How the Sensex and Nifty are actually calculated

On 13 October 2026, the Sensex closed at 73,033.78 and the Nifty 50 at 22,721.95. Both rose about 1.6% that day. But what is that number, and what does "rising 1.6%" mean?

Two baskets of shares

• SensexTracks 30 large companies listed on the BSE. Its base is the year 1978–79, set at 100. Today's 73,000 means the basket is worth about 730 times what it was then.
• Nifty 50Tracks 50 large companies on the NSE. Its base is 3 November 1995, set at 1,000.

An index is just the combined value of its basket, scaled so that the base date equals the starting number. If the basket's total value rises 1.6%, the index rises 1.6%.

Not every share counts: free float

Both indices are weighted by free-float market capitalisation. Market capitalisation is a company's share price times its number of shares. Free float means only the shares that are actually available to trade, excluding big blocks held by promoters, governments or strategic owners.

This is why a company that is 90% government-owned counts for much less in the index than its total size suggests, and why a widely held private bank can be one of the heaviest stocks in both indices.

Why a few stocks move everything

Because the biggest companies carry the biggest weights, a handful of heavyweight stocks, typically the largest private banks, Reliance and the big IT companies, can account for a large share of any day's move. On 13 October, for example, IT stocks led the rally after a strong start to the results season.

That also means the index can rise while most shares fall, or the other way round. That's why analysts watch "market breadth": how many stocks rose versus fell. Midcap and smallcap indices track the next tiers of companies.

Who decides what's in?

The Nifty is run by NSE Indices; the Sensex by Asia Index, a BSE company. Both review their baskets twice a year, adding companies that have grown and dropping those that have shrunk or become less traded. Being added to the Nifty matters: index funds that track it must then buy the stock.

Points vs percent

Headlines love points: "Sensex zooms 1,109 points." But a 1,000-point move meant much more when the Sensex was at 20,000 than at 73,000. Always look at the percentage. On 13 October, 1,109 points was a 1.54% move.