Capitalisation Weighted Index

This index assigns a weight to each stock based on its market capitalisation, which is calculated as:

Market Cap = Share Price × Number of Outstanding Shares

Larger companies thus carry greater influence over the index's performance, while smaller companies contribute less. This methodology provides a representation of overall market performance based on company size.

In a capitalisation-weighted index, weight is directly proportional to market capitalisation. Since market cap is a function of both share price and the number of outstanding shares, companies with a larger total value receive a higher weight in the index. As a result, price movements in large-cap companies have a proportionally greater effect on the index's overall value than movements in smaller companies. This is a structural feature of the calculation method itself, rather than a deliberate preference for any particular company.

Many real-world indices, including several benchmarks, use free-float market capitalisation rather than total market capitalisation to determine weights. Free-float market cap considers only those shares that are readily available for trading in the open market, excluding shares held by promoters, governments, or other strategic holders that are not freely traded. This is calculated as:

Free-Float Market Cap = Share Price × Free-Float Factor × Total Outstanding Shares

Using free-float rather than total market cap means the index weighting more closely reflects the shares actually accessible to the market, rather than a company's entire share base.

Index weights are not static — they are reviewed and adjusted periodically to reflect changes in the market. Rebalancing typically occurs on a fixed schedule, such as semi-annually or quarterly, depending on the index provider's methodology. Adjustments can also be triggered outside the regular schedule by corporate actions such as mergers, stock splits, new share issuances, or delistings. During rebalancing, constituent weights are recalculated using updated price and share data, and constituents may be added to or removed from the index altogether.

This weighting method has characteristics that make it widely used in index construction:

• Reflects the relative market value of constituent companies

• Is automatically self-adjusting, as prices and market capitalisations change
   

Due to its alignment with market structure, this method is commonly used in important benchmarks like the Nifty 50 or S&P 500.

Other index methodologies include:

• Equal-Weighted Index: Each stock has the same weight, regardless of market cap

• Price-Weighted Index: Weight is based purely on stock price (e.g., Dow Jones)

• Fundamentally Weighted Index: Weighting based on financial metrics like earnings or revenue

Each approach varies in risk exposure, sector distribution, and rebalancing needs, making it important to understand how different methods represent the market.

Capitalisation-weighted indices are commonly used in global markets as they closely reflect overall market performance. While they provide a snapshot of the companies' market movements, they also carry structural biases particularly towards large cap companies. Understanding how these indices are constructed and calculated provides context for interpreting market representation and index performance.