Closing Auction Session: All you need to know about SEBI’s new price discovery mechanism for F&O contracts


    By Bajaj Broking Research Team

    Summary :


    SEBI is introducing a Closing Auction Session for stocks with active Futures & Options (F&O) contracts. Here is how the new price discovery mechanism works, how the trading timeline changes, and what it means for your portfolio.

    All you need to know about CAS

    Every trading day, exchanges calculate an official closing price for each listed stock. This single number feeds into Sensex and Nifty index values, mutual fund NAVs, F&O expiry settlements, portfolio P&L, and collateral valuations for pledged shares.

    Starting August 3, 2026, SEBI is changing how this price is determined for stocks that have active F&O contracts.

    Key Takeaway: For F&O stocks, regular continuous trading ends at 3:15 PM instead of 3:30 PM. The official closing price will be calculated through a 20-minute auction session (3:15–3:35 PM) rather than the traditional 30-minute Volume-Weighted Average Price (VWAP).

    Why Is SEBI Making This Change?

    Currently, closing prices are derived using the Volume Weighted Average Price (VWAP) of all trades executed in the last 30 minutes of the session (3:00–3:30 PM). A few anomalous trades during thin liquidity moments can skew this average away from true market valuation.

    SEBI's Closing Auction Session (CAS) aggregates all closing buy and sell interest into a single pool of liquidity. This provides:

    • Fair Price Discovery: Prevents single-order price distortions near the bell.

    • Lower Tracking Error: Helps institutional investors, index funds, and ETFs execute large rebalancing orders efficiently at the exact closing price.

    • Global Alignment: Aligns Indian equity markets with international best practices used in major global exchanges.

    Rollout Phasing & Timing Overview

    Segment

    Regular Trading Ends

    Closing Price Method

    Effective Date

    Stocks with Active F&O

    3:15 PM

    Closing Auction Session (CAS)

    August 3, 2026

    Non-F&O Stocks

    3:30 PM (Unchanged)

    Existing VWAP (Last 30 Min)

    Unchanged

    F&O Derivatives Segment

    3:40 PM (Extended 10 mins)

    Expiry settled on CAS cash close; daily F&O close unchanged

    August 3, 2026

    Minute-by-Minute Breakdown of CAS (3:15 PM – 3:35 PM)

    [3:00-3:15 PM] Normal Trading (Reference Window – VWAP Basis)

    [3:15-3:20 PM] Transition Window (No Order Placement)

    [3:20-3:25 PM] Order Entry Phase I (Market & Limit Orders Allowed)

    [3:25-3:30 PM] Order Entry Phase II (Limit Orders Only; Random Close 3:28-3:30 PM)

    [3:30-3:35 PM] Order Matching at Equilibrium Price
     

    • 3:00–3:15 PM (Reference Window): Regular trading continues. The VWAP during these 15 minutes forms the Reference Price, around which a price band is established for the auction.

    • 3:15–3:20 PM (Transition): Continuous trading stops. Unexecuted limit orders carry forward into the auction only if they fall within the ±3% price band; orders outside the band are cancelled (sequentially, not instantly at 3:15 PM). Stop-loss, iceberg, and Immediate-or-Cancel (IOC) orders are blocked or canceled.

    • 3:20–3:25 PM (Order Entry Phase I): Market and limit orders can be placed, modified, or canceled. The exchange broadcasts indicative equilibrium prices in real time.

    • 3:25–3:30 PM (Order Entry Phase II): Market orders are frozen; only limit orders can be entered or modified. The order entry window closes at a random, system-driven time between 3:28 PM and 3:30 PM to prevent last-second order manipulation.

    • 3:30–3:35 PM (Matching): No further orders are accepted. The system matches orders at the single Equilibrium Price, establishing the official close.

    Fallback Rule: If no equilibrium price can be calculated, the Reference Price becomes the official close. If no trades occur between 3:00–3:15 PM, the day's Last Traded Price is used; if the stock hasn't traded all day, the previous close applies.

    How the Equilibrium Price Is Calculated

    The algorithm seeks the single price level that maximizes total executable share volume. Buyers willing to pay a higher price are considered willing to buy at any lower price; sellers willing to accept a lower price are considered willing to sell at any higher price.

    Calculation Example:

    Price (₹)

    Cumulative Buy Qty

    Cumulative Sell Qty

    Matchable Volume

    ₹496

    2,200

    400

    400

    ₹498

    1,900

    850

    850

    500

    1,500

    1,450

    1,450 (Maximum)

    ₹502

    1,000

    1,700

    1,000

    ₹504

    600

    2,050

    600

    At ₹500, the matched volume reaches its maximum (1,450 shares), making ₹500 the official closing price for the day.

    Action Plan for Traders & Investors

    • Identify Affected Stocks: Check whether your holdings or trading targets have active F&O contracts—this dictates whether your cut-off is 3:15 PM or 3:30 PM.

    • Manage Intraday (MIS) Positions Early: Square off intraday positions and cancel stop-loss and iceberg (disclosed-quantity) orders before 3:15 PM on CAS stocks, as these order types do not carry into the auction window.

    • Adjust Broker Square-Off Schedules: Broker auto-square-off timings for equity cash on CAS stocks will move earlier (typically around 3:00–3:10 PM). Check your stockbroker's official platform announcements for updated schedules.

    • Plan Market Orders Carefully: Market orders for CAS stocks are only accepted up to 3:25 PM. Between 3:25 PM and the random close, use limit orders within the reference price band.

    Frequently Asked Questions

    Published Date : 22 Jul 2026

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    Content Partner - Dalal Street Investment Journal Wealth Advisory Private Limited



    This article is for educational purposes only and should not be considered investment advice. Market investments are subject to risks. DSIJ Wealth Advisory Private Limited is a SEBI-registered Research Analyst (Reg. No: INH000006396) and Investment Adviser (Reg. No: INA000001142). Please consult your financial adviser before investing. 

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