NISM-Series-X-A: Investment Adviser (Level 1) · Investing in Stocks

Stock Market Indices and Market Capitalisation Explained

Updated 11 October 2026 · Fact-checked

A stock market index tracks a basket of shares to show market movement. Most Indian indices, including Sensex and Nifty 50, are weighted by free-float market capitalisation. Index value = free-float market cap today ÷ base market cap × base value. SEBI ranks companies by full market cap into large, mid and small caps.

Understand Stock Market Indices and Market Capitalisation

A stock market index is a single number that shows how a group of shares is moving. It lets you judge the market, or a segment of it, without tracking every stock. It also works as a benchmark to compare a portfolio or fund.

Market capitalisation is the market value of a company's equity. It equals the share price × the number of shares outstanding. A company with 10 crore shares at ₹200 has a market cap of ₹2,000 crore.

Most Indian indices use the free-float market capitalisation weighted method. Free-float means shares available for public trading. It excludes shares held by promoters, governments, strategic holders and other locked-in holders. So free-float market cap = price × number of shares × free-float factor. The free-float factor is the fraction of shares that is freely tradable. A stock with a bigger free-float market cap gets a bigger weight in the index. The reason is simple: only tradable shares reflect what investors can actually buy and sell.

The index is calculated against a base period with a base value. Sensex (BSE) has a base of 100 in 1978-79. Nifty 50 (NSE) has a base of 1000 and a base date of 3 November 1995. Base market cap is adjusted for corporate actions such as new issues and index changes, so the index does not jump for reasons unrelated to price. This adjustment is done through the index divisor, or equivalently by adjusting the base market cap.

Sensex has 30 stocks; Nifty 50 has 50 stocks. Both are large-cap, diversified indices. Under SEBI's categorisation for mutual funds, companies are ranked by average full market cap: the top 100 are large cap, ranks 101 to 250 are mid cap, and 251 onwards are small cap. AMFI publishes the list twice a year. Remember this ranking uses full market cap, not free-float market cap.

Key formulas to remember

Market capitalisation: Market cap = Share price × Number of shares outstanding; Full market cap. SEBI's large, mid and small cap ranking uses this.
Free-float market cap: Free-float market cap = Share price × Shares outstanding × Free-float factor; Free-float factor is between 0 and 1. Used for index weights.
Index value: Index value = (Current total free-float market cap ÷ Base market cap) × Base value; Base market cap is adjusted for corporate actions through the divisor.
Weight of a stock: Weight = Stock's free-float market cap ÷ Total free-float market cap of the index; Higher free-float market cap means higher weight and more influence on the index.
SEBI cap classification: Large cap: rank 1-100; Mid cap: rank 101-250; Small cap: rank 251 onwards; Ranked by full market capitalisation. The list is updated half-yearly by AMFI.

How to solve Stock Market Indices and Market Capitalisation questions

Use this method for any index or market cap question, whether it is a calculation or a concept check.

1Read what is asked: market cap, free-float market cap, index value, weight or classification.
2Note the method named: full market cap, free-float, or price weighted. Default to free-float for Sensex and Nifty.
3Compute market cap = price × shares. Apply the free-float factor only if the question asks for index weight or index value.
4For index value, divide current free-float market cap by base market cap and multiply by the base value.
5For classification, use rank by full market cap: 1-100 large, 101-250 mid, 251 onwards small.
6Check units (₹ crore vs ₹ lakh) and that the free-float factor is a fraction, not a percentage.
7Match your result to the four options and reject any that mix full and free-float market cap.

Quickest way: Ratio shortcut for index questions

When to use it: When a question gives base and current market caps and asks for the index value or percentage change.

Find the ratio current ÷ base. Do not recompute each stock's value if totals are given.
Multiply by the base value. For example, a ratio of 1.5 on base 1000 gives 1500.
For percentage change, use (new index ÷ old index − 1) × 100.
For classification questions, recall only the rank cut-offs 100 and 250 and that the basis is full market cap.

