The Sensex which was first calculated in 1986 is based on the market capitalisation (share price times the number of shares outstanding) of 30 important stocks. The actual stocks have changed over the years along with the Indian economy but in general the Sensex represents the stock prices of the largest and most important companies from a variety of sectors. It is updated every 15 seconds during trading hours.

The S&P CNX Nifty is quite similar except that it is based on 50 large stocks, many of which are also represented in the Sensex.