What Is the S&P 500? How to Invest in It

1. The short answer, and what you actually own

Quick Answer: The S&P 500 is a measuring stick, not an investment. It tracks 500 leading US companies and covers roughly 80% of the country’s stock market value. To own it, you buy a fund that holds those same companies in the same proportions, usually through your investing account.

Ask what is the S&P 500 and you get two answers rolled into one. People say “I’m invested in the S&P 500,” but nobody owns an index. An index is a rule for measuring a group of stocks. S&P Dow Jones Indices maintains that rule and applies it to 500 leading US companies covering roughly 80% of available market value. The number you see on the news is the output.

What you can buy is a fund built to copy the rule. That fund holds real shares of the same 500 companies, in roughly the same proportions, and its price moves with the index. The fund is the product. The index is the blueprint.

That distinction matters more than it sounds. Two funds can copy the exact same blueprint and still hand you different results, because they charge different amounts to do it. We come back to that in section 6.

So the honest answer to what is the S&P 500 has three parts: a list of companies, a rule for weighting them, and a committee that maintains both. The next three sections take those apart in order.

If the word “index” still feels abstract, this short explainer sets it up cleanly before we get into the rules.

2. How a company gets into the index

Quick Answer: Being one of the 500 biggest US companies is not enough. A candidate has to be US-based, profitable, heavily traded, and large enough on a published dollar threshold. Even then a committee makes the final call, which is why size alone never guarantees a spot.

The rules are public. S&P Dow Jones Indices publishes them in a methodology document and updates the dollar thresholds as the market moves. Here is what a company has to clear.

The profit test is the one that surprises people. A famous, fast-growing company can be worth hundreds of billions and still sit outside the index for years because it has not put together four profitable quarters. This is where most short answers to what is the S&P 500 go wrong. The index is not a ranking of the biggest US companies. It is a curated list of large, profitable, liquid ones.

Changes happen quarterly, after the close on the third Friday of March, June, September and December, though a merger or bankruptcy can force a swap at any time.

3. Why weighting by size changes what you own

Quick Answer: The S&P 500 is weighted by company size, not split evenly. Every dollar you put in is divided in proportion to each company’s freely traded market value. The largest holding can move the index more than the smallest hundred combined, which is nothing like how a target date fund spreads risk.

Picture two ways to build a 500-stock portfolio with $10,000.

• Equal weight. Every company gets $20. A small member matters exactly as much as the biggest one.
• Size weight. Each company’s share depends on what its publicly traded stock is worth. The biggest names get hundreds of dollars each; the smallest get a few.

The S&P 500 uses the second method, adjusted so that only shares actually available to public investors count. A company where a founder holds most of the stock gets a smaller slice than its headline value suggests.

This is not a flaw. Size weighting means the index needs almost no trading to stay accurate, which is a large part of why copying it is so cheap. But it does mean your money is not spread evenly across 500 businesses, and that gap has grown wide.

4. Ten companies now carry about 40% of it

Quick Answer: At the end of 2025 the ten largest members made up 40.7% of the index, a record and more than double their share a decade earlier. Over $40 of every $100 you invest goes into ten companies, so an S&P 500 fund is less diversified than the number 500 implies.

The shift is recent and steep. For twenty-five years the top ten sat in a fairly stable band. Then it doubled in ten years.

In 2025 the ten largest members held roughly 41% of the index’s weight while producing about 32% of its earnings: a gap that barely existed in 2015.

There is a second wrinkle. The current leaders are not scattered across unrelated industries the way the 1990 top ten were. Most are tied to the same theme, which means they tend to fall together as well as rise together. That is the part the word “diversified” hides.

5. How to invest in the S&P 500, step by step

Quick Answer: Pick the account before the fund. A 401(k) match beats every other consideration, an IRA comes next, and a taxable brokerage account is last. Only then do you choose between an ETF and an index mutual fund, which hold the same stocks in different wrappers.

How to buy an S&P 500 index fund

Five steps, in the order that actually affects your outcome.

1. Start with the account, not the fund. Money in a 401(k) with an employer match gets a return no fund can match. If you have unmatched money left over, an IRA is next, then a regular brokerage account.
2. Find the S&P 500 option on your menu. In a workplace plan it is usually labelled “S&P 500 index” or “large-cap index.” In a brokerage account you search for an S&P 500 ETF or index mutual fund by name.
3. Check the fee before the ticker. Every fund copying the same index holds the same stocks, so the published annual fee is the main thing separating them. Section 6 shows what the gap is worth.
4. Decide how you will buy. A fixed amount on a fixed date removes the timing decision. Many brokers let you buy partial shares, so a $100 contribution can be fully invested.
5. Automate it and check once a year. Set the contribution to repeat, then leave it. Once a year, confirm the fee has not changed and that the fund still tracks the index it claims to.

