The closing price of a stock is one of the most important numbers in the market. It appears on your
watch-list each evening, but its influence extends well beyond a single figure on a screen. From 3
August 2026, the method used to determine this price is changing for a specific set of stocks, under
a new framework called the Closing Auction Session (CAS). No action is required on your part. This
article explains what is changing, why it matters, and how it may affect the stocks you trade.

The revised end-of-day timeline

For a stock covered by CAS, the final stretch of the trading day follows a defined sequence:

Which stocks are affected?

In this first phase, CAS applies only to stocks that also have Futures & Options (F&O) contracts —
typically the larger, more actively traded names. All other stocks continue exactly as before. In
summary:

– F&O stocks (cash): the closing price is now determined by the CAS auction, with a ±3% price
band applied during the session.
– F&O contracts: stock futures receive a fresh ±3% band and now trade until 3:40 pm; options
remain unchanged.
– Non-F&O stocks: no change: the same trading hours and the existing closing-price
method continue to apply.
To check whether a particular stock is covered, look for the CAS indicator on your Samco platform.

What is changing: the Closing Auction Session!

The objective remains unchanged, to establish a fair closing price. What changes is the method of
arriving at it. Rather than averaging the final 30 minutes of continuous trading, the exchange will
conduct a short, dedicated auction at the end of the day. During this auction, all buy and sell orders
are collected together, and the exchange identifies the single price at which the largest number of
shares can be traded. That price becomes the official closing price, and all matched trades are
executed at it.

How the equilibrium price is determined?

To find the closing price, the exchange tests each possible price and asks a simple question: how
many shares could actually trade at this level? The number of shares that can trade is limited by the
smaller side whichever of buyers or sellers is fewer. The example below illustrates the principle:

At ₹99, there are many buyers but few sellers, so only 300 shares can trade. At ₹101, the reverse is
true. At ₹100, 500 buyers meet 500 sellers, the highest number of matched shares. Accordingly,
₹100 becomes the closing price. This is known as the equilibrium price: the level at which supply and
demand are best balanced. Actual volumes are far larger, but the underlying logic is identical.

Please note that an order placed during the 3:15–3:20 pm transition window may not be accepted.

Why the closing price matters?

The closing price serves as a reference point for several critical calculations across the market:

– Mutual fund NAV: fund valuations and therefore the worth of your investment are
based on closing prices.
– Derivatives settlement: futures and options positions are marked and settled against the
close.
– Index levels: the closing levels of indices such as the Nifty and Sensex are derived from their
constituent stocks’ closing prices.
– Portfolio and margin reporting: end-of-day statements, profit and loss, and margin
calculations all rely on the official close.

Because a single closing price influences millions of portfolios, the method used to arrive at it is
significant.

A common misconception: the close is not the last traded price

It is often assumed that the closing price is simply the last traded price the price at which a stock
last changed hands near the end of the session. In reality, the current closing price is calculated as
the VWAP (Volume Weighted Average Price) of the last 30 minutes of trading. This approach prevents
a single, possibly unrepresentative, final trade from determining the close.

VWAP is an average price in which larger trades carry greater weight than smaller ones. It is
more representative of genuine trading activity and considerably more resistant to
manipulation than any single trade.

In summary

CAS does not change what a closing price represents; it changes how it is determined
through a transparent, single-price auction rather than a rolling average. All eligible orders
meet in one session, and the price at which the most shares trade becomes the close. For most
of the trading day, your experience remains the same.