Ten AI stocks now weigh almost a third of the S&P 500

The S&P 500 is weighted by market value, so the more a company is worth, the more of every index dollar goes into it. When a handful of AI companies grow faster than everything else, the index concentrates, and every passive investor's exposure to AI rises without a single trade.

Where the numbers come from

State Street publishes the full daily holdings of the SPDR S&P 500 ETF Trust (SPY), which tracks the index. We used the file dated October 13, 2026, which lists 502 companies plus a small cash line. Fund weights track index weights closely but not exactly, so treat the figures below as a close proxy for the S&P 500 itself. Alphabet's two share classes are combined.

Two details of the file matter. Alphabet appears twice, as Class A at 3.09% and Class C at 2.47%, which we add together. And the file carries a US dollar cash line of 0.27% that is not a company, which we leave out of every count below nContext: n=502 companies · holdings as of Oct 13, 2026 · SPY daily holdings file, State Street · margin: not applicable, reported weights.

The AI share, counted

We grouped ten companies whose recent growth is tied mainly to AI infrastructure, models or AI software: NVIDIA, Microsoft, Alphabet, Amazon, Broadcom, Meta, Micron, AMD, Palantir and Oracle. Together they carry 32.7% of the fund's weight on October 13, 2026. Apple (7.4%) and Tesla (1.6%), both in the top ten, are left out of that group because AI is not yet the main driver of their revenue; including them would take the figure to 41.7% nContext: n=502 companies · holdings as of Oct 13, 2026 · SPY daily holdings file, State Street · margin: not applicable, reported weights.

The ten largest companies of any kind carry 40.6% of the weight. The memory boom shows up too: Micron, at 1.6%, is the eighth-largest company in the index by weight, ahead of Tesla, Berkshire Hathaway and Eli Lilly nContext: n=502 companies · holdings as of Oct 13, 2026 · SPY daily holdings file, State Street · margin: not applicable, reported weights.

Two ways to picture it

• NVIDIA's weight, 8.2%, is larger than the combined weight of the 253 smallest companies in the fund. One chip designer outweighs half the index by count nContext: n=502 companies · holdings as of Oct 13, 2026 · SPY daily holdings file, State Street · margin: not applicable, reported weights.
• The ten largest companies together, at 40.6%, weigh about as much as the 467 smallest companies combined. The remaining 25 companies sit in between nContext: n=502 companies · holdings as of Oct 13, 2026 · SPY daily holdings file, State Street · margin: not applicable, reported weights.

Why regulators are watching

The Bank of England's Financial Policy Committee flagged this in July 2026. Governor Andrew Bailey said equity valuations 'continue to be high relative to earnings,' that AI-related companies were 'a particular source of growth' which 'has led to increased concentration, especially in US equity markets,' and that the risk of a sharp correction 'remains high.' The Reserve Bank of Australia's October 2026 review adds that the performance of a small group of AI-exposed technology and semiconductor firms has been a key driver of equity market moves since 2025, in the United States, South Korea and Taiwan.

What concentration means for an index investor

Concentration cuts both ways. When the AI leaders rise, a cap-weighted index rises with them; when they fall, diversification across the other 492 companies offers less cushion than the company count suggests, because those companies carry less than 60% of the weight between them nContext: n=502 companies · holdings as of Oct 13, 2026 · SPY daily holdings file, State Street · margin: not applicable, reported weights. Equal-weight versions of the index exist precisely to remove this effect, at the cost of lagging when the largest stocks lead.