Closing Auction Session (CAS): Meaning, Timings & Process

Determining the correct closing price in the stock market is crucial. The Closing Auction Session (CAS) was introduced for this very purpose, helping to facilitate fair price discovery before the market closes. In this article, we will explain the meaning, timings, and process of the Closing Auction Session (CAS) in simple language.

What is the Closing Auction Session (CAS)? 

The Closing Auction Session (CAS) is a special trading process that takes place before the stock market closes, aimed at determining a stock’s official closing price in a more fair and transparent manner. While orders are matched continuously during regular trading, in the CAS, the exchange aggregates all eligible buy and sell orders to identify the price point that allows for the maximum volume of share transactions (Maximum Executable Quantity). This price becomes the official closing price for the day. This method provides a better reflection of the market’s actual demand and supply.

Example: Suppose the following orders were received for a company’s shares during the CAS:

Price	Buy Orders	Sell Orders
₹498	4,000	2,500
₹499	7,000	6,000
₹500	10,000	10,000
₹501	6,000	11,000

In this example, the highest volume of 10,000 shares is matched at ₹500. Therefore, the exchange will declare ₹500 as the official closing price of the share. In other words, the price selected for the CAS is the one where the highest number of buy and sell orders can be successfully matched.

Note: In CAS, the closing price is not always the Last Traded Price (LTP). It is determined based on the price that allows for the best order matching, making the closing price more reliable and fair.

Snapshot of Closing Auction Session (CAS) 

Particular	Details
Full Form	Closing Auction Session (CAS)
Introduced By	Securities and Exchange Board of India (SEBI)
Applicable On	Equity Cash Segment (Phase-wise Implementation)
Initial Coverage	F&O (Derivatives) Eligible Stocks
Effective From	3 August 2026
Session Duration	3:15 PM to 3:35 PM (20 Minutes After Normal Market Trading)
Main Objective	Fair, transparent and efficient Closing Price Discovery
Closing Price Based On	Equilibrium Price (Maximum Matching of Buy & Sell Orders)
Who Can Participate?	Retail Investors, Institutional Investors, FIIs/DIIs and Brokers (as per exchange rules)

Closing Auction Session Timings 

The Closing Auction Session (CAS) takes place on every trading day from 3:15 PM to 3:35 PM. During this 20-minute period, orders are accepted and matched in various stages, and finally, the official closing price of the share is determined based on the closing price discovery process.

Time	What Happens?
9:15 AM – 3:15 PM	Regular trading sessions take place.
3:15 PM – 3:20 PM	Transition Phase, where the Reference Price is calculated and the exchange prepares for the auction.
3:20 PM – 3:25 PM	Both Market Orders and Limit Orders can be placed, modified, or cancelled.
3:25 PM – 3:30 PM	Only Limit Orders are allowed. Order Entry closes randomly between 3:28 PM and 3:30 PM.
3:30 PM – 3:35 PM	Buy and Sell Orders are matched, and the Official Closing Price is determined based on the auction results.

Read Also: What is the Timing for Commodity Market Trading?

How Does the Closing Auction Process Work? 

Understanding the Closing Auction Session process is simple. After the market closes, this process unfolds in specific stages, culminating in the determination of the stock’s official closing price.

Step 1: Trading Session Ends

Regular trading concludes at 3:15 PM. Following this, the respective stocks enter the auction phase.

Step 2: Auction Orders Are Accepted

During the designated period, investors can place new orders and modify existing ones in accordance with the rules. This allows the exchange to gauge the actual buying and selling sentiment in the market.

Step 3: Best Matching Price Is Identified

Once order entry is complete, the exchange identifies the price point that facilitates the highest volume of share transactions. This is the pivotal stage of closing price discovery.

Step 4: Trade Execution

Eligible buy and sell orders are executed at the determined price. Subsequently, all successful trades are finalized.

Step 5: Closing Price Is Published

Finally, the exchange declares this price as the stock’s official closing price. This price is subsequently used in various market processes.

Which Orders Are Allowed During CAS? 

Order Type	Allowed?	Remarks
Limit Order	Yes	The most widely used order in CAS. Participation in the auction is based on this.
Market Order	Yes	It is accepted only during the designated stage. It is not available thereafter.
Stop-Loss Order	No	Stop-loss orders are not accepted in CAS.
Iceberg Order	No	Iceberg orders are not part of the auction session.

Why Did SEBI Introduce the Closing Auction Session? 

SEBI introduced the Closing Auction Session (CAS) to ensure that the closing price of shares is determined more accurately and transparently, based on actual demand and supply. This strengthens the price discovery process in the market and mitigates the impact of last-minute trades on the closing price.

• Better Price Discovery: In the CAS, all eligible buy and sell orders are matched simultaneously. This results in a closing price that allows for the execution of the maximum number of orders.
• Greater Transparency: The auction-based process operates according to established rules. This makes the mechanism for determining the closing price clearer and more impartial.
• Minimizing Last-Minute Price Impact: Large trades occurring just before market close could previously influence the closing price. The CAS helps reduce this impact by considering all eligible orders rather than just a single trade.
• Better Execution for Large Orders: The CAS facilitates the efficient matching of large buy or sell orders from mutual funds, FIIs, DIIs, and other major investors, leading to more effective order execution.
• Alignment with Global Markets: Many major stock exchanges worldwide utilize closing auctions. By implementing the CAS, the Indian stock market has been brought closer to international price discovery systems.

Read Also: Understanding Intraday Trading Timings

Who Will Be Most Affected by CAS? 

The Closing Auction Session (CAS) can impact various market participants differently. Let us understand who might be affected and how.

• Retail Investors: There will be no significant change for long-term investors. However, obtaining a more accurate closing price could lead to better valuation of their investments.
• Intraday Traders: Intraday traders may need to adjust their exit timing and order planning slightly, as the trading process differs at the time of market close.
• F&O Traders: The closing price is crucial for F&O traders, as it is used in the settlement process for certain contracts.
• Mutual Fund & ETF Investors: The closing price is used to determine the NAV of mutual funds and ETFs. Improved price discovery can lead to more accurate valuations.
• Institutional Investors: FIIs, DIIs, and other large investors can benefit from better execution of large orders and a fairer closing price.

CAS vs Previous Closing Price Method 

The table below shows the key differences between the old and new systems.

Parameter	Previous Method	Closing Auction Session (CAS)
Closing Price Basis	Based on the last 30 minutes VWAP	Based on the final price determined in the auction
Price Discovery	Dependent on continuous trading	Determined by the matching of buy and sell orders.
Price Accuracy	In some cases, the closing price could have been affected.	It better reflects actual demand and supply.
Large Orders Impact	The impact of large orders could have been significant.	The impact remains relatively balanced with the auction process.
Transparency	Based on the standard trading process	More transparent due to the structured auction mechanism.
Applicable Stocks	A uniform approach for all stocks	Initially applicable only on F&O Eligible Stocks

Conclusion

The Closing Auction Session (CAS) represents a significant shift towards making the process of determining the closing price in the Indian stock market more transparent and fair. Understanding the process and the timing of the Closing Auction Session enables you to make better decisions at market close and trade with greater confidence under the new regulations.

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