Most NSE stocks get their opening price before the market opens. It comes out of a 15-minute auction between 9:00 and 9:15 AM, which has run on a new timetable since 7 September 2026.
I wanted to know exactly how it works, so I went through NSE's and SEBI's own pages. Here are the rules as they stand today, a made-up order book to show the arithmetic, and what it all means if you're building a trading bot.
What the pre-open session is
For most of the day, the market is a continuous order book: an order trades the moment it meets a matching order. The pre-open session is different. It's a call auction. For a few minutes the exchange only collects orders. Then it works out one price per stock that clears as much of the buying and selling as possible, and every auction trade happens at that price. That becomes the stock's opening price.
SEBI introduced it in 2010 for the stocks in the Nifty and the Sensex and extended it to all other stocks in 2013. Its rules now cover every stock that isn't classified as illiquid; illiquid stocks trade in periodic call auctions instead.
The aim is better price discovery: overnight news and GIFT Nifty shape where people expect a stock to start, and the auction weighs all their orders together before the first price is set.
The timetable since 7 September 2026
NSE changed the pre-open structure from 7 September 2026, five weeks after it started the closing auction. It's still 15 minutes long:
| Time (IST) | What happens |
|---|---|
| 9:00 to 9:05 | Limit and market orders can be entered, modified and cancelled |
| 9:05 to 9:10 | Limit orders only. New market orders are rejected; ones already in can't be modified or cancelled |
| 9:08 to 9:10 | Order entry closes at a random moment chosen by the system |
| 9:10 to 9:12 | Orders are matched and trades confirmed |
| 9:12 to 9:15 | Buffer before normal trading |
| 9:15 | Continuous trading starts |
Before the change, SEBI's rules gave 8 minutes to order entry, closing at a random point between the 7th and 8th minute, then 4 minutes to matching and 3 to the buffer, with market orders allowed throughout. That's why older explainers and forum answers say the open is set "at 9:08".
The random close is there to prevent a flood of orders in the final seconds. Other rules:
- Only limit and market orders. Stop-loss, immediate-or-cancel (IOC) and disclosed-quantity orders aren't allowed.
- Algo market orders are allowed only in the first five minutes.
- Trades are final. Pre-open trades can't be cancelled.
- Live indicators. During the session NSE shows each stock's indicative equilibrium price, the quantity that would trade, total buy and sell quantities and the imbalance, plus indicative opening values for its indices, Nifty 50 included.
How the opening price is chosen
When order entry closes, NSE applies these rules in order:
- Most quantity. The equilibrium price is the price at which the largest quantity can trade. Limit and market orders both count.
- Smallest imbalance. If several prices allow that quantity, it takes the one with the smallest unmatched quantity.
- Closest to the previous close. If still tied, the price closest to the previous day's close wins. After a corporate action, the adjusted close or base price stands in.
- The previous close itself, if it sits exactly halfway between the two closest tied prices.
Orders are then matched in a set sequence: market against market, then remaining market orders against limit orders, then limit against limit, by price and then time.
Two edge cases:
- Only market orders on both sides: they're matched at the previous day's close (or adjusted close or base price).
- No price found: the first trade in the normal market after 9:15 becomes the opening price, and market orders move into the normal market at the previous day's close.
Unfilled orders aren't thrown away. They move to the normal market with their original time stamps: limit orders at their limit price, market orders at the discovered equilibrium price.
What about the Nifty? An index doesn't trade, so it has no auction of its own. NSE shows an indicative opening value during the session, and since an index is calculated from its stocks' prices, the Nifty's opening value reflects what its 50 stocks discover in the auction. I couldn't find an NSE page saying which price is used for a constituent that finds no auction price, so I'm treating that detail as unconfirmed.
A worked example with a made-up order book
Everything in this section is made up to show the method. It isn't a real stock or a real morning.
