Closing Auction Session (CAS): Meaning, Timings and How the Closing Price Is Set

The closing price is the most-used number the stock market prints each day. It feeds the Nifty 50 and Sensex, sets mutual fund and ETF NAVs, marks F&O positions and values every portfolio statement. Since 3 August 2026, for stocks that have F&O contracts, that number comes from a Closing Auction Session (CAS) instead of an average of the last half-hour of trades.

SEBI mandated the change and NSE, BSE and MSEI run it. Here is what the auction is, the new timetable, how the price is actually worked out, and what changes for your orders.

Key takeaways

• Who is covered: only stocks with F&O contracts (phase 1). All other stocks keep the old VWAP close.
• When: continuous trading in F&O stocks stops at 3:15 P.M. The auction runs 3:15–3:35 P.M.
• How: orders are collected, not executed. After entry closes, everything matches at one price — the equilibrium price, where the most shares can trade.
• That price is the official close. It is used for indices, NAVs and derivatives marking.
• Derivatives trade longer: equity F&O now runs until 3:40 P.M.
• Unlisted shares are not affected. They trade off-market and have no exchange close.

New market timings

Intraday traders: brokers have moved their auto square-off earlier for F&O stocks, because continuous trading in them now ends at 3:15 P.M. Cut-offs vary by broker — roughly 3:05 to 3:12 P.M. is typical — while non-F&O stocks keep a later cut-off. Check your own broker's notice rather than assuming the old 3:20 P.M.

What is the Closing Auction Session?

In the normal Continuous Trading Session (CTS), an order executes the moment a matching order exists on the other side. The price moves trade by trade.

The closing auction works differently. For about ten minutes, the exchange only collects buy and sell orders. Nothing executes. When the entry window shuts, the exchange looks at the whole order book at once and picks the single price at which the largest number of shares can change hands. Every matched order — buyer and seller alike — trades at that one price, and that price becomes the stock's closing price for the day.

The point is that the close reflects the full pool of end-of-day demand and supply, rather than whichever trades happened to print in the final minutes.

The four stages of CAS

Why the random close? If everyone knew entry shut at exactly 3:30:00, the incentive would be to wait and drop a large order in the final second to swing the price. A random end time removes that target.

The reference price is the volume-weighted average price (VWAP) of the stock's trades between 3:00 and 3:15 P.M. It is not the closing price. It anchors the auction: orders must sit within a ±3% band around it, and orders outside the band are rejected. If the auction produces no trade, the reference price is used as the close.

How the equilibrium price is found: a worked example

Suppose the reference price is ₹514, so the band runs from about ₹498.60 to ₹529.40. At the end of order entry the book looks like this:

There are also market orders: 100 shares to buy and 150 shares to sell. Market orders have no price limit, so they count as willing to trade at every price.

Two rules drive the calculation:

• A buyer with a limit of ₹520 is happy to pay any lower price too. So the buy demand at a price is every limit buy at that price or higher, plus market buys.
• A seller with a limit of ₹510 is happy to receive any higher price too. So the sell supply at a price is every limit sell at that price or lower, plus market sells.

The shares that can actually trade at each price is the smaller of the two:

At ₹515, 1,000 shares can trade — more than at any other price. So ₹515 is the equilibrium price. Every matched buy and sell executes at ₹515, including the buyer who bid ₹520 and the seller who asked ₹510, and ₹515 becomes the official close.

When volumes are tied at two prices, the exchange applies its published tie-break rules. Within a price, market orders are matched first and limit orders then go by time priority. Anything left unmatched lapses at the end of the session.

How the closing price was set before

Before CAS — and still today for non-F&O stocks — the close is the VWAP of trades in the last 30 minutes of trading (3:00–3:30 P.M.):

VWAP = Σ (price × volume of each trade) ÷ total volume

VWAP = ₹12,47,800 ÷ 1,000 = ₹1,247.80, the closing price.

VWAP weights by volume, so it is fairer than a simple average. But it only sees trades that actually executed. In a thin or volatile last half-hour, a handful of prints — or a large order pushed through at the end — could set the close for a stock that underpins index weights and fund NAVs.

Why SEBI introduced the closing auction

• Better price discovery. Pooling all end-of-day orders produces a close that reflects total demand and supply, not a few late trades.
• Harder to manipulate. A single auction price, a ±3% band and a random close make it much costlier to push the closing print.
• Large orders execute with less impact. Index funds and ETFs that must trade at the close get a deep, single pool of liquidity instead of chasing the price down or up.
• Tighter tracking. A more representative close helps passive funds track their benchmarks more accurately.
• Global practice. The NYSE, LSE and most major exchanges already set official closes through closing auctions.

Continuous trading vs the closing auction

What it means for you

• Long-term investors mostly will not notice. Your holdings are simply valued at a more robust closing price.
• Want to trade at the close? Place a limit order within the band between 3:20 and roughly 3:28 P.M. After 3:25 P.M. you cannot add or change a market order, and the window can shut from 3:28 P.M.
• Don't expect an instant fill. You will see execution only after matching, from 3:30 P.M.
• Stop-loss and iceberg orders do not work the way they do in continuous trading during the auction; check how your broker handles them.
• Missed the auction? The post-close session (3:50–4:00 P.M.) lets you buy or sell at the closing price, subject to a matching order.

Does CAS affect unlisted shares?

No. Unlisted shares — NSE India Limited, MSEI and every other pre-IPO name — do not trade on an exchange order book, so there is no closing auction and no official close. Prices are indicative desk quotes, and deals settle as off-market demat transfers.

CAS matters to unlisted exchange shareholders in a different way: it is a new market-structure feature every exchange has to run, and the extra ten minutes of derivatives trading adds to the busiest, most fee-rich segment of an exchange's business. Background: NSE vs MSEI unlisted shares.

FAQs

Is the Closing Auction Session used for all stocks? No. Phase 1 covers only stocks with F&O contracts. All other stocks keep continuous trading until 3:30 P.M. and a VWAP-based close until the exchanges notify otherwise.

Can retail investors take part? Yes. Any investor can place orders in the auction through their broker, like any other order.

Will my auction order execute immediately? No. Orders placed between 3:20 and 3:30 P.M. are only collected. Matching happens after entry closes, and trades are confirmed between 3:30 and 3:35 P.M. — if your order is on the tradable side at the equilibrium price.

Can I modify or cancel my order? Limit orders: yes, until entry closes (randomly between 3:28 and 3:30 P.M.). Market orders: only until 3:25 P.M.; after that they are locked.

What happens if my order does not match? It lapses at the end of the session. You can still try the post-close session at the closing price.

What if the auction produces no trade? The reference price — the 3:00–3:15 P.M. VWAP — becomes the closing price.

Why does the closing price matter so much? It is used to calculate indices, compute mutual fund and ETF NAVs, mark and settle derivatives, value portfolios and set the next day's reference. A cleaner close improves all of them.

Disclaimer: This is general information on market mechanics, not investment advice. Session timings, bands and broker cut-offs are set by SEBI, the exchanges and your broker and can change — follow the latest exchange circulars. Unlisted shares are illiquid and prices are indicative, not exchange-quoted.