NISM-Series-VIII: Equity Derivatives · Understanding Index

Index Construction Methodologies: Weighted Index Methods Explained

Updated 11 October 2026 · Fact-checked

Index construction methodology decides how much each stock influences an index. In market cap weighting, weight follows total market value. Free-float weighting uses only publicly available shares. Price weighting follows share price. Equal weighting gives each stock the same weight. Index value = Current market cap ÷ Base market cap × Base value.

Understand Index Construction Methodologies

An index is a single number that tracks the combined movement of a basket of stocks. The method used to build it decides which stocks move it the most.

In a market capitalisation weighted index, each stock's weight is its market cap: share price × total issued shares. Bigger companies have more influence. A 1% move in a large company shifts the index more than a 1% move in a small one.

A free-float market capitalisation weighted index counts only the shares available for trading by the public. Shares held by promoters, governments and other strategic holders are excluded. Free-float market cap = share price × shares in the free float. Nifty 50 and Sensex use this method, so you should know it well.

In a price weighted index, the weight depends on the share price alone. The index is the sum of prices divided by a divisor. A high-priced stock dominates, whatever the size of the company. In an equal weighted index, each stock gets the same weight, and the index is rebalanced to keep weights equal.

An index needs a base period and a base value. The base period is the date from which the index starts. The base value is the number assigned to the index on that date, such as 100 or 1000. Later values are measured against it.

Key formulas to remember

Market capitalisation: Market cap = Share price × Total number of issued shares; Used in full market cap weighted indices.
Free-float market capitalisation: Free-float market cap = Share price × Number of shares in free float; Free-float shares = issued shares less promoter, government and other locked-in holdings.
Index value (cap based): Index value = (Current market cap ÷ Base market cap) × Base value; Use free-float market cap for both current and base if the index is free-float based.
Price weighted index: Index = Sum of prices of constituent stocks ÷ Divisor; Divisor is adjusted for splits and constituent changes so the index does not jump.
Equal weighted index: Weight of each stock = 1 ÷ Number of stocks; Index moves with the simple average of the stocks' percentage returns, until weights drift and are rebalanced.
Index return: Index return % = (New index ÷ Old index − 1) × 100; Use this to find the percentage change between two index values.

How to solve Index Construction Methodologies questions

Use this method for any question on index methodology or index value.

1Identify the method named in the question: market cap, free-float, price weighted or equal weighted.
2List the data given: prices, total shares, free-float shares or free-float factor, base market cap, base value.
3For cap based indices, compute each stock's market cap as price × shares. Use only free-float shares if the method is free-float.
4Add the market caps to get the current total.
5Apply: Index = (Current total ÷ Base total) × Base value.
6For price weighted, add the prices and divide by the divisor. For equal weighted, use the average of the percentage changes.
7Check the answer. If the market cap rose, the index must rise above the base value, and the reverse if it fell.

Quickest way: Ratio shortcut for index value

When to use it: Use when the question gives current and base market caps, or asks how the index changes after a price move.

Find the ratio of current to base market cap. Do not compute full figures if units cancel.
Multiply the ratio by the base value.
For a single stock price move, find the stock's share of total market cap, then multiply by its % change to get the index impact.
For equal weighted indices, simply average the percentage changes of the stocks.
Eliminate options that move in the wrong direction first.

Common mistakes in Index Construction Methodologies

Using total issued shares in a free-float index.
Students remember the market cap formula and skip the word free-float.
Fix: Read the method first. If free-float is named, multiply price by free-float shares only.
Treating a high-priced stock as large in a market cap index.
Price weighting and cap weighting get mixed up.
Fix: In cap weighted indices, size is price × shares. Price alone matters only in price weighted indices.
Mixing free-float and full market cap between current and base figures.
The base market cap is given in one form and the current in another.
Fix: Use the same basis for both numerator and denominator.
Forgetting to multiply by the base value.
The ratio looks like a final answer.
Fix: The ratio is only a multiple. Index = ratio × base value, for example 1.2 × 1000 = 1200.
Confusing base period with base value.
Both terms sound alike.
Fix: Base period is the date or time. Base value is the number set for the index on that date.
Assuming an equal weighted index is dominated by large companies.
Students carry over cap weighted thinking.
Fix: Equal weighting gives each stock the same weight at rebalancing, so small and large firms count equally.

Worked examples

Example 1

A free-float market cap weighted index has a base value of 1000 and a base free-float market cap of ₹50,00,000 crore. The current free-float market capitalisation of all constituents is ₹60,00,000 crore. What is the index value today?

Show the solution

Identify the method: free-float market cap weighted, with current and base free-float caps given.
Ratio = 60,00,000 ÷ 50,00,000 = 1.2.
Index = 1.2 × 1000 = 1200.

Answer: 1200

Example 2

A stock has a share price of ₹200, total issued shares of 10 crore and promoter holding of 60%. What is its free-float market capitalisation, assuming all non-promoter shares are free-float?

Show the solution

Free-float shares = 40% of 10 crore = 4 crore.
Free-float market cap = ₹200 × 4 crore.
= ₹800 crore.

Answer: ₹800 crore

Exam tips

Read the method word first. Free-float and full market cap give different answers from the same data.
Questions often give a free-float factor. Multiply it by issued shares to get free-float shares.
If the index is based on a ratio, check for a direction error before calculating long figures.
Know which method Nifty 50 and Sensex use: free-float market cap weighted.
Learn definitions of base value and base period as they often appear as direct recall questions.

Practice questions from Understanding Index

Index Construction Methodologies in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Index Construction Methodologies: frequently asked questions

What is the difference between market cap weighted and free-float index?

A market cap weighted index uses all issued shares of a company. A free-float index uses only shares available for public trading. It excludes holdings such as promoter and government stakes, so it better reflects what investors can actually buy.

How do you calculate index value using free-float market capitalisation?

Find the free-float market cap of each stock as price × free-float shares and add them. Divide by the base free-float market cap and multiply by the base value.

What is the difference between price weighted and equal weighted index?

A price weighted index gives more influence to stocks with higher share prices. An equal weighted index gives each stock the same weight at rebalancing, regardless of price or size.

What are base value and base period of an index?

The base period is the date from which the index is measured. The base value is the number assigned to the index on that date, such as 100 or 1000. All later index values are compared with it.