Micron Became One of the 10 Biggest Holdings in S&P 500 Index Funds. What Does That Mean for the Stock?

Memory maker Micron Technology (NASDAQ:MU) now ranks among the biggest companies in the stock market. At the end of September, it was the ninth-largest holding in the Vanguard S&P 500 ETF (NYSEMKT:VOO), making up 1.82% of the fund.

This put it above Tesla, at 1.52%.

Funds made to track the S&P 500 (SNPINDEX:^GSPC) hold the same stocks at around the same weights, so Micron's place shows up in all of them. The whole Vanguard fund, which has about $1.7 trillion in the ETF and its other share classes, owned about 29.7 million Micron shares on Sept. 30, or around 2.6% of the company.

The rise has been quick. Micron's market value passed $1 trillion for the first time on May 26, and the stock has more than quadrupled in the last year. But shares are also down about 8% since Sept. 30, to roughly $983 as I write. I think the weight itself matters less for the stock than it may seem.

Image source: Getty Images.

The weight follows the price

The S&P 500 is weighted by float-adjusted market value. That means S&P Dow Jones Indices counts the market value of each company's shares that are available to public investors, and a stock's weight is its share of that total.

Put another way, Micron's spot in the top 10 came from its stock price, which climbed much faster than the rest of the market.

That also explains how Micron passed Tesla. At the end of September, Tesla's shares were worth around $1.4 trillion all told, more than Micron's $1.2 trillion or so.

But Micron had the higher weight, because a far larger chunk of Tesla's stock falls outside what the index counts as available to the public.

The weight moves both ways, too. With Micron's shares down since Sept. 30, its weight in the State Street SPDR S&P 500 ETF Trust was 1.54% on Oct. 26. That was below Tesla's 1.67%, and it put Micron just outside the top 10 that day.

Does the top 10 draw in more buyers?

Not on its own. After all, an index fund already holds every stock in proportion to its weight, so when Micron's price rises, the fund's stake gains value without the fund buying more shares.

Nothing new happens the day a stock moves from 11th place to ninth.

What does change is where new money goes. At a weight of around 1.6%, every $1,000 investors put into an S&P 500 fund now sends about $16 into Micron, much more than a year ago.

Of course, it works the other way, too. When money leaves these funds, their Micron holdings shrink in the same proportion. Index ownership hasn't kept the stock from sharp swings. Micron fell 6% on Oct. 22 alone, and it's about 19% under its record close, $1,213.56 on June 25.

Memory prices still matter most

Micron's stock, and its weight with it, climbed mostly because its earnings did, with demand from AI projects boosting its memory sales.

A year ago, the memory maker's quarterly revenue was $11.3 billion. Micron's fiscal 2026 ended on Sept. 3, and revenue in that year's last quarter reached $54.2 billion, up 379% year over year and 31% above the fiscal third quarter's $41.5 billion.

For the first quarter of fiscal 2027, management expects around $61.5 billion. Yes, that'd set another record. But it implies sequential growth of around 13%, a slowdown from the 31% rate of the last quarter.

Earnings per share for fiscal 2026 came to $74.33, up around 880% from $7.59 in fiscal 2025.

At around $983, the stock trades at about 13 times earnings, about half the 24.7 times earnings Vanguard listed for its S&P 500 fund at the end of September. Investors are paying much less for every dollar of Micron's profit than for the index overall, probably because memory prices have long moved in cycles.

For anyone who holds an S&P 500 fund, the top-10 spot mainly means Micron's swings show up more. A 10% fall in Micron would knock around 0.16% off the fund.

Does the top-10 spot change anything for the stock, then? Not much by itself. The index just gives Micron more weight when the stock beats the market and less when it trails. What moves the price is still memory demand.

And the company is still growing. At about 13 times earnings, I'd consider buying the shares. But anyone who holds an S&P 500 fund already has some Micron, and a fall in memory prices could hurt both.

Daniel Sparks has clients with positions in Tesla. The Motley Fool has positions in and recommends Micron Technology, Tesla, and Vanguard S&P 500 ETF. The Motley Fool has a disclosure policy.