S&P 500 vs. Nasdaq-100 vs. Dow 30

Three questions, three rulebooks

The S&P 500 measures a large-cap segment of the U.S. equity market. The Nasdaq-100 draws eligible large non-financial companies from Nasdaq listings and applies its own modified weight rules. The Dow contains 30 selected U.S. companies and gives a higher-priced constituent more influence per percentage point than a lower-priced one, subject to its divisor.

Nasdaq's current methodology can admit multiple eligible share classes of one company and may temporarily have more than 100 securities. '100' refers to the index design, not a promise that the security count is always exactly 100.

A hypothetical move looks different

Suppose a security has a 6% weight in a simplified cap-weighted index and rises 2%. Its approximate contribution is 6% × 2% = 0.12 percentage points before other constituents and methodology effects. In a price-weighted example, a $200 stock and a $100 stock have a 2-to-1 influence for the same percentage move; a $1 move in either stock changes the price sum by the same dollar amount.

These examples explain the weighting mechanism, not current constituent weights or an actual session's index points.

Do not swap the index for its ETF

IVV, QQQ, and DIA are funds designed to track the S&P 500, Nasdaq-100, and Dow respectively. Fund holdings are useful dated references, but their weights, cash, fees, and timing can differ from official index values. The difference is especially important when comparing DIA fund holdings with the Dow's price-weighted formula.

Key takeaway

Name the benchmark, the weighting method, the data source, and the measurement time before comparing index moves. Market Watch 360 separates these views so the same stock move can be interpreted under the relevant methodology.