What is the Sensex?
The Sensex – short for Sensitive Index, officially the S&P BSE SENSEX – is the flagship stock market index of the Bombay Stock Exchange (BSE). It tracks the share prices of 30 large, well-established, financially sound companies listed on the BSE, chosen to represent the major sectors of the Indian economy. When you hear that "the market is up" or "the market crashed," the Sensex is usually one of the two numbers being quoted.
First published in 1986, the Sensex is the oldest stock index in India and one of the most widely watched barometers of investor sentiment. The 30 constituents are blue-chip names spanning banking, IT, energy, FMCG, pharmaceuticals, automobiles and more – together they account for a large slice of the BSE's total market value, so their collective movement is treated as a proxy for how the whole exchange (and broadly, corporate India) is doing on any given day.
A single Sensex number on its own means little – what matters is how it moves. If the index rises, it means the 30 companies' shares, weighted by size, have gained value on average; if it falls, they have lost value. Its NSE counterpart is the Nifty 50, and the two usually move in the same direction.
How the Sensex is calculated
The Sensex is a free-float market-capitalisation-weighted index. In plain terms, bigger companies (by tradable market value) move the index more than smaller ones, and the index level is the combined free-float market value of all 30 stocks expressed relative to a fixed starting point.
That starting point is the base period of 1978-79, which was assigned a base value of 100 (set on 1 April 1979). Every Sensex level you see today is measured against that base. The core formula is:
Sensex = (Total free-float market cap of the 30 stocks ÷ Base market cap) × 100
In practice, the "Base market cap ÷ 100" part is handled by a single number called the index divisor, so the live calculation is simply the current free-float market cap of the 30 stocks divided by the divisor. The divisor is adjusted whenever the composition or share structure changes – for example when a company is added or removed, or when there is a bonus issue, rights issue or stock split – so that the index level stays continuous and isn't artificially jolted by these technical events.
The index switched from the older full market-capitalisation method to the free-float method on 1 September 2003, bringing it in line with leading global indices. Understanding market capitalization first makes the free-float idea below much easier to follow.
What "free-float" means
Free-float refers to the portion of a company's shares that is actually available for trading in the open market. It deliberately excludes holdings that are locked away and not traded day to day – chiefly promoter (founder/parent) stakes, government holdings, and other strategic or locked-in shares.
A company's free-float market cap is therefore smaller than its total market cap:
Free-float market cap = Share price × Total shares × Free-float factor
The free-float factor is the fraction of shares freely tradable. Suppose a company trades at ₹500 with 100 crore shares (total market cap ₹50,000 crore), but promoters hold 60% and only 40% floats freely. Its free-float market cap is ₹500 × 100 crore × 0.40 = ₹20,000 crore – and that ₹20,000 crore, not the full ₹50,000 crore, is the weight it carries in the Sensex.
- Reflects real tradable value – the index moves in line with shares investors can actually buy and sell, not paper value locked with promoters.
- Reduces concentration – promoter-heavy companies don't dominate the index just because their founders hold a large, untraded stake.
- Globally standard – most major world indices (and the Nifty 50) use the same free-float approach, making cross-market comparison fairer.
How the 30 companies are chosen and reviewed
The 30 constituents are not random – they are selected and maintained by an index committee against a published rulebook. Candidates are drawn from the broader S&P BSE 100 universe and screened on size, liquidity and how well they represent the market.
- Listing & universe – the stock must be listed on the BSE and part of the larger eligible universe (the S&P BSE 100).
- Size – it should rank among the top companies by free-float and total market capitalisation (broadly the top ~75).
- Liquidity – the stock must be actively and consistently traded, so the index reflects prices that real buyers and sellers transact at.
- Minimum float weight – a constituent should carry a minimum free-float weight (around 0.5%) so each name is meaningful to the index.
- Sector balance – the committee aims for the 30 to broadly represent the major sectors of the economy rather than cluster in one industry.
The committee reviews the index periodically (typically semi-annually, around June and December) and can swap out a company that no longer fits – replacing it with one that better reflects current market conditions. Changes can also happen between scheduled reviews if a constituent undergoes a major corporate action such as a merger, acquisition or delisting. This rotation is why the Sensex line-up of decades ago looks very different from today's.
Sensex vs Nifty 50
India's two headline indices are the Sensex and the Nifty 50. They measure the same thing – the performance of India's largest, most liquid companies – but belong to the two different exchanges, which is why both exist side by side.
| Feature | Sensex | Nifty 50 |
|---|---|---|
| Exchange | BSE (Bombay Stock Exchange) | NSE (National Stock Exchange) |
| Number of stocks | 30 | 50 |
| Launched | 1986 (base year 1978-79 = 100) | 1996 (base year 1995 = 1000) |
| Method | Free-float market-cap weighted | Free-float market-cap weighted |
| Maintained by | BSE Index Services Pvt. Ltd (BSE) | NSE Indices Ltd |
| Common ticker label | INDEXBOM: SENSEX | INDEXNSE: NIFTY_50 |
Because the two share many of the same large companies, they move very closely together – when one is up for the day, the other almost always is too. The Nifty 50, with 50 stocks, casts a slightly wider net; the Sensex, with 30, is the older and narrower benchmark. Neither is "better": they are simply each exchange's flagship index. Many of the Sensex's 30 companies also sit in the NSE's flagship – you can browse those large-caps on our Nifty 50 stocks list reference.
How to invest in the Sensex
You can't buy "the Sensex" directly – it's a number, not a share. But you can invest in instruments that mirror it so your returns track the index almost exactly, minus a small fee.
- Sensex index funds – mutual funds that hold the same 30 stocks in the same proportions as the index. You can invest through a lump sum or a monthly SIP, just like any mutual fund.
- Sensex ETFs – exchange-traded funds that track the Sensex and trade on the exchange like a stock; you buy and sell units through a demat and trading account during market hours.
- Sensex derivatives – futures and options on the index, used mainly by traders and for hedging rather than long-term investing; these carry higher risk and are not beginner-friendly.
For most long-term investors, a low-cost index fund or ETF bought steadily via a monthly SIP is the simplest way to capture the index's long-run growth without picking individual stocks. You can estimate how a regular monthly contribution might grow over time with our SIP calculator. Remember that index funds rise and fall with the market, so returns are never guaranteed.
Trading and SIP execution only happen when the market is open – check the share market timings and the holiday calendar on our stock market holidays page before placing orders.
"BSE Sensex," "S&P BSE Sensex" and "INDEXBOM: SENSEX" – all the same index
You'll see the Sensex written several different ways across Google, news sites and trading apps. These are labels for the very same index, not different products:
- Sensex / BSE Sensex – the everyday names. "BSE" just signals it belongs to the Bombay Stock Exchange.
- S&P BSE SENSEX – the official branded name. The index was co-branded with S&P Dow Jones Indices for years; it is now calculated and maintained by BSE Index Services Pvt. Ltd, which became a wholly owned subsidiary of BSE in 2024, while the "S&P BSE" branding remains in use.
- INDEXBOM: SENSEX – the ticker code Google Finance uses ("BOM" = Bombay). On other platforms you may see ^BSESN. These are just symbols for the same Sensex number.
So whether a source says "Sensex," "BSE Sensex," "S&P BSE SENSEX" or shows "INDEXBOM: SENSEX," they are all quoting the identical 30-stock BSE benchmark described on this page. If you also follow the futures market, note that GIFT Nifty tracks the Nifty 50 (the NSE benchmark), not the Sensex.