S&P 500 | Glossary

The S&P 500 is an index of 500 large US companies, weighted by size, tracked by the cheapest funds in America. It is not the whole market: it is roughly 80% of it, with the biggest ten companies owning about 38% of the index.

Markets & InvestingLast reviewed 2026-10-06

What it is

The S&P 500 is an index of roughly 500 large US companies, selected by a committee at S&P Dow Jones Indices and weighted by market capitalization, so the biggest companies carry the biggest weights. Launched in its modern form in 1957, it covers about 80% of US public equity value, which is why the market up usually means the S&P up. The selection criteria are boring on purpose: US domicile, positive earnings, adequate liquidity, and enough float to trade. Companies are added and removed a few times a year.

You cannot buy the index itself; you buy a fund that holds its constituents. Dozens of funds track it, from the first US ETF in 1993 to index funds charging 0.015% a year. The index has returned roughly 10% a year with dividends reinvested over the last century, and its dividend yield runs near 1.3%. It is the benchmark almost every US fund manager is measured against, and the one most of them lose to.

Key takeaways

The S&P 500 holds roughly 500 large US companies, weighted by market cap, not price-equal.
It covers about 80% of US public equity value; total-market funds cover the rest.
The ten largest companies held about 38% of the index in 2025, a multi-decade high.
Long-run return runs roughly 10% a year nominal with dividends reinvested, about 7% real.
No one invests in it directly; index funds and ETFs hold its constituents for a fraction of a percent.

How it works

The weighting is the mechanism. Each company's weight is its float-adjusted market cap divided by the index total, so a 7% weight means 7% of every dollar invested in the fund rides on that company. Price moves in the mega-caps move the whole index mechanically, which is why six stocks can produce most of a year's gain. The index is repriced continuously by the market and reconstituted by committee, usually quarterly, when companies are added or removed.

The composition is decided, not automatic. A committee applies earnings, liquidity, float, and domicile criteria, which is why the index holds roughly 500 companies rather than a formula's exact count. Sectors drift with the market: information technology carried the largest sector weight in 2025 at more than 30%, with financials and health care far behind. The drift is the market's opinion compounded, not a decision anyone made on your behalf.

Funds track it by holding every constituent at its weight, and the tracking difference is the fee plus a few basis points of friction. Equal-weight versions of the same index hold every company at the same size and produce measurably different returns. Neither version is more correct; they are different bets on the same 500 names.

Why it matters

The S&P 500 is the default answer to what should I own, and the default is mostly correct: a single purchase of diversified, profitable, US large-cap equity at nearly no cost. What the default is not is evenly spread. Roughly 38% of it sat in ten companies in 2025, so the fund is a mega-cap portfolio with a long tail.

I watched a colleague named Brent (yes, that Brent) describe his two stocks as safer than the index because he knew the companies. His two names fell 40% in a year the index fell 18%. The index is not safe; it is spread, and spread is the only free protection on offer.

The highest-leverage thing most Americans can do is own the index cheaply, automate contributions, and decide deliberately whether the concentration suits them. Project the outcome with the investment calculator or check the return history with the investment return calculator.

Real-world examples

$100,000 in the S&P 500 for 30 years

At the century-long average of 10% with dividends reinvested, $100,000 grows to roughly $1.74 million in 30 years. At a more cautious 7% real return, the same money grows to roughly $761,000 in today's purchasing power. The spread between the two numbers is inflation, the partner who takes a cut of every projection. The decade you actually get decides which number is closer to yours, which is why the century average is an expectation and not a promise. The reinvested dividends are roughly two points of the 10%.

What the 38% concentration means in dollars

A $100,000 index fund investment holds roughly $38,000 in the ten largest companies and about $3,800 in the largest one. When the mega-caps gained 40% in a year and the other 490 names gained 8%, the index returned roughly 20%, not the average company's 8%. The index return is the weighted return, not the typical return. The weight is the exposure, and the exposure is where the return and the risk both live. An equal-weight fund holds the same names at a twentieth of the size each.

The equal-weight comparison

An equal-weight S&P 500 fund holds the same 500 companies at about $200 each per $100,000 invested. In the mega-cap-led rallies of 2023 through 2025 the cap-weighted index won; after the dot-com unwind, from 2000 through 2007, the equal-weight version won by a wide margin. Same names, different weights, different decade outcomes. The choice between them is a position, not a default, and it should be taken deliberately rather than by whichever fund the platform listed first.

Common mistakes

Mistake: Calling the S&P 500 the whole market.

Why it bites you: It is roughly 80% of US public equity value; small caps and mid caps sit outside it, and total-market funds exist precisely for that gap.

Mistake: Assuming the index is evenly diversified.

Why it bites you: Ten companies held roughly 38% of it in 2025; the fund's risk is concentrated where the market cap is, and a tech drawdown moves the whole index.

Mistake: Judging the index by one year.

Why it bites you: The index fell 18% in 2022 and gained roughly 25% in each of 2023 and 2024; annual numbers are noise, and the century average is the only signal worth acting on.

Mistake: Buying the index and a dozen overlapping funds.

Why it bites you: A growth ETF, a mega-cap fund, and the S&P 500 hold the same top names three times, which concentrates the portfolio while feeling like diversification.

Ivy's take

I spent an evening with the index's own methodology documents and a century of return data so you do not have to, and the honest summary is that the S&P 500 is the closest thing investing has to a public utility. It is not clever, it is not balanced, and it is not the whole market. It is 500 large companies at whatever weights the market's prices produce, for nearly nothing.

Brent owned two stocks he knew the names of and called it research. I own 500 and call it a default. When the concentration bothers me, I add an international sleeve and an equal-weight fund, and I rebalance on a schedule that has never once been exciting.

Related jargon, defined

Market capitalization: Share price times shares outstanding; the index's weighting input.
Cap-weighted index: An index holding each company in proportion to its market cap.
Equal-weight index: An index holding every constituent at the same size.
Dividend yield: Annual dividends divided by price; near 1.3% for the index.
Total return: Price change plus reinvested dividends, the century-long 10% figure.
Index fund: A fund holding the index's constituents at index weights.

Frequently asked questions

What companies are in the S&P 500?+

Roughly 500 large US companies chosen by committee against earnings, liquidity, float, and domicile criteria. The list changes a few times a year, and the largest weights in 2025 were the mega-cap technology names.

Can I invest in the S&P 500 directly?+

No. You buy a fund that holds its constituents, from ETFs charging a few basis points to index mutual funds under 0.02% a year.

What is the average return of the S&P 500?+

Roughly 10% a year nominal with dividends reinvested over the last century, about 7% after inflation. Individual decades have ranged from strongly negative to strongly positive.

Why is the S&P 500 so concentrated in a few companies?+

Because it is cap-weighted and the largest companies grew fastest, their weights compounded to about 38% for the top ten by 2025. The index did not choose the concentration; prices did.

Is the S&P 500 the same as the Dow?+

No. The Dow holds just 30 companies and weights them by share price, which makes it a different index with a different return; the S&P is the professional benchmark.

Disclaimer: This information is general in nature and does not constitute financial or legal advice. Always consult a qualified professional for your specific situation.

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Index Fund Market Capitalization Exchange-Traded Fund Diversification Dividend Yield

S&P 500