Understand Nifty 50, Sensex composition — Stock-Market notes

WHAT is an index? (Definition)

WHY these numbers? 30 and 50 are large enough to be diversified (one company crashing won't wreck it) yet small enough to stay liquid and easy to track.

HOW are companies chosen? (Selection criteria)

Key filters (both indices are similar in spirit):

Large market capitalization — big companies dominate the economy.
High liquidity — measured by impact cost (how much price moves when you place a trade). Low impact cost = liquid.
Listed & traded for a minimum period, domiciled in India, regular financials.
Sector representation — Banking, IT, Energy, FMCG, Auto, etc., so no single sector dominates unfairly.

Companies are reviewed periodically (Nifty semi-annually) and rebalanced — laggards drop out, risers enter.

HOW is the index NUMBER computed? (Derivation from scratch)

Both Nifty and Sensex use the Free-Float Market-Capitalization Weighted method. Let's build it up.

Step 1 — Market cap of one company. $\text{Market Cap} = (\text{Share Price}) \times (\text{Total Shares outstanding})$ Why? It's the total rupee value of the company.

Step 2 — Free-float adjustment. Not all shares trade freely. Promoters, government, and locked-in holders don't sell daily. Only the free-floating shares (available to public) drive the price. So we weight by these. $\text{Free-Float Factor (IWF)} = \frac{\text{Freely tradeable shares}}{\text{Total shares}}$ $\text{Free-Float Mcap} = \text{Price}\times\text{Total Shares}\times\text{IWF}$ Why free-float? Because promoter-held shares can't actually be bought by investors, so they shouldn't inflate a company's weight in a tradeable index.

Step 3 — Sum the basket, compare to base. $\text{Index} = \frac{\sum_{i=1}^{N}\text{Free-Float Mcap}_i}{\text{Base Market Cap}}\times \text{Base Index Value}$

For Sensex: $N=30$ , Base Value $=100$ . For Nifty: $N=50$ , Base Value $=1000$ .

Figure — Understand Nifty 50, Sensex composition

Worked Examples

Worked example Example 1 — Build a mini 2-stock index

Base day: Company A price ₹100, 1000 shares, IWF 0.5. Company B price ₹200, 500 shares, IWF 0.8. Set base index = 100.

Step — base free-float mcap. Why? We need the denominator. A: $100\times1000\times0.5 = 50{,}000$ . B: $200\times500\times0.8 = 80{,}000$ . Base = $130{,}000$ .

Today: A rises to ₹120, B stays ₹200 (shares & IWF same). A: $120\times1000\times0.5 = 60{,}000$ . B: $80{,}000$ . Today total = $140{,}000$ .

Step — index. Why? Ratio to base × base value. $\text{Index} = \frac{140000}{130000}\times100 = 107.69$ Market is up 7.69%.

Common Mistakes (Steel-manned)

Flashcards

How many companies are in Sensex vs Nifty 50?

Sensex = 30 (BSE); Nifty 50 = 50 (NSE).

What weighting method do both Sensex and Nifty use?

Free-float market-capitalization weighting.

Define free-float market cap.

Price × Total shares × Investable Weight Factor (fraction freely tradeable).

Why exclude promoter-held shares from index weight?

They aren't freely tradeable, so they shouldn't inflate a company's weight in a tradeable index.

Nifty 50 base date & base value?

3 Nov 1995, base value 1000.

Sensex base period & base value?

1978–79, base value 100.

Why can't you compare Sensex level (75000) vs Nifty level (23000) directly?

Different base values/dates; only % change is comparable.

What is 'impact cost' used for in selection?

A liquidity measure — how much price moves when a trade is placed; low impact cost = liquid = eligible.

Why is a 'divisor' adjusted during bonus/split?

To keep the index continuous when no real wealth is created by the corporate action.

Master index formula?

Index = (Σ Pᵢ·Sᵢ·IWFᵢ / Base Mcap) × Base Value.

Which exchange & maintainer for Nifty 50?

NSE, maintained by NSE Indices Ltd.

Does one company crashing wreck the index?

No — diversification across 30/50 stocks and many sectors cushions it.

Recall Feynman: explain to a 12-year-old

Imagine a class of 5000 students, too many to track. So you pick the 50 tallest, popular kids and every day you measure their average height, weighted by how much each one actually plays outside. If today that average is bigger than the first-day average, you say "the class grew!" That measured number is the index. The tallest kids (biggest companies) move the number most. If someone splits into twins (a bonus/split), you tweak your math so the number doesn't jump for a fake reason. Sensex watches 30 kids, Nifty watches 50.

Connections

What is a Share and Ownership — index members are shares you can own.
Market Capitalization — the weighting engine of the index.
Free-Float vs Total Shares — the IWF concept.
BSE vs NSE Exchanges — homes of Sensex and Nifty.
Index Funds and ETFs — how you invest in the whole basket at once.
Corporate Actions - Bonus, Split, Dividend — why the divisor exists.
Sector Diversification — why the basket spans industries.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, market mein hazaaron companies listed hain — sabko roz track karna impossible hai. Isliye humne ek chhota sa basket banaya: sabse bade aur sabse zyada trade hone waali companies. Us basket ki total value track karte hain, aur usko bolte hain index. Sensex BSE ki 30 companies ka basket hai, aur Nifty 50 NSE ki 50 companies ka. Jab basket upar jaata hai, hum bolte hain "market up hai".

Ab weight kaise milta hai? Har company ka weight uske free-float market cap se decide hota hai — matlab Price × Total Shares × IWF. IWF sirf un shares ko ginta hai jo public khareed-bech sakti hai; promoter ya government ke locked shares count nahi hote. Isliye Reliance/HDFC jaisi badi companies ka weight zyada, aur choti company ka kam. Ye galatfehmi mat rakhna ki "50 stocks hain toh har ek 2%" — bilkul nahi, weighting size ke hisaab se hoti hai.

Ek important baat: Sensex ~75000 aur Nifty ~23000 dikhte hain, par iska matlab Sensex "bada/behtar" nahi hai. Dono ki base value alag hai (Sensex base 100, Nifty base 1000). Sirf percentage change compare karo, absolute number nahi. Aur jab koi company bonus ya split deti hai, tab maintainer divisor adjust karta hai taaki index bina wajah jump na kare — kyunki actual wealth toh change hui hi nahi.

Yaad rakhne ka trick: "3-B-B, 5-N-N" — 30 stocks, BSE, Base 100 (Sensex); 50 stocks, NSE, 1995 base 1000 (Nifty). Aur "Free-Float = jo float karta hai (trade hota hai) wahi ginta hai." Bas itna samajh lo, index ka pura concept clear ho jaayega!

Test yourself — Shares, Ownership & Indices

Connections