Closing Auction Session (CAS): What it is and How it Works

Effective August 3, 2026, eligible stocks in the cash segment have moved to a new Closing Auction Session (CAS) to determine their closing price. The new mechanism is designed to make closing price discovery more transparent and better reflect overall market demand and supply. 

The Closing Auction Session is a well-synchronised process for determining the closing price based on actual supply and demand. So, how does the Closing Auction Session work? Here’s everything an investor needs to know. 

#What is the Closing Auction Session?

The Closing Auction Session (CAS) is a separate auction session conducted by the stock exchange to determine the closing price of eligible stocks. The CAS process begins at 3:15 PM and carries on through the transition to the post-close session at 3:35 PM. There are different stages for reference-price calculation, order entry, order matching, and post-close trading. 

It starts after the continuous trading session and uses an order-matching mechanism similar to that of the pre-open session. CAS aims to ensure that the closing price better reflects real demand and supply forces and to minimise the risk that a few last-minute trades influence the price. 

In the initial phase of CAS implementation, only stocks present in the Futures & Options (F&O) segment are covered. The mechanism is expected to be extended to other stocks in subsequent phases. 

#Closing Auction Session Timing

The Closing Auction Session begins at 3:15 PM and concludes at 3:35 PM. The session is divided into multiple stages, including reference price calculation, order entry, order matching, and the post-close session. Each stage has a defined time window and plays a specific role in determining the stock's closing price. 

#Closing Auction Session (CAS) Timings

Source: NSE

#New Market Timings After CAS

After CAS implementation, the new market timings for F&O-enabled stocks. Stocks outside the CAS framework, or non-CAS stocks, continue to follow the old closing price discovery mechanism. Because of the CAS, the equity derivatives segment also continues trading until 3:40 PM. 

#New Market Timings for Stocks

#New Market Timing for Derivatives

Because of the CAS, the equity derivatives segment also continues trading until 3:40 PM. The extension allows the derivatives market to remain open after the cash-market closing auction and provides time for prices in the two segments to adjust. 

#How is the Closing Price Determined in CAS?

Under the CAS framework, the closing price is determined through a series of steps. 

#Step 1: CAS Transition and Reference Price is Established

At 3:15 PM, continuous trading for CAS-eligible stocks stops, and a transition window begins until 3:20 PM. 

During this period, the reference price is calculated from the 3:00 PM - 3:15 PM VWAP. If there were no trades, the last traded price is used; if there were no trades all day, the previous day’s close becomes the reference price. 

A ±3% price band is then applied. For example, a ₹500 reference price means orders can be placed between ₹485 and ₹515. Orders outside this range are not accepted. 

#Step 2: Buy and Sell Orders are Collected

Between 3:20 PM and 3:25 PM, investors can place both market and limit orders.

Then, from 3:25 PM to 3:30 PM, only limit orders are allowed. And, in the last two minutes of trading, order entry is randomly closed to prevent last-second timing.  

#Step 3: Order Matching and Trade Confirmation Stage

From 3:30 PM to 3:35 PM, the exchange determines the equilibrium price, the price at which the maximum number of shares can be matched. 

If multiple prices allow the same maximum volume, the exchange chooses the one with the least unmatched quantity. If there is still a tie, the price closest to the reference price is selected.

This equilibrium price becomes the closing price.

#Closing Auction Session Example 

Suppose the reference price for stock XYZ is set at ₹500. The CAS price band would be ₹485-₹515. During the auction period, investors can place buy or sell orders at different prices, but these orders must be within the CAS price band to be accepted. 

During the CAS order entry phase, the following trade orders were placed:

The exchange then compares the cumulative buy and sell quantities at each price to determine how many shares can actually be traded. For example, at ₹495, there are 8,000 shares available to buy and 2,000 shares available to sell. Therefore, only 2,000 shares can be executed, leaving 6,000 shares unmatched.

Similarly, at ₹500, the cumulative buy quantity is 5,000 shares, while the cumulative sell quantity is 7,000 shares. This means 5,000 shares can be executed, with 2,000 remaining unmatched.

At ₹502, the cumulative buy and sell quantities are both 10,000 shares. Therefore, the maximum possible quantity of 10,000 shares can be executed, with no unmatched quantity.

Since ₹502 allows the maximum executable volume, it becomes the equilibrium price. This equilibrium price is then used as the stock's closing price.

#Order Matching Priority

When the auction enters the matching phase, orders are matched according to a defined priority. The usual sequence is:

1. #Market orders vs market orders: These are matched based on time priority.
2. #Market orders vs limit orders: The best available limit price gets priority, followed by time priority.
3. #Limit orders vs limit orders: The best price gets priority, followed by time priority.

The auction matches different types of orders by priority to determine a single equilibrium price before the orders are executed. 

#Which Orders Are Allowed During CAS?

During the auction process, CAS permits market and limit orders. However, stop-loss and iceberg orders cannot be placed during CAS.

Also, eligible unexecuted limit orders from the continuous trading session can automatically carry forward into CAS. However, stop-loss orders and orders placed outside the applicable CAS price band are cancelled.

If an investor modifies a carried-forward order during CAS, it is treated as a fresh order for relevant risk checks.

#Why was the Closing Auction Session for Stocks introduced?

The CAS was introduced to replace the old volume-weighted average price (VWAP) method. In the VWAP method, a few trades in the final minutes of the trading hour could influence the closing price. 

CAS ensures that a stock’s closing price is less susceptible to manipulation through last-minute trades, thereby improving transparency in price discovery and enabling Indian exchanges to settle trades in line with globally accepted standards. 

For CAS-eligible stocks, the earlier VWAP-based closing-price mechanism has been replaced by an auction-based mechanism. Under the CAS, buy and sell orders are collected, and the closing price is determined through an equilibrium price mechanism based on the maximum executable volume.

#5 Key Points on CAS Investors Should Know

• CAS is a closing price discovery mechanism. Therefore, for most long-term investors, CAS has no impact on their investment strategy. 
• CAS covers stocks that are eligible for trading in the Futures & Options (F&O) segment. It does not apply to every stock listed on the exchange. 
• For CAS-eligible stocks, the period from 3:15 PM onward differs from normal continuous trading. Orders are not matched immediately. Market orders are available only during the initial order-entry period and are frozen after 3:25 PM. After 3:25 PM, only limit orders can be entered, modified, or cancelled. 
• Orders can be placed only within the applicable CAS price band around the reference price. This limits the extent to which orders can move the potential closing price during the auction. 
• The closing price can differ from the last traded price. The final trade executed before the auction does not necessarily determine the closing price. The auction can discover a different equilibrium price based on the overall buy and sell orders. 

#The Bottom Line

CAS changes a small but important part of market mechanics. The closing price is now discovered, rather than simply observed. For investors, the key distinction is that the last traded price and the official closing price can differ. If you trade near 3:15 PM, understanding the auction window, order restrictions, and the equilibrium-price mechanism is therefore more useful than treating the closing price as just another end-of-day quote.