date it refers to. Worth saying that these are snapshot numbers and they move, so if you're using any of them for something that matters, pull the current factsheet rather than trusting this page to still be right. The mechanics change far more slowly than the numbers do, and the mechanics are the useful part here.
The S&P 500 holds 503 constituents, not 500, because several companies have more than one share class in the index. Its total market capitalisation is $64.51 trillion and the ten largest companies account for 38.5% of it. Membership is decided by a committee, not a formula, which is the single most misunderstood fact about the world's most tracked index. It is not the 500 largest US companies, and it never has been.
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The rest of this page is for members
Below this point there are 6 sections and 3 named sources, roughly 1050 words of it. Every figure carries the source it came from and the date the data is from.
You can keep browsing every statistic in the library for free. The intro and the summary are always open.