The pre-open session doesn't work like the normal market — the exchange collects all orders first and works out a single equilibrium price before any trade happens. Once that price is set, your market order is matched at that price, the same as if you'd placed a limit order there.
How does the pre-open session work?
For NSE and BSE equity, the pre-open session runs from 9:00 AM to 9:15 AM, before the normal market opens. Orders are collected but nothing executes immediately — the exchange gathers them all, then runs a call auction to work out the single price at which the maximum quantity of shares can trade, the equilibrium price.
One thing to watch: market orders are only accepted until 9:05 AM. After that the window is limit-orders-only and a new market order is rejected. For the minute-by-minute breakdown, and what decides which orders match first, see What is the pre-open session?
Why does my market order convert to a limit order?
A market order has no price of its own — it just says "execute me at the best price." In the pre-open session, that "best price" is the equilibrium price the auction discovers. So your market order gets matched at the equilibrium price, exactly like a limit order placed at that price would.
What if there isn't enough matching demand?
If the auction can't find a matching counter-order at the equilibrium price for your full quantity, the unmatched part of your order carries forward to the normal market session that opens at 9:15 AM.
Things to keep in mind
- This applies to the equity pre-open window only — once the normal market session begins, market orders execute normally against the live order book.
- You still can't control the exact price a pre-open market order fills at, since it depends on what the auction discovers — if you want a hard price limit, use a Limit order instead.