A stock closed at ₹100.00 yesterday. When order entry closes, the book holds these limit orders, plus market orders to buy 100 shares and to sell 100 shares.
| Price (₹) | Buy limit orders (shares) | Sell limit orders (shares) |
|---|---|---|
| 101.50 | 200 | 400 |
| 101.00 | 300 | 500 |
| 100.50 | 400 | 300 |
| 100.00 | 500 | 400 |
| 99.50 | 300 | 300 |
| 99.00 | 400 | 200 |
Count each order at every price it would accept. A bid at ₹101.50 will also buy at ₹101 or ₹100, so the cumulative buy at a price is every buy order at that price or higher, plus the market buys. The cumulative sell is every sell order at that price or lower, plus the market sells. The tradable quantity is the smaller of the two.
| Price (₹) | Cumulative buy | Cumulative sell | Tradable quantity | Unmatched |
|---|---|---|---|---|
| 101.50 | 300 | 2,200 | 300 | 1,900 to sell |
| 101.00 | 600 | 1,800 | 600 | 1,200 to sell |
| 100.50 | 1,000 | 1,300 | 1,000 | 300 to sell |
| 100.00 | 1,500 | 1,000 | 1,000 | 500 to buy |
| 99.50 | 1,800 | 600 | 600 | 1,200 to buy |
| 99.00 | 2,200 | 300 | 300 | 1,900 to buy |
- Most quantity: 1,000 shares can trade at both ₹100.50 and ₹100.00. A tie.
- Smallest imbalance: 300 shares are left over at ₹100.50 and 500 at ₹100.00. The opening price is ₹100.50.
₹100.00 is exactly yesterday's close, and it still lost: the previous close only breaks a tie after the imbalance rule. The order of the rules matters.
All 1,000 shares trade at ₹100.50, whatever each person asked for:
- Buyers: everyone willing to pay ₹100.50 or more fills: the 100-share market order plus the 200, 300 and 400 bid at the three highest prices. The ₹101.50 bidder pays ₹100.50.
- Sellers: 1,300 shares were offered at ₹100.50 or less, but only 1,000 can go. The market sell fills first, then 200 at ₹99.00, 300 at ₹99.50 and 400 at ₹100.00. The ₹99.00 seller receives ₹100.50.
- Left over: the 300 shares offered at exactly ₹100.50 move into the normal market with their original time stamps, along with the unfilled bids at ₹100.00 and below and the offers at ₹101.00 and above.
So the stock opens 50 paise above yesterday's close, and normal trading at 9:15 starts with a best bid of ₹100.00 and a best offer of ₹100.50, both left over from the auction.
IPO days, re-listings and futures
The special pre-open session runs from 9:00 to 10:00 AM for newly listed IPO shares on their first day (SME IPOs included), re-listed shares on the day trading restarts, and stocks with derivatives on the ex-date after a corporate restructuring. Orders are taken from 9:00 to 9:45, with a random close between the 35th and 45th minute; matching runs from 9:45 to 9:55 and the buffer to 10:00. Only limit orders are accepted. The third tie-break uses the base price instead of the previous close; for an IPO that's the issue price. If no price is found, an IPO stock moves to normal trading at its issue price, while a re-listed stock's orders are cancelled and it goes through the special session again on a later day.
Futures have their own pre-open. Since December 2025, NSE has run it for current-month index and stock futures, adding next-month futures in the last five trading days before expiry. It follows the same timetable as the equity session, including the September change, with the previous close as the tie-break reference. Options aren't included, so their first prices come from normal trading after 9:15. According to Zerodha, futures on a stock with a corporate action such as a merger skip the pre-open that day.
