Sensex

What it is

The S&P BSE Sensex, short for Sensitive Index, is India's oldest stock market index, launched in 1986 with a base value of 100 and a base year of 1978–79. It tracks 30 of the largest and most actively traded companies on the Bombay Stock Exchange, spanning sectors like banking, IT, energy, FMCG and autos. Names such as Reliance, HDFC Bank, TCS, Infosys and ICICI Bank have long been part of it.

How it works

The Sensex is a free-float market-capitalisation weighted index — bigger companies (by the value of shares actually available to trade) move the index more. A committee reviews the constituents periodically, swapping out companies that no longer fit and adding rising large-caps. When the news says "Sensex is up 400 points," it reflects the weighted price movement of these 30 stocks.

Why it matters

The Sensex is the headline barometer of Indian equities and investor sentiment, quoted everywhere from TV tickers to WhatsApp forwards. It is a benchmark for fund managers, the basis for index funds and ETFs, and a long-run proxy for the Indian economy — it has compounded at strong double-digit-equivalent levels over decades despite many crashes.

Common mistakes

Alongside the NSE's Nifty 50, the Sensex is one of two indices every Indian investor watches, and you can invest in it cheaply through Sensex index funds and ETFs that simply mirror its 30 holdings; derivatives on the Sensex also trade on the BSE. That said, don't confuse the index level with anything about valuation — a higher Sensex number isn't "expensive" by itself, and the round-number milestones that make headlines mean little in isolation. Also remember the Sensex represents only 30 giants, so it can rise even while many mid- and small-cap stocks fall, and a few heavyweight stocks can swing it disproportionately on any given day. For broad exposure, treat it as a large-cap gauge, not the whole market — pair it with mid- and small-cap exposure if you want to capture the full breadth of Indian equities, and judge your own portfolio against the relevant benchmark rather than the headline index alone.