Closing Auction Session (CAS) Explained: New NSE/BSE Closing Price Rules From August 2026

What Used to Decide Your Stock's Closing Price

Here's something most traders get wrong: the closing price you see was never just the last traded price (LTP) of the day. Even before CAS, the exchange took every trade between 3:00 p.m. and 3:30 p.m. and worked out a volume-weighted average. That average, not the single last trade, became the official close.

This had a real weak spot. Picture a thinly-traded stock. One small trade at 3:29 p.m., at an odd price like ₹980, could sit in that average alongside thousands of shares that traded near ₹1,000 all day. In a low-volume name, that one trade could tug the closing price further than it should. Anyone holding an index fund tracking that stock, or an F&O contract expiring that day, absorbed that tug without ever placing the order themselves.

That number does more work than most people realise. It sets the day's closing value of the Sensex and Nifty, it is what fund houses use to strike each mutual fund scheme's NAV, it decides the final settlement of F&O contracts expiring that day, it is the price your broker's P&L statement values your holdings at, and it is the price applied when you pledge shares as collateral for margin. A closing price that can be nudged is therefore a problem reaching well beyond the stock itself.

What's Changing From August 3, 2026

CAS replaces that average with an auction. For a short window, every buyer and seller places their order into one shared pool. Nobody's single trade sets the price on its own. The exchange looks at the whole pool at once and finds the one price where the most shares actually change hands. That price, not an average of scattered trades, becomes the close.

You will feel this as a customer mainly through timing and a few order-type rules, covered in detail below. The stock itself doesn't change. What changes is the last 20 minutes of the trading day, and how the number labelled "Close" gets calculated.

Which Stocks Come Under CAS, and the New Market Timings

SEBI is rolling this out in phases. In Phase I, the equity cash segment is split in two: Category I covers stocks that have active F&O contracts, and Category II covers every other listed stock. Only Category I stocks go through CAS from August 3, 2026. Category II stocks keep the old volume-weighted average method, and SEBI has left the door open to bringing them in at a later phase. The framework applies across NSE, BSE and MSEI. Market opening time does not change at all, and the commodity and currency segments are untouched.

Why Is SEBI Making This Change?

SEBI's own reasoning, echoed across exchange and broker commentary, comes down to four points:

• Cutting down on closing-price gaming: a single large, well-timed order late in the old 30-minute window could distort the average, especially in a thin stock. An auction removes that single point of leverage.
• Catching up with global practice: NYSE, the London Stock Exchange, Euronext, Hong Kong Exchange and the Australian Securities Exchange all already close their markets with a call auction of this kind. India's cash market was one of the larger exchanges still relying on a plain average instead.
• Fairer price discovery: pooling every order into one auction lets the real weight of buying and selling interest set the price, rather than whichever trades happened to land in a scattered 30-minute stretch.
• Less tracking error for index funds and ETFs: fund managers who must match an index's closing value every day get a steadier, harder-to-nudge number to work with.
• Easier execution for large orders: today an institution buying in size has to slice its order up to avoid pushing the price against itself. In an auction, a large buyer is far more likely to meet a large seller at a single price, which cuts execution cost.

The CAS Timeline, Minute by Minute

Equity derivatives keep trading until 3:40 p.m., giving F&O traders a few extra minutes to react once the new closing price lands. The morning pre-open session is being restructured too, from September 7, 2026, under the same SEBI circular. It stays 15 minutes long, but the phases change: market and limit orders from 9:00 a.m. to 9:05 a.m., limit orders only from 9:05 a.m. to 9:10 a.m. with a system-driven random close between 9:08 a.m. and 9:10 a.m., matching from 9:10 a.m. to 9:12 a.m., and the transition into continuous trading from 9:12 a.m. to 9:15 a.m. Opening and closing price discovery will finally work the same way.

Reference Price, LTP and Closing Price: What's the Difference?

These three terms sound similar and confuse a lot of traders. Here's the plain difference:

• LTP (Last Traded Price): the price of the single most recent trade, updating all day. It has never been the official closing price, before or after CAS.
• Reference price: a new, CAS-specific number. The exchange calculates it from trades between 3:00 p.m. and 3:15 p.m., and every order during the auction must stay within ±3% of it. Think of it as the auction's starting anchor, not the final answer.
• Closing price: the actual official close, now called the equilibrium price under CAS. It comes from matching every order in the auction pool, and it can land anywhere inside the ±3% band around the reference price, not necessarily at the reference price itself.

So the flow runs: LTP keeps ticking all day, a reference price gets set from a short window near the end, and the closing price gets decided afterward by the auction, inside a band built around that reference price.

How the Equilibrium Closing Price Gets Set

Once order entry freezes, the exchange looks for the single price that matches the largest possible quantity of buy and sell orders. That price becomes the equilibrium price, the stock's official close for the day. If more than one price could match the same maximum quantity, the exchange picks the price with the smallest gap between total buy and sell quantity, and if that's still tied, the price closest to the reference price.

Here's a simple version of the math. Say a stock's reference price is ₹1,000, so the auction can clear anywhere between ₹970 and ₹1,030. Keep in mind that a buyer willing to pay ₹1,010 will happily buy at ₹1,000 too, and a seller willing to accept ₹1,000 will happily sell at ₹1,010, so buying interest piles up as the price falls and selling interest piles up as it rises. Suppose the book looks like this: at ₹1,000 there are 9,000 shares of buying interest against 4,000 shares of selling interest; at ₹1,005, 7,000 against 7,000; at ₹1,010, 3,000 against 9,500. The quantity that can actually trade at each price is the smaller of the two sides, so 4,000 shares at ₹1,000, 7,000 at ₹1,005 and 3,000 at ₹1,010. ₹1,005 clears the most, so ₹1,005 becomes the closing price, five rupees above the reference price. Real order books have far more price points, but the logic stays the same: find the price that matches the most shares.

