Capitalization-Weighted Index

A capitalization-weighted index is a stock market index in which each company's influence on the index's value is based on the total dollar value of its shares, not just its share price. The bigger a company is in market terms, the more it moves the index.

To build one, you take each company's market capitalization, which is its share price multiplied by the number of shares available to trade (often just the freely tradable "float" rather than every share in existence), and add those figures up across all companies in the index. That combined total is then scaled down by a divisor, a number the index provider adjusts over time so that things like stock splits, new share issuance, or companies being added and removed don't cause artificial jumps or drops in the index value.

The practical effect is that a handful of very large companies can dominate how the index behaves. If a company worth $2 trillion drops 3%, it can move a cap-weighted index far more than a $10 billion company dropping 10%, even though the smaller company's percentage move was bigger. This is different from a price-weighted index, where a stock's raw share price (not its total size) determines its pull on the index, and different from an equal-weighted index, where every company counts the same regardless of size.

The nuance that trips people up is assuming an index reflects "the average stock." In a cap-weighted index it often doesn't. A handful of mega-cap names can carry the index while most of the underlying stocks are flat or falling, which is why traders watching an index like the S&P 500 sometimes need to check what's happening under the surface before assuming the headline number tells the whole story.