Common mistakes in Stock Market Indices and Market Capitalisation

Using full market cap to compute index weights
Students remember market cap and forget that Sensex and Nifty are free-float based.
Fix: Whenever the word index appears with weights, multiply by the free-float factor first.
Treating free-float factor as the promoter holding
The two numbers are complements, so they get mixed up.
Fix: Free-float factor = 1 − non-tradable holdings. If promoters hold 60% and nothing else is locked, the factor is 0.40.
Classifying large, mid and small cap by free-float market cap
Students carry the index method over to the SEBI classification.
Fix: SEBI ranking uses full market capitalisation. Free-float is for index construction only.
Mixing up the number of stocks and base values of Sensex and Nifty
Both are large-cap benchmarks with similar descriptions.
Fix: Sensex: BSE, 30 stocks, base 100. Nifty 50: NSE, 50 stocks, base 1000. Link BSE with 30 and NSE with 50.
Remembering the cut-offs as 100, 150, rest
The mid cap band is 150 companies wide, so students confuse width with rank.
Fix: Mid cap is rank 101 to 250. Small cap starts at rank 251.
Assuming the index changes when a company's shares are split without a price effect
Students ignore the divisor adjustment.
Fix: Index adjustments for corporate actions keep the index continuous. Only market price movement should move the index.

Worked examples

Example 1

A company has 20 crore shares outstanding at a price of ₹500. Promoters and other locked-in holders own 55% of the shares. What is the company's free-float market cap?

Show the solution

Full market cap = 20 crore × ₹500 = ₹10,000 crore.
Free-float factor = 1 − 0.55 = 0.45.
Free-float market cap = ₹10,000 crore × 0.45 = ₹4,500 crore.

Answer: ₹4,500 crore. The index uses this figure for weight, not ₹10,000 crore.

Example 2

An index has a base value of 1000 and a base market cap of ₹40,00,000 crore (free-float). The current free-float market cap is ₹58,00,000 crore. What is the index value, and what is a stock's weight if its free-float market cap is ₹2,90,000 crore?

Show the solution

Ratio = 58,00,000 ÷ 40,00,000 = 1.45.
Index value = 1.45 × 1000 = 1450.
Weight = 2,90,000 ÷ 58,00,000 = 0.05.
Weight = 5%.

Answer: Index value is 1450 and the stock's weight is 5%.

Exam tips

Expect direct recall: number of stocks, base year, exchange, and the 100 / 250 rank cut-offs. Memorise them.
When an option says weights use full market cap for Sensex or Nifty, it is a trap. Free-float is the answer.
Calculation questions are short ratios. Watch units and the free-float factor.
In caselets, an index is often the benchmark for a client's fund. Pick the index that matches the fund's segment, such as a large-cap index for a large-cap fund.
Each wrong answer costs 25% of the question's marks, so skip only if you cannot remove two options.

Practice questions from Investing in Stocks

Stock Market Indices and Market Capitalisation in other exams

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

CMA IntermediateStock Market Indices and Market Efficiency

Stock Market Indices and Market Capitalisation: frequently asked questions

How is the Nifty 50 index calculated using free float?

Each stock's free-float market cap is its price × shares × free-float factor. The total across the 50 stocks is divided by the base market cap and multiplied by the base value of 1000. Adjustments for corporate actions keep the index continuous.

What is the difference between Sensex and Nifty?

Sensex is the BSE index with 30 stocks and a base value of 100. Nifty 50 is the NSE index with 50 stocks and a base value of 1000. Both use the free-float market cap method.

How does SEBI classify large, mid and small cap?

Companies are ranked by full market capitalisation. The top 100 are large cap, ranks 101 to 250 are mid cap, and rank 251 onwards are small cap. AMFI publishes the list half-yearly.

Why is free float used instead of full market cap?

Shares held by promoters or locked in are not available for trading. Free float reflects the shares investors can actually buy, so the index is more realistic and harder to distort.