Index weighting is the methodology used to assign each constituent's influence on an index. Common approaches include market-cap weighting, equal weighting, price weighting, and factor-based weighting.
How it works
A cap-weighted index gives larger companies more influence; an equal-weight index gives each constituent the same starting weight. Rebalancing rules determine how those weights are reset over time.
Index return is therefore a weighted combination of constituent returns, not a vote based on how many components rose or fell.
Why it matters
A few large constituents can dominate headline index performance even when most members move in the opposite direction.
Weighting also affects concentration, turnover, sector exposure, and how an ETF or other product must trade to track the index.
A simple market example
In a five-stock example where A has 60% weight and the other four have 10% each, A rising 4% while the others fall 2% leaves the cap-weighted index positive even though four stocks declined.
Common mistakes
Assuming an index is the simple average return of all constituents.
Using a headline index move as proof that the majority of stocks moved in the same direction.
Frequently asked questions
What is market-cap weighting?
Weights are based primarily on each constituent's market capitalization, subject to the index provider's methodology.
What is equal weighting?
Each constituent receives the same target weight at rebalancing rather than a weight proportional to size.
Is an index the same as an ETF?
No. An index is a calculation methodology; an ETF is a tradable fund that may seek to track one.
Educational content only. Definitions describe common market usage and may vary by jurisdiction, instrument, or institution.