The Sensex is the benchmark index of the BSE, made up of 30 of the largest and most traded companies on the exchange, weighted by free-float market value. It started with a base of 100 in the financial year 1978-79, crossed 80,000 for the first time on 4 July 2024, and was trading around 72,000 in early October 2026. Along with the Nifty 50, it is the number most Indians mean when they ask how the market did today.
The Sensex, short for Sensitive Index, is the oldest stock index in India and the headline number for the BSE. It was launched in 1986 and calculated back to a base of 100 for 1978-79. A reading of 72,000 means the basket of 30 companies is worth about 720 times what it was then. Asia Index Private Limited maintains it. That firm became a wholly owned BSE subsidiary in May 2024, after BSE bought out its former partner S&P Dow Jones Indices.
How it is built
The index holds 30 stocks, chosen for size, liquidity and sector balance. Since 2003 the weights have been based on free-float market cap. That is the value of the shares actually available for trading, leaving out promoter and government holdings. So a company with a large public float carries more weight than one with the same total value but a 75% promoter stake. The list is reviewed twice a year, in June and December. Changes are announced a few weeks before they take effect. The mechanics, including the divisor and what gets a stock added or dropped, are covered in what the Sensex tells Indian traders.
Milestones
The Sensex first crossed 1,000 in 1990, 10,000 in February 2006 and 21,000 in January 2008, just before the global financial crisis cut it by more than half. It passed 50,000 in January 2021 during the post-COVID bull market and 80,000 on 4 July 2024. It then gave back some of those gains. In early October 2026 it was trading near 72,000. That left the Nifty 50 on a trailing price-to-earnings ratio of about 19, below its five-year average.
Sensex and Nifty 50
The two indices overlap almost completely. Every Sensex company is also in the Nifty 50, so the two move in step. A 1% day on one is nearly always a 1% day on the other. The Nifty 50 is wider at 50 stocks. It is the one most index funds and derivatives traders follow, because the NSE has deeper liquidity. The Sensex's second life is in derivatives. SEBI limited each exchange to one weekly index expiry in late 2024. Since then Sensex weekly options have been the main alternative to Nifty options, with their own expiry day. The differences for a trader are set out in Nifty 50 vs Sensex.
What the number means
The Sensex is a price index. It rises when its 30 companies get more valuable and falls when they get cheaper. It ignores dividends. A total return version that reinvests dividends is also published, and it is the fairer yardstick for a long-term investor. The index says nothing about mid caps or small caps. They can rise while the Sensex falls, and the other way round. Banks, IT and energy dominate it. A bad quarter for those three sectors can pull it down while most of the market is fine.
How traders use it
As a market gauge first. A fall of 20% from the peak marks a bear market. As a tradeable instrument second, through Sensex futures and options on BSE. Those have smaller lot values than Nifty contracts and suit traders with less capital. And as the benchmark that any large-cap portfolio or fund should be measured against.
FAQs
How many stocks are in the Sensex?
Thirty. The Nifty 50 has fifty.
What was the Sensex base year?
The financial year 1978-79, with a base value of 100.
Who calculates the Sensex?
Asia Index Private Limited, a wholly owned subsidiary of BSE since May 2024.
Can the Sensex be bought?
Not directly, but index funds and ETFs that track it can, and Sensex futures and options trade on BSE.