One point trips up almost everyone new to auctions: every trade in CAS goes through at that single equilibrium price, whatever your own limit price said. If you had a buy limit at ₹1,010 and the auction settles at ₹1,005, you buy at ₹1,005, not ₹1,010. A keener limit price improves your chance of being included in the match; it does not get you a different price.

In a real book the two sides rarely balance exactly at the equilibrium price, so not everyone sitting at that price gets filled. The exchange works through the queue in a set order: market orders rank ahead of limit orders and are matched against each other first, by the time they were placed; leftover market orders are then matched against eligible limit orders on price-time priority; and finally limit orders are matched against each other, again on price-time priority. Limit orders carried over from the continuous session keep the time priority they already had, so modifying one late in the day costs you your place in the queue. Anything still unmatched when the session ends simply lapses.

Two fallbacks are worth knowing. If a stock did not trade at all between 3:00 p.m. and 3:15 p.m., the reference price falls back to the day's last traded price, and if it did not trade all day, to the previous day's close adjusted for any corporate action. And if the auction itself produces no equilibrium price, because nothing matched, the reference price becomes the closing price by default.

How Will This Impact Your Futures and Options Trades?

This is where CAS reaches beyond the cash market, and it's worth separating two things that often get mixed up.

Daily mark-to-market settlement is not affected. Every day, an open futures position gets marked to market using that futures contract's own closing price, worked out from the futures contract's last half-hour of trading, not from the underlying stock's cash-market close. CAS doesn't touch this daily process at all.

Expiry-day final settlement is affected, directly. When a stock future or option expires, its final settlement price is defined off the underlying stock's closing price in the cash market. Once CAS becomes that closing price for F&O-eligible stocks, expiry-day settlement for those contracts flows from the CAS equilibrium price instead of the old VWAP. If you hold a stock future or option through expiry, your payout now depends on where that stock's CAS auction lands, not on a 30-minute average.

Index derivatives feel this too, indirectly. Nifty and Bank Nifty's closing value is built from the closing prices of their constituent stocks. Since most heavyweight index constituents carry F&O contracts and now close via CAS, the index's own closing value, and therefore index F&O settlement on expiry day, also shifts to reflect CAS pricing underneath it.

What Happens to Your Existing Orders?

Orders placed during the regular session usually carry forward into CAS without you doing anything. Two kinds don't survive the transition: stop-loss orders and iceberg orders. Both get cancelled automatically once CAS begins. Any order priced outside the ±3% band also gets cancelled or rejected, whether it's new or carried over. Carried-over limit orders keep the time priority they already had, which is worth remembering before you tweak one at 3:14 p.m. On the derivatives side, the CAS band governs futures orders between 3:15 p.m. and 3:40 p.m. and the usual dynamic price-band widening is suspended for that stretch; option orders are not subject to it.

How You'll Actually Notice the Difference

• If you're a delivery investor: almost nothing changes for you day to day. The number labelled "Close" on your holdings is simply computed differently now, and should be a fairer, harder-to-nudge figure.
• If you trade intraday (MIS): this is where you’ll feel it most. On SAHI, from August 3, 2026, MIS positions in stocks under CAS are squared off at 3:10 p.m., ten minutes earlier than the old 3:20 p.m. cutoff. Stocks outside CAS, along with index and stock F&O contracts, move to 3:23 p.m. So a CAS-stock position you planned to hold until 3:20 p.m. will already be closed. Auto square-off timings are set by each broker, so check separately if you also trade elsewhere. SAHI’s policy is set out in the auto square-off FAQ.
• If you hold F&O positions: derivatives trading now runs until 3:40 p.m., and if your contract expires that day, your final settlement traces back to the new CAS close on the underlying, not the old VWAP.
• If you invest through index funds or ETFs: you likely won't notice anything directly, but the fund itself should track its index a little more accurately, since the closing prices feeding its NAV are now auction-based.
• If you run algo or API-based strategies: build in the loss of stop-loss and iceberg orders at the CAS transition. Any logic relying on those needs to close positions before CAS starts, not during it.

Common Mistakes to Avoid With CAS

• Placing a stop-loss order and forgetting it won't carry into CAS: it gets cancelled automatically, so your protection disappears right when the closing price is being set.
• Assuming your MIS position closes at the old cutoff time: brokers have moved these times earlier for CAS stocks. Check the new time, don't assume the old one still applies.
• Placing an order far outside the ±3% band and expecting it to sit there: it gets auto-rejected instead of waiting quietly in the book.
• Trying to time an order to the last possible second: the random close inside the 3:25–3:30 p.m. window exists specifically to stop this.
• Assuming your F&O expiry-day payout works exactly like it used to: the settlement number now comes from the CAS auction, not the old 30-minute average.

Sources: SEBI circular no. HO/47/11/11(3)2025-MRD-POD2/I/2765/2026 dated January 16, 2026 and the NSE implementation circular from May 2026, introducing the Closing Auction Session and related pre-open session changes; NSE settlement-price documentation on daily MTM vs expiry-day final settlement. Confirm your broker's exact MIS cutoff times and verify live rules with the exchanges before CAS goes live.