Market-cap weighting
What is Market-cap weighting?
Market-cap weighting assigns each security a weight based on its equity market value relative to the total market value of all index or portfolio constituents.
Why it matters
Many major equity indexes use market-cap weighting because it scales naturally with the investable market and generally requires less trading than fixed-weight methods. As a company's value rises relative to others, its weight increases automatically. The resulting index represents how public equity value is distributed, subject to eligibility and free-float adjustments.
How it works
A basic weight equals a company's market capitalization divided by the combined market capitalization of all constituents. Index providers often use free-float-adjusted capitalization, excluding strategic or otherwise unavailable shares. Corporate actions, constituent changes, and index rules can also affect weights. Funds tracking the index trade to reflect those official calculations.
Example
If three eligible companies have float-adjusted market values of $600 billion, $300 billion, and $100 billion, their weights are 60%, 30%, and 10%. A price rise in the largest company increases its weight without a rebalance trade. New share issuance or a change in free float may require an index adjustment.
How to interpret it
A capitalization-weighted portfolio is not equally diversified across businesses or risk sources. Large companies and sectors can dominate, particularly after prolonged outperformance. Compare top-holding weights, sector concentration, valuation, and factor exposure. Relative performance against such a benchmark is heavily influenced by positions in its largest constituents.
Limitations
The method allocates more capital to companies after their market values rise and does not directly consider fundamentals, expected return, or risk. It can become concentrated during market booms. Nevertheless, replacing it with another weighting method introduces different active assumptions, costs, and exposures rather than creating a universally neutral portfolio.
Practical checklist
Confirm whether weights use full or free-float capitalization and whether individual, sector, or country caps apply. Review the index provider's eligibility, reconstitution, and corporate-action rules. For an index fund, distinguish benchmark concentration from tracking differences introduced by fees, sampling, taxes, or trading. Investors concerned about dominant constituents should quantify the alternative portfolio's factor exposures and implementation costs before changing weighting methods. Review concentration after every index reconstitution.
Also known as: capitalization weighting, cap weighting