SEBI’s Closing Auction Session (CAS): What Changes From 3 August 2026

The closing price of a stock does more than mark the end of the trading day. It feeds into benchmark indices like the Nifty 50 and Sensex, decides the NAV of your mutual funds and ETFs, settles F&O contracts, and values entire portfolios.

To make this reference point fairer and harder to influence in the final minutes of trading, SEBI has introduced the Closing Auction Session (CAS), replacing the older VWAP-based method for a set of eligible securities.

This blog explains what CAS is, how the new price discovery process works, and what it means for you as an investor.

What Is the Closing Auction Session (CAS)?

How It Differs From Continuous Trading

In the regular Continuous Trading Session (CTS), buy and sell orders are matched and executed the moment prices align. CAS works differently. During its auction window, eligible buy and sell orders are simply collected, not executed immediately.

How the Price Gets Determined

At the end of the auction, the exchange works out a single equilibrium price that maximises the quantity of shares that can be traded, applying a set of prescribed auction rules. Every eligible order matched during the auction executes at this one price, which then becomes the official closing price for the day.

The exchange arrives at this price using:

• Maximum executable quantity
• Indicative Equilibrium Price (IEP)
• Predefined tie-breaking rules
• Aggregate buy and sell order book

Key Highlights of CAS

• Effective date: CAS goes live from 03 August 2026.
• Applicability: Initially limited to equity securities in the cash segment that have listed F&O contracts.
• Timings: Conducted from 3:15 PM to 3:30 PM on all trading days; the final closing price is set during the closing process after the auction.
• Price discovery: Orders are collected during the session and matched at a single equilibrium price, not executed one by one.
• Official closing price: The equilibrium price where the maximum tradable quantity gets matched becomes the day’s closing price.
• No change for non-CAS securities: These continue with the existing VWAP-based methodology until notified otherwise.
• Extended F&O hours: The Equity Derivatives segment stays open until 3:40 PM to stay aligned with CAS.

What Are the CAS Timings?

CAS runs as a separate 20-minute session between 3:15 PM and 3:35 PM, broken into the following sub-periods:

Note: The reference price for CAS is the Volume Weighted Average Price (VWAP) of trades executed during the last 30 minutes of the Continuous Trading Session. This only serves as the starting reference for the auction — it is different from the final closing price discovered through CAS.

How Does CAS Determine the Closing Price?

CAS arrives at the official closing price through a four-step, auction-based process.

Step 1: Order Collection

During the order entry period, investors place, modify, or cancel eligible buy and sell orders. No trades execute during this phase.

Step 2: Price Discovery

Once the order entry window closes, the exchange evaluates every buy and sell order at each price level and calculates the executable quantity at each price, then identifies the price where the highest trading volume can be matched. If more than one price qualifies, predefined tie-breaking rules decide the final equilibrium price.

Step 3: Order Matching

Once the equilibrium price is set, all eligible buy and sell orders are matched simultaneously at that single price.

Step 4: Official Closing Price

The equilibrium price becomes the stock’s official closing price for the trading day.

Example of CAS Price Discovery

Suppose the following orders are available during the CAS for ABC Ltd:

The exchange then evaluates cumulative buy and sell quantities at each price:

Key takeaway: Under CAS, the official closing price is not the last traded price. It is the equilibrium price at which the maximum number of buy and sell orders can be matched, keeping end-of-day price discovery fair and transparent.

A Second Example

Suppose the following orders are available during CAS:

How Was the Closing Price Calculated Before CAS?

Under the previous NSE methodology, the closing price was the Volume Weighted Average Price (VWAP) of all trades executed in the last 30 minutes of the Continuous Trading Session, weighted by trade volume, and applicable to all equity shares.

Why Has SEBI Introduced CAS in Place of VWAP?

Under the previous methodology, the closing price relied on VWAP of trades in a narrow window near market close, leaving it exposed to influence from a handful of last-minute trades.

Under CAS, the closing price is drawn from the entire eligible order book, so it reflects overall market demand and supply rather than a thin slice of late trading activity. This makes the closing price more transparent, more robust, and less susceptible to manipulation.

What Is the Objective of CAS?

CAS is designed to determine a fair, transparent, market-driven closing price by pooling all eligible buy and sell orders into a single auction at the end of the trading day.

Unlike continuous trading, where prices shift with every matched order, CAS aggregates orders first and then determines one equilibrium price where the maximum quantity of shares can trade.

What Are the Key Benefits of the Closing Auction Session?

CAS offers five key benefits: better price discovery, greater transparency, reduced price impact, improved index and ETF accuracy, and alignment with global best practices.

• Better price discovery: Pooling all eligible orders into one auction helps the market find a closing price that truly represents end-of-day demand and supply.
• Greater transparency: Every participant submits orders in the same auction window, and the final price is set through a predefined exchange mechanism.
• Reduced price impact: Large institutional orders near market close are absorbed into a common auction pool, minimising their impact on price.
• Improved accuracy for indices and ETFs: A more representative closing price helps indices, ETFs, and index funds track their benchmarks more accurately.
• Alignment with global practices: Leading exchanges like the NYSE and LSE already use closing auction mechanisms, and CAS brings Indian markets in line with this global norm.

Why Does CAS Matter for Investors?

For retail and institutional investors alike, CAS strengthens market efficiency by ensuring the official closing price comes from a robust auction process rather than a handful of last-minute trades. This leads to:

• More reliable closing prices
• Improved market integrity
• Better execution opportunities for large orders
• Greater confidence in index and portfolio valuations
• Enhanced fairness and transparency in end-of-day trading

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

Conclusion

As Indian capital markets continue to evolve, the Closing Auction Session is set to play a meaningful role in end-of-day price discovery, making closing prices more efficient, reliable, and aligned with global standards. Knowing how CAS works helps you place, modify, and time your orders with more confidence as the trading day wraps up.

What is CAS: FAQs