Closing Auction Session (CAS): Meaning, Timings and How It Works

Why Isn't the Closing Price Just the Last Trade of the Day?

Fair question. The intuitive answer — take the price of the last trade — turns out to be a bad idea in practice.

Imagine a stock that trades quietly around ₹880 all day. Then at 3:29 PM, a handful of trades go through at ₹862 in a thin moment. If the closing price were the last traded price, ₹862 becomes the official number — even though the stock had nothing to do with ₹862 for the rest of the day.

To avoid this, the current system uses the volume-weighted average price of the last 30 minutes. More shares traded, more weight. Harder to game than a single price. Better — but still not perfect.

So What's Actually Wrong with the VWAP Method?

The old VWAP method works, but only up to a point. It looks at just the last 30 minutes, and in thin or volatile conditions a handful of trades can drag the close away from where the stock actually spent the day. CAS is designed to fix that.

The key reasons for making the switch:

1. Better price discovery. Pooling every eligible order into one auction produces a price that reflects the whole market at once — not whoever happened to trade in the last half hour.
2. Large orders can execute with ease. Today, a mutual fund wanting to buy a big block has to slice it into small pieces to avoid moving the price against itself. In CAS, a large buyer is far more likely to find a large seller at the same price — in the same pool, at the same time. Big trades become cheaper and less disruptive.
3. Less tracking error for passive funds. A closing price that better reflects actual market consensus means index funds and ETFs can mirror their benchmarks more tightly. Small investors in these funds benefit directly.
4. India joins the rest of the world. The NYSE, the London Stock Exchange, and most other major markets have been running closing auctions for years. This brings India's equity markets in line with the global standard.

How CAS fixes it

CAS pools every eligible buy and sell order into one auction and finds the single price where the most shares can actually trade. That becomes the close.

Because everyone's orders are in the same pool at the same time, one large order can't move the price the way it can in continuous trading. It just becomes part of the auction, matched at the same price as everything else.

It also gives passive funds a clean window to put their closing orders in — where they'll be matched against genuine counterparties rather than chasing a moving price through the open market.

A Simple Worked Example

The exchange scans every price level and asks: where can the most shares actually change hands? The answer is always the smaller of the cumulative buy and sell quantities at each price.

Say the reference price going into the auction is around ₹100. Here's what the order book might look like:

Price Level	Buy Qty	Sell Qty	Cumulative Buy	Cumulative Sell	Executable Volume
₹103	100	500	100	1,500	100
₹102	200	400	300	1,000	300
₹101 (EQ)	400	300	700	600	600
₹100	300	200	1,000	300	300
₹99	500	100	1,500	100	100

At ₹101, 600 shares can trade — more than at any other level. ₹101 becomes the equilibrium price, every eligible order executes there, and that's the official closing price for the day.

What if two prices tie?

If more than one price yields the same maximum executable volume, the exchange picks the one with the smaller imbalance between buy and sell quantities. If that's still tied, it goes with the price closest to the reference price. And if the reference price sits exactly midway between two candidates, the reference price itself is the closing price. If no price can match any shares at all, the reference price becomes the close by default and no trades are executed.

What is the Reference Price?

The reference price is the VWAP of trades in the relevant stock between 3:00 PM and 3:15 PM — the last 15 minutes of normal trading. It does two things: it's the anchor the auction starts from, and it sets the ±3% band within which orders must be placed. Orders outside that band are rejected.

It's not the closing price. The auction will almost always produce a different number. But if the auction produces no result at all, the reference price steps in as the official close.

Situation	Reference Price Used
Trades executed 3:00–3:15 PM	VWAP of those trades
No trades between 3:00–3:15 PM	Last traded price of the day
No trades at all during the day	Previous day's official closing price
Corporate action on the day	Adjusted closing price or base price

The New End-of-Day Map

From 3 August, not everything closes at the same time. This will cause some confusion. Here's what's actually happening:

1. Stocks with F&O contracts — normal trading stops at 3:15 PM, then a 20-minute auction runs until 3:35 PM.
2. All other stocks — nothing changes. Trading continues until 3:30 PM as before.
3. Futures & Options contracts — trading extended to 3:40 PM.

