Market concentration: what mega caps mean for S&P 500 investors

When you buy an S&P 500 index fund you own about 500 companies, but not equally. On October 29, 2026, the ten largest holdings of the SPDR S&P 500 ETF (SPY) made up 37.4% of the fund, and the largest three, NVIDIA, Apple and Microsoft, made up 21.3%. More than a fifth of a "500-stock" fund rode on three companies. That is what investors mean by market concentration.

This article explains how concentration arises in a market-cap-weighted index, how large it is today, how it compares with history, and what it changes for someone who owns the index: the diversification they get, the risks they carry and the alternatives that exist. It doesn't argue that concentration is good or bad. It shows how to see it and think about it.

How a cap-weighted index concentrates

Most major indexes, including the S&P 500 and the Nasdaq-100, are market-cap weighted: each company's weight is its market value divided by the total market value of all members (see Market capitalization: formula, free float and enterprise value and Stock market indexes: S&P 500, Dow, Nasdaq and Russell explained). The S&P 500 uses float-adjusted values, counting only the shares available to the public, not those held by founders, governments or other insiders.

There is no cap on a single company's weight in the S&P 500. Weights move with prices every second, without any trading by the fund: if one company's shares double while the rest of the index is flat, its weight roughly doubles. That is the strength of cap weighting (it needs almost no rebalancing, so it is cheap and tax-efficient, and it matches what investors as a whole own) and the source of concentration. Winners grow into larger and larger shares of the index.

How concentrated is the S&P 500 now?

Source: State Street Global Advisors, SPY fund page, holdings as of October 29, 2026; the fund held 504 lines.

Three things stand out:

• The top three are each bigger than most sectors. NVIDIA alone (7.77%) outweighed the whole energy sector (3.77%), and NVIDIA plus Apple (15.42%) outweighed health care (10.02%) or financials (12.05%).
• The top is a technology story. Information technology was 38.4% of the fund, and most of the rest of the top ten (Amazon, Alphabet, Meta, Tesla) sit in the consumer discretionary and communication services sectors but are technology-driven businesses. Eli Lilly is the only top-ten name outside that group.
• The other ~490 companies share the remaining ~63%. The average company outside the top ten has a weight of roughly 0.13%.

Compared with history

Concentration rises and falls with market leadership. In the 1960s and 1970s a handful of industrial and oil giants, and IBM, carried heavy weights. At the peak of the dot-com boom in 2000, commonly cited estimates put the top ten at roughly a quarter of the S&P 500. Concentration then fell for more than a decade as the technology bubble deflated and leadership broadened, and rose steadily from the mid-2010s as the largest technology companies grew their profits faster than the rest of the market. Readings near 40% in 2025 and 2026 are, by the measures index providers and researchers publish, the highest in the modern history of the index.

Two facts keep this from being a simple bubble comparison. First, today's largest companies are, on the whole, highly profitable, with earnings that have grown along with their prices. Second, concentration says nothing on its own about valuation: a company can be 8% of the index and fairly priced, or 3% and overpriced. Concentration describes exposure, not mispricing.

What concentration means for an index investor

Your diversification is narrower than the count suggests

Diversification works because different companies' fortunes are partly independent. When a few companies carry a large share of the weight, and those companies share a theme (in 2026, technology spending and artificial intelligence), the index behaves more like a portfolio of fewer bets. One way to measure it is the effective number of stocks, 1 ÷ the sum of squared weights. An equal-weighted 500-stock index scores 500. A cap-weighted one with today's top-heavy weights scores far lower, in the dozens rather than the hundreds.

A few earnings reports can move the index

The largest companies' reports now move the whole market. When a company with a 7% weight moves 8% the day after its report, the index moves about half a percent from that one name. What moves stock prices? Earnings, rates, flows, news and sentiment shows the post-earnings moves for six of these companies over 2025 and 2026, averaging 6% a session.

Concentration cut both ways in the data

The most direct way to see concentration's effect is to compare the cap-weighted S&P 500 (SPY) with the equal-weighted S&P 500 (RSP), which holds the same companies at the same weight each (about 0.2%) and resets them every quarter.

Price returns from Finabase weekly closes. RSP pays a somewhat higher dividend yield, so a total-return comparison narrows the gap slightly.