Nothing in that list requires you to predict anything. That is the point.

6. What it costs, and what a small fee does over 30 years

Quick Answer: The cheapest S&P 500 funds charge a few hundredths of a percent a year, which is a couple of dollars per $10,000. The difference between 0.03% and 1.00% is small on a statement and large over decades, which is why the expense ratio is the first number to check.

Below is the arithmetic, run on a single $10,000 lump sum held for 30 years with no further contributions. We assume 7% a year before fees. That is an assumption for the model, not a forecast.

Read the last column again. A 1.00% fee costs nothing you would notice in year one and takes about $18,700 by year thirty. That is the whole argument for index funds in one line, and it is arithmetic rather than opinion.

7. Does copying the index actually beat paying a manager?

Quick Answer: Over twenty years, about 91% of actively managed US large-cap funds failed to beat the S&P 500. Over one year, roughly a quarter beat it. The longer the window, the worse the odds get, which is why a plain index fund is a reasonable default in your IRA or 401(k).

S&P Dow Jones Indices publishes this comparison twice a year, measuring the same funds against the index they are trying to beat. Here is the record through mid-2025.

Notice the three-year row is friendlier than the five-year row. Short windows are noisy, and a good stretch does not carry. The number that should shape a retirement decision is the twenty-year one, and it says nine in a hundred.

8. What the S&P 500 leaves out

Quick Answer: The index holds no bonds, no international stocks, no small companies and no real estate outside listed REITs. It covers about 80% of US stock value, which is a lot of one thing and none of several others. Anything it misses has to come from elsewhere in your portfolio.

Four gaps are worth naming.

• No bonds. When stocks fall, an S&P 500 fund falls with them. There is no ballast inside it.
• No international exposure. Members must be US-domiciled companies, so a portfolio built only on this index is a bet on one country.
• No small or mid-sized companies. Those sit in separate indexes with their own thresholds.
• Twenty percent of US stock value. The roughly one-fifth of the market outside the index is not represented at all.

None of this makes it a bad holding. It makes it an incomplete one. The common mistake is treating a single S&P 500 fund as a finished portfolio rather than as the largest piece of one.

9. The verdict

Quick Answer: Buy the cheapest S&P 500 fund on your menu, hold it inside a tax-advantaged account first, and treat it as your US large-company slice rather than your whole plan. Then decide separately what else your investing plan needs.

Our position, stated plainly. If someone asks you what is the S&P 500 and you answer “500 big American companies,” you are two-thirds right and missing the part that matters. The index earns its reputation on cost and on the twenty-year record in section 7. It has not earned the word “diversified” as fully as it did a decade ago, because ten related companies now carry about two-fifths of it.

So use it, cheaply, and know what it is: a well-run, low-cost, increasingly top-heavy slice of US large-company stocks. If that concentration keeps you up at night, the fix is a broader fund or a separate holding elsewhere, not abandoning indexing.

Companies cannot pay for placement in our rankings, and we do not sort funds by anyone’s commission.

10. Frequently Asked Questions

1. Is the S&P 500 the same thing as the stock market?

No. It covers about 80% of US stock market value, which leaves roughly a fifth of it out, plus every company listed outside the United States. It is the most widely used stand-in for the US market, not the market itself.

2. Can I buy the S&P 500 directly?

You cannot. An index is a published calculation, not a security. What you buy is an ETF or index mutual fund that holds the same companies in the same proportions, and those funds are available in most 401(k) menus and every major brokerage.

3. How many companies are actually in it?

Five hundred companies, but slightly more than 500 individual stocks. A handful of members have two classes of shares trading separately, and both classes can be included, so the stock count runs a few above the company count.

4. Is one S&P 500 fund diversified enough on its own?

It is diversified across 500 US companies but concentrated at the top, with about 40.7% of the index in ten names at the end of 2025. It also holds no bonds and no international stocks. For most people it works as the core holding, not the entire portfolio.

5. What return should I expect from it?

Nobody can tell you honestly. The index has produced strong long stretches and losing ones, and past decades do not set future ones. The 7% used in our fee table is a modelling assumption chosen to show the effect of costs, not a prediction of what you will earn.

This article is information, not financial advice. Index rules, fund fees and holdings change, so confirm current figures in the fund’s prospectus before acting. More about how we work at DollarVisor and in our disclaimer.