Closing auction session: how the closing price is struck in an auction from 3 August 2026

01What actually changed

The SEBI circular opens by describing the method it is replacing. It writes it in the present tense: the closing price of stocks in the equity cash segment is determined on the basis of the volume weighted average price of trades executed during the last thirty minutes of the continuous trading session. That sentence was accurate on the day it was written. It is no longer accurate for a large set of stocks.

Two documents fix the date it stopped being accurate. The SEBI circular states that the framework shall be implemented in the cash segment from 3 August 2026. NSE's notice to members covers the equity derivatives segment. It says the matching changes there shall be effective in live from the same date. A mock trading session was held on the Saturday before.

The closing price did not move by a small amount. It changed kind. It went from a summary of trades already done to a price discovered in an auction.

02How the closing price is calculated: an auction, not an average

A volume-weighted average price describes trades that have already happened. Every trade in the window goes into it. Each goes in at whatever price it was done at, weighted by its size. Nobody necessarily traded at the average at all. It is one figure standing in for many different prices.

An equilibrium price is the other kind of number. Orders are collected first, and nothing executes while they are being collected. Then the system looks for the one price at which the largest quantity can be matched. That price becomes the closing price. Every trade in the auction is executed at it. It is one price, not many.

Which price that is, is not left to judgement. The circular sets out the ladder in order:

1. The price at which the maximum volume is executable.
2. If more than one price ties on volume, the price leaving the smallest unmatched quantity in absolute terms.
3. If prices still tie, the one closest to the reference price.
4. If the reference price sits exactly midway between the tied pair, the reference price itself becomes the closing price.
5. If no equilibrium price is discovered at all, the reference price becomes the closing price.

The reference price is not a guess either. It is the volume-weighted average of the trades done in the stock between 3:00 pm and 3:15 pm IST. An average of past trades still exists inside the mechanism. It is now the anchor the auction is measured against. It is also the fallback when the auction yields nothing. It is no longer the answer.

An average answers "what did the last stretch of trading work out to". An equilibrium price answers "at what single price can the most shares change hands right now". Those are different questions, and they have different answers.

03The closing auction session: four stages and their rules

The auction is a session in its own right, not a tail on the end of continuous trading. It has four stages. The ones a participant can act in are shorter than the session as a whole.

The third stage does not end on the clock. The system shuts it at an instant picked at random between 3:28 pm and 3:30 pm IST. The exact moment order entry closes is not knowable in advance.

• Only limit orders and market orders are accepted, and both count towards the equilibrium price.
• Quantity cannot be hidden. An order must be disclosed in full, so iceberg orders are not accepted.
• Orders that rest until a trigger price is reached are not accepted in the auction. They are not carried into it from continuous trading either.
• Unexecuted limit orders from continuous trading are carried into the auction, unless their price falls outside the band applying inside it. They hold higher time priority than orders placed during the auction. Modifying one gives that priority up.
• Market orders are matched first. They match against each other in time order, at the equilibrium price. They then match against limit orders, in price-time order. The remaining limit orders are matched among themselves.
• The price band inside the auction is plus or minus 3% of the reference price.

While the auction runs, the exchange publishes a different set of numbers: the indicative equilibrium price, the cumulative buy and sell quantity, the imbalance at the equilibrium price, the imbalance counting only market orders, and an indicative index. That set differs from continuous trading on purpose. There is no last traded price to publish while nothing is executing.

04Closing auction session time: the clock at the end of the day

Every boundary below is set by a numbered paragraph of one of the two circulars. The fourth column says which. A timetable written before August 2026 gets these boundaries wrong. It has only one closing time to show.

The first row is the one that surprises people who read only the timings. The exchange did not just add a session. It also restated which half hour counts as the last half hour of continuous trading for the price-band framework. That definition moved earlier, to sit before the auction begins.

There is no longer a single time at which "the market closes". There are several. Which one applies depends on the segment. Inside the cash segment, it depends on the individual stock.

05Only some stocks, which is why the index carries two values

The rollout is phased. Both circulars say so in the same words: initially, the closing price of stocks in the cash segment on which derivative contracts are available shall be determined based on the auction. Every other security keeps the old method. That is the volume-weighted average of the last thirty minutes of continuous trading.

That has a consequence the exchange spells out. An index can hold constituents on both sides of the phase line. While the auction is running, some of its constituents are in the auction and the rest are still trading continuously. The exchange therefore publishes two numbers. One is the actual index value, worked out from last traded prices. The other is an indicative index close. That one uses the indicative equilibrium price for stocks in the auction, and a volume-weighted average from 3:00 pm onwards for the stocks that are not.

06The number that settles an expiring contract

This is why the change reaches people who never place an order in the cash segment. The same SEBI circular modifies the master-circular paragraph that defines a settlement price. It now reads: the settlement price shall be the closing price of the underlying index on the day of expiry, and the closing price of the underlying index shall be based on the closing price of the constituents of the underlying index.

The parallel paragraph for stock derivatives is changed the same way. Those contracts settle at a price calculated by the clearing corporations. It is based on the volume-weighted average of the closing prices of the stock in the cash segment, across all stock exchanges.

So the chain runs in one direction. The auction sets a constituent's closing price. The constituents' closing prices set the index's closing price. The index's closing price on expiry day is what an expiring contract settles at. The mechanism at the head of that chain did not exist in the cash segment's close before this year.

Expiry-day settlement did not change its definition. What changed is where the closing price that definition points at comes from.

07What did not change, and one date that has not arrived

• The closing price in the futures and options segment itself. The exchange's guidelines record that it continues to be determined by the existing mechanism.
• The closing price of securities outside the first phase. That is still the volume-weighted average of the last thirty minutes of continuous trading.
• Option price bands, which the exchange states are unchanged across the whole trading day, 9:00 am to 3:40 pm, the auction included.

The same SEBI circular also rewrites the pre-open session. It gives that half of itself a different implementation date: 7 September 2026. From that date the pre-open runs as four stages inside 9:00 am to 9:15 am. Order entry for limit and market orders runs from 9:00 am. Limit orders alone run from 9:05 am, with a random close in the last two minutes. Matching runs from 9:10 am for two minutes. Orders then move into continuous trading from 9:12 am for three minutes.

08What this post does not cover

• This post describes what the circulars say. It does not describe how any one broker shows the auction in an order window. That varies.
• The first phase is defined by a property, not by a fixed list published here. The property is a stock having derivative contracts available on it. That property changes as contracts are introduced and withdrawn.
• Operating detail sits in standard operating procedures. The exchanges and clearing corporations were directed to write them. Later circulars can amend any of this. Each source below carries the date it was last checked.
• Market structure explains who trades where and how orders meet. It does not predict the next price.

The documents this post is built on

Each card is a real document, named by its own reference number and date, with the part of this post that rests on it and the day a human last opened it.

Make it concrete