cap-weight

An index construction method where components are weighted according to their total market capitalization.

A cap-weight (or capitalization-weighted) index is a stock market index construction method where individual components are weighted according to their total market capitalization. In this framework, the weight of each stock is directly proportional to its total market value, meaning larger companies naturally command a larger share of the index. This structure is designed to mirror the actual market value of all outstanding shares in the investable universe. To calculate a component's weight, its individual market capitalization is divided by the sum of the market capitalizations of all companies in the index. Because market capitalization fluctuates daily with stock prices, the weights of the underlying holdings shift automatically. This self-rebalancing nature makes cap-weighted strategies highly cost-efficient for exchange-traded funds and mutual funds, as they require minimal portfolio turnover compared to other weighting methodologies. Investors care about cap-weighting because it serves as the default representation of market beta and forms the basis of major benchmarks like the S&P 500. However, a key vulnerability is market concentration. During periods when a few mega-cap companies outperform the rest of the market, these giant stocks can dominate the index's performance. This concentration can mask underlying weakness in the average constituent and expose passive investors to systemic sector risks.

An index consists of only three companies: Company A with a market cap of $500 billion, Company B with $300 billion, and Company C with $200 billion. The total index market cap is $1 trillion ($500B + $300B + $200B). Company A's weight is calculated as $500 billion / $1 trillion = 50%, Company B's weight is 30%, and Company C's weight is 20%.