The auction itself runs in four stages:

Time	Stage	What's happening	Non-F&O stocks
9:15 AM–3:15 PM	Normal CTS	Regular trading, as always	Same
3:15–3:20 PM	Transition	CTS ends for F&O stocks. Reference price calculated. Orders placed here are held and forwarded at 3:20 PM.	CTS continues
3:20–3:25 PM	Order Entry I	Limit and Market orders can be placed, modified, or cancelled.	CTS continues
3:25–3:30 PM	Order Entry II	Limit orders only. Market Protection Orders (MPP) can still be placed, modified, or cancelled — they're sent as limit orders to the exchange. Window closes randomly between 3:28 and 3:30 PM.	CTS ends at 3:30 PM
3:30–3:35 PM	Matching	All orders matched at the equilibrium price. Official closing price set.	VWAP close calculated
3:35–3:40 PM	Close published	Official closing price published.	—
3:40 PM	F&O closes	Equity derivatives segment closes.	—
3:50–4:00 PM	Post-close	Trades executed at the official closing price.	Same

The random close

The order entry window shuts at a randomly chosen moment between 3:28 and 3:30 PM — never at a fixed second. This is deliberate. If everyone knew the exact closing instant, some would flood the book with last-second orders to nudge the price. The randomness makes that unplayable.

What Orders are and Aren't Allowed?

Not all order types are permitted in CAS. Here's the full picture:

Order Type	3:20–3:25 PM	3:25–3:30 PM	Notes
Limit Orders	Yes	Yes	Allowed throughout. Open orders from before 3:15 PM are carried forward automatically.
Market Protection Orders (MPP)	Yes	Yes	Allowed — Angel One converts Market orders to Market Protection (limit) orders before sending to the exchange. These can be placed, modified, or cancelled in both windows.
Stop Loss (SL) Orders	No	No	Not permitted. Any open SL orders from before 3:15 PM are automatically cancelled when CAS begins.
Iceberg / Disclosed Quantity	No	No	Not permitted. Cancelled automatically at 3:15 PM.
IOC Orders	No	No	Not permitted. Rejected if placed during CAS.
GTT Orders	No	No	GTT triggers stop for CAS-eligible stocks from 3:15 PM.

One thing worth knowing about orders placed during the auction: nothing executes straight away. The exchange collects all orders during 3:20–3:30 PM and matches them in one go at 3:30–3:35 PM. If your order is sitting there looking unfilled, that's expected.

Also: all orders must be within ±3% of the reference price. Outside that, they're rejected. The order pad on Angel One will show you the valid range so you always know where to place.

What Changes if You Trade Intraday?

This is the one that needs your attention most.

Because the auction begins at 3:15 PM, your intraday positions need to be out before then. Here are the revised auto square-off times at Angel One:

Product	Previous Square-Off	Revised Square-Off
Equity MIS Orders	3:15 PM	3:10 PM
Equity Bracket Orders (BO)	3:10 PM	3:05 PM
Index and Stock F&O Contracts	3:20 PM	3:20 PM

Also: any pending Stop Loss, Iceberg, or GTT orders on F&O stocks are automatically cancelled at 3:15 PM. If you still need to exit after that, you'll have to place a fresh limit order during the auction.

What Changes if You Trade F&O?

A few specific things:

1. You have until 3:40 PM — 10 minutes longer than before.
2. If you hold stock futures or options to expiry, the final settlement price now comes from the CAS closing price of the underlying, not the old 30-minute VWAP.
3. The ±3% price band applies to stock futures too, from 3:15 PM to 3:40 PM.
4. After 3:15 PM, Nifty and Sensex will look frozen. That's because all 50 constituent stocks are in the auction — no new trades are executing, so the index doesn't move. It will update once the CAS closing prices come through around 3:35 PM. Options traders should note: you're effectively pricing against a static underlying during the auction window.