Nifty 50, Sensex, S&P 500 — what are indexes and how do they work?

Here's a question that trips up even people who have been investing for years: if someone says 'I bought Nifty 50', what did they actually buy? The answer is — nothing directly. You cannot buy the Nifty 50 itself. It's a number. A measuring stick. What they actually bought is an index fund or ETF that tracks the Nifty 50 — a fund that holds all 50 stocks in the same proportions as the index. The index is the recipe. The fund is the dish.

What exactly is an index?

An index is a list of selected stocks, each given a weight, designed to represent the performance of a market or segment of a market. Think of it like a class topper list — instead of ranking students, it ranks companies by size. The Nifty 50 ranks India's 50 largest companies by something called 'free-float market capitalisation' — which is the total market value of shares that are actually available to trade (excluding promoter holdings that are locked up). The bigger the company, the more it influences the index.

How the Nifty 50 is constructed

Every six months, NSE reviews the composition of the Nifty 50. Companies that have grown large enough can enter. Companies that have shrunk, become illiquid, or faced regulatory issues can exit. As of 2025, the top five companies by weight in the Nifty 50 are typically Reliance Industries, HDFC Bank, ICICI Bank, Infosys, and TCS — these five alone can account for 35–40% of the entire index. So when Reliance has a big day, the Nifty moves meaningfully regardless of what the other 45 companies are doing.

The Sensex — what's the difference?

The Sensex is the BSE's equivalent of the Nifty — but it tracks just 30 companies instead of 50. It's older (launched in 1986, versus Nifty's 1996 launch) and historically more famous, which is why you see it on TV news. In practice, the Nifty and Sensex move together almost perfectly — they share most of the same large companies. The Nifty 50 is more widely used by institutional investors, fund managers, and for derivatives trading. If you're building a portfolio or comparing mutual fund performance, Nifty 50 is your benchmark.

Beyond the headline index — the Nifty family

The Nifty 50 is just one index. NSE operates dozens of them. Nifty Next 50 — the 51st to 100th largest companies, often the future Nifty 50 constituents. Nifty Midcap 150 — companies ranked 101 to 250. Nifty Smallcap 250 — companies ranked 251 to 500. Nifty 500 — the entire top 500. Sectoral indices like Nifty Bank, Nifty IT, Nifty Pharma — tracking specific industries. Each of these has index funds tracking them. When you see a 'banking sector fund' in your mutual fund app, it's almost certainly tracking the Nifty Bank index.

US indexes — why you keep hearing about them

The S&P 500 is to America what the Nifty 50 is to India — except the companies it tracks (Apple, Microsoft, Nvidia, Amazon, Alphabet) are often global businesses that operate in India too. When the S&P 500 falls 3% in a night, Indian FIIs who manage global money often rebalance by selling Indian stocks the next morning, which pulls our markets down even if nothing changed in India. The Nasdaq is heavier in technology — so a fall in tech stocks globally hits it harder than the S&P 500. The Dow Jones is the oldest (1896) but tracks only 30 companies and is less representative of the modern economy.

Index funds — owning the index without picking stocks

Here's the practical payoff of understanding all this. An index fund holds every stock in its target index, in the same proportion. A UTI Nifty 50 Index Fund holds all 50 Nifty companies. When Reliance is 9% of the Nifty, it's 9% of the fund. When markets rise and Reliance's weight grows to 10%, the fund automatically reflects that — no fund manager decision required. The expense ratio on these funds is 0.1–0.2%, versus 1–2% on actively managed funds. For large-cap investing where the market is efficient and hard to beat, index funds have consistently outperformed most active managers over 10+ year periods. This is not a controversial claim — it's backed by decades of data.