The Pre-Open Session Explained: How the Opening Price Is Set
Published 2026-10-22 · Aadarna Stock Broking Pvt. Ltd.
Key takeaways
- The pre-open session is a short call auction held before normal trading. It helps the exchange find one opening price based on all buy and sell orders collected.
- It has an order entry phase, where you can place, change or cancel orders, and a matching phase, where orders are matched at a single equilibrium price.
- Matched orders trade at the equilibrium price, even if your limit price was better. Unmatched orders usually move into the normal market.
- The opening price can differ a lot from the previous close, and the pre-open session does not remove price risk.
Every trading day, before the regular market opens, the stock exchanges run a short session called the pre-open session. Many new investors see prices moving on their trading app before the market has 'officially' opened and wonder what is happening.
This article explains why the pre-open session exists, how its phases work in general terms, and what happens to an order you place during it. Exact timings and rules are set by NSE and BSE and can change, so always check the latest details on their official websites.
Why does the pre-open session exist?
Overnight, many things can happen: company announcements, news from other countries, or policy changes. If trading began straight away, the first few trades could happen at prices far away from where most buyers and sellers actually want to trade. That can cause sudden, sharp price swings at the open.
The pre-open session tries to reduce this. Instead of matching orders one by one, the exchange first collects orders for a short time. It then finds a single price, called the equilibrium price, at which the largest number of shares can be bought and sold. This process is known as a call auction.
- Price discovery: the opening price reflects the combined demand and supply from all collected orders.
- Smoother start: it aims to reduce wild swings in the first moments of trading.
- Fairness: everyone who participates gets the same price for matched orders.
Exchanges decide which segments and securities go through a pre-open session. Call auctions are also used in certain special situations, such as when a newly listed share starts trading or when trading resumes after a market-wide halt. Details are available on NSE and BSE.
What are the phases of the pre-open session?
In general terms, the pre-open session has three parts. The exact duration of each part is set by the exchanges.
- Order entry phase: You can place new buy or sell orders, modify them or cancel them. No trades happen yet. The exchange shows an indicative equilibrium price, which keeps changing as new orders come in. The order entry window closes at a randomly chosen moment near its end. This random close makes it harder for anyone to place last-second orders just to push the price.
- Order matching and confirmation phase: You can no longer place, change or cancel orders. The exchange calculates the final equilibrium price and matches eligible orders at that one price. Trade confirmations are then sent.
- Buffer or transition period: A short gap that allows a smooth move from the pre-open session into the normal market, where continuous trading begins.
How is the equilibrium price worked out?
The equilibrium price is the price at which the maximum quantity of shares can be traded. Let us look at a simple, hypothetical example with round numbers.
For example, say these orders are collected for a share during the order entry phase:
- Buy orders: 100 shares at ₹102, 200 shares at ₹101, 300 shares at ₹100.
- Sell orders: 150 shares at ₹99, 200 shares at ₹100, 250 shares at ₹101.
At ₹100, buyers willing to pay ₹100 or more want 600 shares, and sellers willing to accept ₹100 or less offer 350 shares. So 350 shares can trade. At ₹101, only 300 shares can trade, and at ₹99, only 150. Since ₹100 allows the most shares to change hands, ₹100 becomes the equilibrium price.
All 350 shares trade at ₹100. The buyer who was willing to pay ₹102 also pays only ₹100. If more than one price allows the same maximum quantity, the exchanges have set rules to decide the final price.
What happens to orders you place in the pre-open session?
How your order is treated depends on its type and whether it gets matched. The broad treatment is as follows:
- Matched limit orders: A limit order is an order with a price you set. If your limit price allows it to be matched, it trades at the equilibrium price, not at your limit price.
- Market orders: A market order has no price set by you. It is matched at the equilibrium price and generally gets priority over limit orders.
- Unmatched limit orders: These are not cancelled. They move into the normal market at your original limit price and wait there.
- Unmatched market orders: These are generally converted into limit orders at the equilibrium price and moved to the normal market. If no equilibrium price is found, the exchange applies its own rule, such as using a reference price.
In our example, the buyer at ₹100 wanted 300 shares but only 50 could be filled after higher bids were served. The remaining 250 shares move into the normal market as a buy order at ₹100. The seller at ₹101 got no match, so that order also moves into the normal market at ₹101.
Some order types, such as stop-loss orders, may not be accepted during the pre-open session. Also, orders placed after market hours (often called after-market orders) may be sent either to the pre-open session or to the normal market, depending on the platform. Check how Angel One's platform handles these before relying on them.
What risks should you keep in mind?
The pre-open session makes the opening price more orderly, but it does not remove risk.
- Gaps from the previous close: The equilibrium price can be much higher or lower than yesterday's closing price, especially after big news.
- Market orders have no price limit: A market order placed in the pre-open session will trade at whatever equilibrium price is discovered. You do not know that price in advance.
- The indicative price keeps changing: The price shown during order entry is only a guide. The final price may be different.
- Thin participation: In some shares, very few orders come in during pre-open, so the discovered price may not reflect wider interest.
- Charges still apply: Trades executed in the pre-open session attract charges as per Angel One's prevailing tariff, just like other trades.
What should you check before using the pre-open session?
- Read the current pre-open timings and rules on the NSE and BSE websites.
- Check which order types your trading platform allows during pre-open.
- Understand the difference between a limit order and a market order, and what price risk each carries.
- Ask Angel One's support or your Authorised Person how pre-open and after-market orders are shown and handled on the platform. Details are also available on Angel One's website.
Aadarna is an Authorised Person of Angel One Ltd. We can explain how the process works, but we do not advise on whether or when to place any trade.
Frequently asked questions
Can I cancel my order during the pre-open session?
Yes, but only during the order entry phase. Once the order entry window closes and matching begins, you cannot modify or cancel your order. Any part that remains unmatched moves into the normal market, where you can manage it as usual.
Why did my buy order trade at a lower price than I entered?
In a call auction, all matched orders trade at the single equilibrium price. If your limit price was higher than the equilibrium price, you still pay only the equilibrium price. This is how the call auction is designed.
Is the pre-open price a prediction of how the share will move during the day?
No. The equilibrium price only shows where buy and sell orders matched at that moment. Prices can move in any direction once normal trading starts, and the pre-open session does not tell you where they will go.
Do I need to place orders in the pre-open session?
No, it is optional. You can place orders in the normal market instead. Whether to participate is your own decision, so first understand how your orders will be treated and the price risk involved.