What is Closing Auction Session in Stock Market

Starting 3 August 2026, eligible stocks in India’s equity cash market began using the Closing Auction Session (CAS) to determine their official closing price. The full form of CAS is Closing Auction Session, a dedicated process where buy and sell orders are brought together to discover the closing price. SEBI introduced this framework through its circular issued on 16 January 2026.

For investors, the important change is that the price at which continuous trading ends may not be the same as the final closing price.

Key Takeaways

• CAS full form is Closing Auction Session, a dedicated auction process used to determine the official closing price of eligible stocks.
• CAS was introduced on 3 August 2026 for stocks covered under the new closing-price framework.
• For eligible stocks, continuous trading ends at 3:15 PM, followed by the CAS process until 3:35 PM.
• During CAS, buy and sell orders are brought together to discover an equilibrium price at which the maximum possible quantity can be matched.
• The final CAS closing price may differ from the LTP at 3:15 PM, as the closing price is determined through the auction.
• Initially, CAS applies to stocks available for trading in the equity derivatives segment.
• The new mechanism is intended to make closing-price discovery more transparent and efficient, particularly for activities that rely on official closing prices, such as index calculation and settlement.

What Is the Closing Auction Session?

It is a dedicated auction at the end of the trading day used to determine the official closing price of eligible stocks.

Instead of trades continuing normally, the exchange collects eligible buy and sell orders and identifies the price at which the maximum quantity can be matched.

For example, if buyers are willing to purchase a stock around ₹500 and sellers are offering it around ₹502, the auction considers the available orders and determines the appropriate closing price based on the exchange's price-discovery rules.

In simple words, CAS is a structured way of finding the market's closing price using demand and supply.

Impact on Market Timing

The introduction of CAS changes the trading schedule for stocks covered under the new mechanism. While non-CAS stocks continue with their existing market timings, eligible F&O stocks move to the closing auction after 3:15 PM. The equity derivatives segment also gets an extended trading window.

Stocks

Derivatives

In practical terms: If you trade an F&O-eligible stock, 3:15 PM is now the end of its continuous trading session. The stock then moves into CAS, where the closing price is determined through the auction process. Non-F&O stocks continue to follow their existing trading schedule.

Why Is SEBI Introducing CAS Now?

The closing price is important for several market-related purposes, including valuation and settlement. SEBI's objective is to make closing-price discovery more structured and improve the reliability of the final market price.

An auction allows buying and selling interest to come together specifically around the market close instead of relying only on trades executed during the earlier closing period.

Closing Auction Session (CAS) Timings

For eligible stocks, the broad CAS schedule is:

• 3:15 PM–3:20 PM: Transition/reference-price stage
• 3:20 PM–3:25 PM: Order-entry period
• 3:25 PM–3:30 PM: Restricted order-entry period
• 3:30 PM–3:35 PM: Order matching and trade confirmation

The order-entry period also has a system-driven random closure near its end.

How Was the Closing Price Determined Earlier?

Earlier, the closing price for applicable securities was based on the Volume Weighted Average Price (VWAP) of trades during the final 30 minutes of continuous trading.

For example, if a stock traded 1,000 shares at ₹100 and 5,000 shares at ₹102, the ₹102 trade would have a greater influence on the VWAP because a larger number of shares were traded at that price.

CAS uses a different approach by considering the orders participating in the closing auction.

CAS vs VWAP-Based Closing Price

In short: The earlier method calculated the closing price from executed trades, while CAS uses an auction to discover the closing price based on buying and selling interest.

Continuous Trading Session (CTS) vs Closing Auction Session (CAS)

During the Continuous Trading Session (CTS), buy and sell orders can be matched continuously throughout market hours.

During CAS, orders are collected during the permitted period and then matched through the closing auction mechanism.

Think of it this way: CTS is continuous price discovery, while CAS is end-of-day auction-based price discovery.

Which Stocks Are Covered Under CAS?

CAS does not initially cover every listed stock. In the first phase, it applies to stocks in the equity cash segment for which derivative contracts are available.

Therefore, investors should check the latest list provided by the exchange or their broker instead of assuming that every stock enters CAS at 3:15 PM.

Conclusion

The Closing Auction Session changes how the final price of eligible stocks is discovered in India's equity market. From 3 August 2026, these stocks move into CAS after continuous trading ends at 3:15 PM, with the auction process running until 3:35 PM.

For long-term investors, the change is mainly something to understand. For intraday traders, however, knowing the new timings and order rules is important because the price seen at 3:15 PM may not be the final closing price.

FAQs

What is the Closing Auction Session in simple terms?

The Closing Auction Session is a special end-of-day process where buy and sell orders are brought together to discover the official closing price of eligible stocks.

When does CAS start in India?

For eligible stocks, CAS starts at 3:15 PM and ends at 3:35 PM.

Does CAS apply to all stocks or only some?

CAS initially applies to stocks in the equity cash segment for which derivative contracts are available. It is being implemented in phases.

What happens to my limit order if it doesn't match during CAS?

An unmatched order does not automatically become a completed trade. Its treatment depends on the applicable exchange and order-handling rules. Investors should check their broker's order status after the auction.

Can I cancel an order during CAS?

Order modification and cancellation depend on the stage of CAS and the type of order. The restrictions become tighter as the auction progresses, so investors should check the applicable rules before placing an order.

How is the CAS closing price different from LTP?

LTP is the price at which the most recent trade occurred. The CAS closing price is determined through the closing auction. Therefore, a stock's LTP and final closing price can be different.

Does CAS affect mutual fund NAV calculation?

It can indirectly affect NAV because the closing value of stocks held by an equity mutual fund is relevant to portfolio valuation. However, CAS does not replace the separate rules governing mutual fund NAV cut-off times and applicability.

What if a stock has no trades during the CAS window?

The exchange has prescribed rules for cases where a normal auction price cannot be discovered. The applicable reference-price and closing-price mechanism is then followed rather than simply leaving the stock without an official closing price.

Disclaimer: This article is for educational purposes only and should not be considered investment advice. Investors should verify the latest SEBI and stock-exchange rules before making investment or trading decisions.