How it compares with the new closing auction
Since 3 August 2026, stocks with F&O contracts get their closing price from a closing auction session instead of the average of the last half hour. It follows the same pattern, and Business Standard reported that NSE changed the pre-open to line it up with the closing auction.
| Pre-open auction | Closing auction | |
|---|---|---|
| Covers | Stocks not classed as illiquid, plus current-month futures | Cash-market stocks that have derivatives contracts |
| Whole session | 9:00 to 9:15 | 3:15 to 3:35 (3:15 to 3:20 sets the reference price) |
| Market and limit orders | 9:00 to 9:05 | 3:20 to 3:25 |
| Limit orders only | 9:05 to 9:10 | 3:25 to 3:30 |
| Random close | 9:08 to 9:10 | 3:28 to 3:30 |
| Matching | 9:10 to 9:12 | 3:30 to 3:35 |
| Tie-break reference | Previous day's close | Volume-weighted average price of trades from 3:00 to 3:15 |
| Price band | Not covered here | 3% either side of the reference price |
| Not allowed | Stop-loss, IOC, disclosed quantity | Stop-loss, iceberg |
Unexecuted limit orders from continuous trading carry into the closing auction, except stop-loss and iceberg orders and those outside the band. Stocks without derivatives trade continuously until 3:30. Today's post-market report shows the effect: in five-minute data the Nifty's value sat still from 3:15 PM and moved 32.45 points when the auction result came in.
What this means if you're building a bot
None of this depends on any particular strategy, but it changed how I read the first minutes of a chart.
The gap is set before you can trade it. The jump from yesterday's close to today's open is decided in the auction, so by 9:15 the overnight news is already in the price. That's why the first 1-minute candle can start far from the previous close, and why GIFT Nifty describes the gap well but says little about the rest of the day.
"The open" can mean two things. A stock's day open is the auction price, but trading at 9:15 starts from whatever the auction left on the book plus new orders. In the made-up example the auction printed ₹100.50, yet the best bid at 9:15 was ₹100.00. A Zerodha staff member explained on the Kite Connect forum in 2018 that for equities the day candle's open may not match the 9:15 minute candle's open for this reason. If a rule uses "the open", as anything built on the opening range does, decide which open you mean and check what your data source stores.
What you can see before 9:15. NSE publishes indicative prices, quantities and index values during the session; how much reaches you depends on your broker. For Kite Connect, Zerodha staff said on the developer forum in 2021 and 2022 that pre-market ticks stream over the WebSocket, that the equilibrium price appears in the tick's "open" field, and that market depth is available. Those answers predate the September 2026 change, so I'd log what arrives and check the timestamps before relying on it. I haven't checked other brokers.
Pre-open orders need their own handling. Market orders lock at 9:05. Nothing fills before 9:10, and every fill gets the same price, which can be better than your limit. An unfilled limit order is still live at 9:15 with its original time stamp, so code that forgets it will be surprised. An unfilled market order becomes a limit order at the auction price. For the wider rules on algo orders, see my algo trading guide.
The same applies at the other end of the day: from 3:15 PM, F&O stocks are in auction mode, not continuous trading.
Sources
- NSE: Pre-open session (equity market)
- NSE: Pre-open session (equity derivatives)
- NSE: Special pre-open session and member FAQs on the special pre-open session
- NSE: Closing auction session
- NSE circular 123/2026 (4 September 2026): go-live of the pre-open changes in equity derivatives on 7 September 2026
- SEBI master circular for stock exchanges, trading chapter: call auction in the pre-open session (the rules before September 2026, including the IPO session)
- SEBI circulars: introduction of call auction in the pre-open session (July 2010), extension of the pre-open session to all scrips (February 2013), closing auction session and changes to the pre-open auction (January 2026)
- Corporate Law Reporter: summary of SEBI's 2013 circular (used for the 2010 Nifty and Sensex pilot, as SEBI's PDFs didn't load for me)
- Zerodha (broker sources, used where NSE's pages were silent): copy of the exchange circular on the closing auction and pre-open changes, closing auction explainer, pre-open for index and stock futures, pre-market and post-market sessions
- Business Standard: NSE to revise pre-open auction framework from 7 September, NSE to start pre-open session for equity derivatives from 8 December 2025
- Kite Connect developer forum: day open versus the 9:15 candle (2018), getting pre-market data through the API (2021 and 2022)
- The order book in the worked example is made up.
This guide explains exchange rules for information only. It is not investment advice or a recommendation to trade. I am not registered with SEBI as an investment adviser or research analyst.