Microsoft, Alphabet, Amazon Earnings Test 15% S&P 500 Weight

Microsoft, Alphabet, and Amazon report quarterly earnings within two days of each other, a scheduled test for S&P 500 funds where the three companies collectively hold roughly 15% of the index weight. Microsoft and Alphabet are scheduled to report on Wednesday, Oct. 28, while Amazon follows on Thursday, Oct. 29.

The S&P 500 has climbed around 12% in 2026 as of this writing, excluding dividends. Despite this gain, the three megacaps have trailed the broader index. Microsoft shares are up around 8% this year, Amazon is up about 7%, and Alphabet Class A shares are up about 2%. In contrast, Apple and Nvidia, which together made up around 14.6% of Vanguard's S&P 500 index fund at the end of 2025, have contributed more significantly to the index's rise. Apple is up about 28% and Nvidia about 14%, adding roughly 3 percentage points to the index's gain based on end-2025 weights. The three laggards added about 1 point in total.

The shift in relative performance has altered the fund's composition. Nvidia and Apple now make up about 15.5% of the index, while Microsoft, Alphabet, and Amazon have dipped to about 15%. This concentration means that for a holder with $10,000 in an S&P 500 fund, approximately $1,500 is invested in these three stocks. Historical volatility suggests these reports can move a fund meaningfully; each of the three stocks has moved around 10% or more the day after at least one report in 2026.

Recent post-earnings reactions highlight this volatility. Microsoft dropped about 10% after its January fiscal second-quarter report but jumped about 16% following its July fiscal fourth-quarter report. Alphabet Class A shares climbed around 10% after April results and fell about 7% after July results. Amazon slid about 6% after its February report before soaring about 15% in July. If all three stocks repeated their July reactions, the impact on a $10,000 fund would be an increase of about $110, or a bit over 1%, assuming the rest of the index stayed flat. A repeat of their January and February reactions would decrease the fund by about $80.

While these moves are significant for individual holdings, they represent roughly what the whole index can move in one session; the S&P 500 dropped about 1.2% on Oct. 22 alone. Nvidia, Apple, Microsoft, Alphabet, and Amazon now constitute about 30% of the index. Alphabet Class A shares trade about 21% below their 52-week high, while Microsoft is about 5% below its own high. The upcoming reports may shrink or widen the gap between these laggards and the rest of the index, though the direction remains uncertain.