The PSX opening price is selected from pre-open orders using four rules, in order: maximize the shares that can trade, minimize any remaining imbalance, minimize the net change from the applicable reference price, then choose the highest price if the tie remains. During the break after pre-open, the system calculates that price and matches and confirms pre-open orders at it. If there are no pre-open orders, no opening price is computed in that phase.
From queued orders to an opening price
PSX’s pre-open has two parts: an Order Entry Period and a Break Period. During order entry, orders are entered and queued. Market orders and order execution, modification or cancellation are disallowed. During the break, the system calculates the opening price, matches pre-open orders at that price and confirms them. New order entry, modification and cancellation are disallowed during the break.
Where an imbalance exists, the Trading System chooses an opening price. Once selected, trading takes place at that price. The regular open session starts after the break.
The four price-selection rules
Rule 8.14.5 sets out these decisions in sequence:
- Maximize the shares that can trade. Compare the price levels and select the one at which the greatest number of shares can be traded.
- Minimize residual volume imbalance. If multiple price levels tie on tradable shares, prefer the one that leaves the smallest remaining imbalance.
- Minimize net change from the reference price. If the tie remains, use the price with the smallest net change from the previous day’s close or the previous session’s last trade, whichever applies.
- Select the highest price. If all earlier rules still leave a tie, choose the highest tied price.
Each rule applies only if the preceding rule leaves more than one candidate. This is a system-defined sequence for matching orders, not a forecast of where the security will trade later in the session.
What if there are no pre-open orders?
If no orders were entered during pre-open, the system does not compute an opening price in the break. Once a trade occurs in the open state, its first trade price becomes the opening price, except for cross trades. If no such trade occurs, no opening price is computed.
What happens to unfilled pre-open orders?
Under Rule 8.14.6, pre-open orders receive time priority according to their actual entry time. Unfilled orders retain that priority into the trading day—and, on Friday, into the trading session. Limit orders continue to be governed by the normal procedures.
PSX’s Trading Hours page also states that pre-open orders cannot be cancelled, modified or suspended until the break ends. For the session sequence and schedule, see PSX trading hours explained. For a separate guide to the Exchange’s market types, see PSX market types explained.
The opening-price process explains how the first price is established; it does not determine whether a trade fits an individual investor’s circumstances. The position-size calculator is a separate arithmetic tool. Visit the PSX hub for company research links.
Sources
- PSX Regulations, updated 23 September 2026 — Chapter 8, clauses 8.14.1–8.14.7; checked 3 November 2026.
- PSX — Trading Hours — break-period label and restrictions on pre-open orders; checked 3 November 2026.