Closing Auction Session (CAS) Explained:

The New Way NSE Calculates Closing Prices?

The Indian stock market has introduced a new system called the Closing Auction Session (CAS) for F&O stocks. From now on, the closing price of eligible stocks will no longer be calculated using the old method. Instead, an auction-based mechanism will determine the final price. Don't worry if this sounds complicated. Let's understand it in simple terms.

05-08-2026/ WEDNESDAY

Why is the Closing Price Important?

Every trading day, the exchange needs to determine one official closing price for every stock. This price is important because it is used for: Calculating indices like Nifty and Sensex, Determining mutual fund NAVs, Settling F&O contracts on expiry and Calculating margins and portfolio values. Even a small change in the closing price can impact thousands of investors.

The Old Method vs The New Method

Old Method (VWAP)

Previously, the closing price was calculated using Volume Weighted Average Price(VWAP). Imagine a stock trading between ₹1,000 and ₹1,010 throughout the day. Suddenly, at 3:29 PM, someone sells a few shares at ₹980. Although the market mostly traded around ₹1,000, this last-minute trade could affect the closing price.

New Method (CAS)

Now, all buy and sell orders are collected into one common pool after market hours, and the exchange determines a single price where maximum trades can happen. This process is called the Closing Auction Session (CAS).

Why Did NSE Introduce CAS?

Better Price Discovery - The closing price now reflects the opinion of the entire market instead of a few last-minute trades.

Easier Execution for Large Investors - Large institutions like mutual funds can execute large orders more efficiently.

Global Practice - Major stock exchanges such as the NYSE and LSE already use auction-based closing mechanisms.

Which Stocks Are Affected?

CAS currently applies only to F&0 eligible stocks.

CAS Timeline Explained

How Is the Closing Price Calculated?

What Happens If Two Prices Have the Same Volume?

The exchange follows these rules:

The price with the lowest imbalance wins.

If still tied, the price closest to the reference price wins.

If the reference price is exactly in the middle, the reference price itself is selected.
Order Priority During CAS

Orders are executed in the following order:

Market orders matched with market orders.
Market orders matched with limit orders.
Limit orders matched with limit orders.

Important Rules for Traders

Allowed Orders

✅ Market orders

✅ Limit orders

Not Allowed

❌ Stop-loss orders

❌ Iceberg orders

❌ Orders outside the ±3% price band

Final Takeaway:

However, for intraday traders and active market participants, it is important to understand:

Trading in F&O stocks ends at 3:15 PM.
The closing price is now determined through an auction.
MIS positions close earlier.
Only certain order types are allowed during CAS.

The goal of CAS is simple: to create a fairer and more accurate closing price that reflects the opinion of the entire market.

Drop your thoughts below!

"Together let's chart our financial independence"

Gajapriya Annadurai
Full Time Trader & Investor, Founder @Stark School of Finance.