When the largest companies led, in 2023 and through most of the five years, the cap-weighted index pulled far ahead. When they fell hardest, in 2022, equal weighting lost less. Neither approach is "right"; they are different bets. Cap weighting bets on the market's own valuation of each company. Equal weighting bets more on smaller members and needs regular selling of winners and buying of laggards to stay equal.

Concentration makes active managers' job harder, in both directions

A fund manager who holds a "normal" diversified portfolio of large US stocks will rarely hold 7.8% in one company. In years when the giants lead, that underweight costs performance against the index; in years when they lag, it helps. 2025 was a year the giants led, and 79% of US large-cap active funds trailed the S&P 500, according to S&P's SPIVA year-end 2025 scorecard. Why most active funds underperform: costs, arithmetic and luck covers what drives that result over longer periods.

Rules sometimes force selling

Some funds and indexes cap concentration by rule, which can cause large, mechanical trades:

• US tax diversification rules for funds (the regulated investment company tests) limit how much of a fund's assets can sit in a few large positions, which is why many sector ETFs cap their biggest holdings and must rebalance when one grows too large.
• The Nasdaq-100 has rules for a special rebalance when its largest members' combined weight passes set thresholds. One was triggered in July 2023, reducing the weights of the biggest technology companies.

The S&P 500 has no such caps, so concentration there can keep rising for as long as the largest companies outgrow the rest.

Ways investors spread the weight

None of these is a recommendation; each changes the exposure in a different way, with its own costs:

The Index investing: passive vs. active funds and what the data shows, Asset allocation: how stock, bond and cash mixes have behaved and Growth vs value investing: styles, P/E and the long record guides go deeper on these choices.

Common misreadings

• "High concentration means a crash is coming." Concentration measures exposure, not valuation. It can stay high for years, and it has fallen in the past through other companies catching up, not only through the giants falling.
• "I own 500 stocks, so I'm diversified." In count, yes. In exposure, a large share of your result depends on a handful of companies and one broad theme.
• "Equal weight is safer." It was in 2022, but it holds more smaller companies, which can be more volatile, and it lagged badly over the five years shown.
• "My other funds diversify it." Many growth, technology and even "quality" funds hold the same few companies at large weights. Overlap adds up across funds.

How Finabase shows concentration

• The heatmap. The S&P 500 heatmap sizes every member by market cap and colors it by the day's change, grouped by sector. The handful of giant tiles shows the concentration at a glance, and on a day when they all move together, why the index moved. Other indexes, such as the S&P 100 and the Dow, have their own maps.
• Sectors. The sector page shows each sector's return over periods from a day to a year, and each sector's own page, such as information technology, lists its member stocks.
• Charts and comparison. Open RSP's chart and add SPY as a comparison to see the equal-weight versus cap-weight gap for any range.

Frequently asked questions

What percentage of the S&P 500 is the top 10?

As of October 29, 2026, the ten largest holdings of SPY made up 37.36% of the fund (nine companies, because Alphabet's two share classes both appear). The figure changes daily with prices; fund providers publish it on their websites.

Is the S&P 500 too concentrated?

There is no official limit, and whether it is "too" concentrated is a judgment. It is more concentrated than at any time in its modern history, which means index investors depend more on a few companies than before. Whether that is a risk depends on how those companies' profits hold up.

Does an equal-weight S&P 500 fund do better?

Sometimes. Over the five years to October 2026 it trailed the cap-weighted index by a wide margin (about 40% against 67% in price), but in 2022 it fell less. Equal weighting usually charges more and trades more.

Why does cap weighting concentrate the index?

Because each company's weight grows with its share price. Companies whose shares rise faster than the rest automatically take a bigger share of the index, without the index fund buying anything.

Summary

A market-cap-weighted index gives the biggest companies the biggest weights, and as winners grow they take over a larger share. On October 29, 2026, SPY's ten largest holdings were 37.36% of the fund and information technology was 38.4%, the most concentrated the S&P 500 has been in its modern history. For index investors that means narrower diversification than the company count suggests and more dependence on a few companies' earnings. It cut both ways in the data: cap-weighted SPY beat equal-weighted RSP by about 28 points in price over five years, but fell more in 2022. Alternatives such as equal-weight, total-market and international funds spread the weight differently, each with its own costs.