What Is the S&P 500? The Complete Guide

The S&P 500 is the single most referenced number in American finance. When someone says “the market was up today,” they usually mean the S&P 500. But most investors who quote it could not explain what it actually measures. This guide fixes that.

What the S&P 500 Actually Is

The S&P 500 is a stock market index that tracks the performance of 500 large companies listed on U.S. stock exchanges. It is maintained by S&P Dow Jones Indices, and a committee decides which companies get in — this is not an automatic list.

To qualify, a company generally needs a large market capitalization, high trading liquidity, and a record of profitability. When a company no longer meets the bar, it gets removed and replaced. The index is rebalanced periodically, so the lineup slowly evolves with the economy. What was a railroad index a century ago is a technology-heavy index today.

Think of it as a carefully curated portfolio representing roughly 80% of the total value of the U.S. stock market.

The 500 Companies

The index spans all 11 sectors of the economy — technology, healthcare, financials, consumer goods, energy, and more. You will recognize most of the big names: Apple, Microsoft, Nvidia, Amazon, Alphabet. But it also includes hundreds of companies you have never heard of, from industrial suppliers to regional banks.

Technology is by far the largest sector, which is why the index tends to rise and fall with Big Tech’s fortunes. That concentration is worth understanding before you invest (more on that below).

Market-Cap Weighting: Why Apple Moves the Index More Than Everyone Else

The S&P 500 is market-cap weighted, meaning bigger companies have bigger influence. Each company’s weight is proportional to its total market value.

A concrete example: if Apple makes up roughly 7% of the index and a small constituent makes up 0.01%, then a 1% move in Apple pushes the index about 700 times harder than a 1% move in the small company. The index is not a democracy — it is a weighted average dominated by its giants.

This is very different from an equal-weighted index, where every company counts the same. The standard S&P 500 you see quoted everywhere is the cap-weighted version.

S&P 500 vs the Dow vs the Nasdaq-100

Three indexes, three different things:

• S&P 500: 500 large U.S. companies, cap-weighted. The broadest and the standard benchmark for “the U.S. market.”
• Dow Jones Industrial Average: Only 30 companies, and it is price-weighted — a $500 stock moves the Dow more than a $50 stock regardless of company size. It is the oldest index but the quirkiest methodology.
• Nasdaq-100: The 100 largest non-financial companies listed on the Nasdaq exchange. Heavily tilted toward technology — think of it as the S&P 500’s more aggressive tech-heavy cousin.

For most investors, the S&P 500 is the one that matters. Fund managers are judged against it, retirement plans are built around it, and financial media quotes it as the market’s pulse.

Historical Returns: What ~10% a Year Really Means

Over the long run, the S&P 500 has delivered average annual returns of roughly 10% before inflation (about 7% after inflation), including reinvested dividends. That is the single most important number in this guide.

But “average” hides a lot. The ride includes gut-punch drawdowns: the index fell by roughly half during the 2008 financial crisis, dropped about 25% in 2022, and has suffered a 10%+ decline in most individual years. There was also a stretch from 2000 to 2009 where it went essentially nowhere.

The lesson: the returns come from staying invested through the ugly periods, not from avoiding them. Nobody can time them reliably.

How to Invest in the S&P 500

You cannot buy the index itself — it is just a calculation. Instead, you buy an index fund that mirrors it. The most popular options are ETFs like VOO, SPY, and IVV, which all track the S&P 500 with annual expense ratios around 0.03%. That means you pay about $3 per year for every $10,000 invested.

Buying one is straightforward: open a brokerage account, search the ticker, and place the order. Our beginner’s guide to buying stocks walks through the mechanics, and our ETF vs mutual fund comparison explains why most investors pick the ETF version over a traditional mutual fund.

The Fine Print: Concentration Risk

One honest caveat: the S&P 500 is less diversified than it looks. A small handful of mega-cap technology companies now account for an unusually large share of the index, so when Big Tech sneezes, the “broad market” catches a cold. You are more exposed to a single sector’s fortunes than the number “500” suggests.

That does not make it a bad investment — it has been a great one. It just means you should know what you own: a bet heavily weighted toward America’s largest, mostly technological, companies.

The Bottom Line

The S&P 500 is 500 large U.S. companies, weighted by size, averaging about 10% a year over the long run with plenty of painful drops along the way. For most everyday investors, a low-cost S&P 500 index fund is the simplest, cheapest way to own a piece of the American economy — one purchase, 500 companies, 0.03% in fees. Everything else is optimization.

Not financial advice.