29 Aug 2026 · 6 min read
A lot of traders still find CAS confusing, and honestly, the official explanations don't help much. Here's the plain version — no jargon, and no assuming you already know what VWAP means.
CAS is short for Closing Auction Session — a few minutes at the end of the trading day whose only job is deciding a stock's official closing price. Everything before that runs like a normal trading day. CAS only touches the very last stretch.
The old system had a real weak spot. A stock's closing price used to be just the average price over the last 30 minutes of trading. On a quiet day, that made it easy to nudge — place a burst of orders right at the end and the closing price moves, even though nothing about the company changed. Traders call this marking the close.
It matters because that one number feeds into index calculations, derivatives settlements, and how mutual funds value their holdings for the day. CAS exists to make that number harder to game.
CAS runs for 20 minutes, 3:15 to 3:35 PM, and moves through a few phases without you needing to do anything special. The exchange starts by working out a reference price — the average price from trades between 3:00 and 3:15 PM. From 3:20 to 3:25 PM you can place normal orders, market or limit. After 3:25 PM, only limit orders go through, and here's the clever part: this window doesn't close at a fixed time. It shuts randomly somewhere between 3:28 and 3:30 PM, so nobody can time an order to land right before the cutoff. Stop-loss and iceberg orders don't work during any of this — just plain limit and market orders.
By 3:35 PM, the exchange has matched everything and settled on one price — whichever price lets the most shares change hands, capped within 3% of that opening reference price — and that becomes your official close. Trading doesn't end there either; a short pause runs till 3:50 PM, then the usual post-close session picks up exactly as it always has, till 4 PM.
Not every stock gets this treatment — only ones with F&O contracts, which NSE calls Phase I, hinting more could get added down the line. Everything else still closes the old way, off the last 30 minutes' average. And if none of this rings a bell, that's not you being out of the loop — CAS itself only arrived in 2026.
If you're running an SIP or holding for the long haul, none of this changes your day. You don't need to watch the clock. It only starts to matter if you're placing an order in that last half hour before the market shuts.
A few brokers pushed SEBI to roll CAS back after the early confusion. SEBI Chairman Tuhin Kanta Pandey shut that down in August 2026: "CAS is here to stay for sure," he said — SEBI will keep tweaking rough edges based on feedback, but the system itself isn't going anywhere.
None of this changes how you invest day to day. It's just worth knowing what that closing price actually is — and that it's not some temporary rule that might vanish next quarter.
About the author
Co-Founder | Research & Investment
B.E. in Civil Engineering · M.B.A. in Finance · NISM-certified Research Analyst · NISM-certified Equity Derivatives
With a passion for financial markets spanning more than a decade, Aditya's work is centred around understanding businesses, markets, and the sectors in which they operate. He believes that meaningful wealth creation is built over the long term through disciplined investing, continuous learning, and informed decision-making. His primary focus is equity and sector-specific research — he continuously studies companies, industries, and market trends, with this research forming an important part of the stock and mutual fund selection process at Vision Investment.
“Good investing begins with good research — and good research never stops.”
Co-Founder
Co-Founder | Mutual Funds, Insurance & Taxation
B.Com. in Taxation · LL.B. · NISM-certified Equity Derivatives · AMFI-certified Mutual Fund Distributor
With more than a decade of interest and experience in financial markets, Dip brings a complementary perspective to Vision Investment, combining investment knowledge with a strong understanding of taxation, mutual funds, and insurance. He has been involved in GST-related matters since the introduction of GST in India, developing practical experience in the evolving tax and compliance environment. His primary areas of focus are mutual fund investments, insurance solutions, and taxation-related matters — helping clients understand the financial and tax implications of their decisions.
“The right financial decision is not only about returns — it is about understanding the complete picture.”
