Partial fills sneak up on scalpers who trade size. A market order for 5 lots gets 3 filled at one price and 2 at a worse one, or never. The average entry is now different from the clicked price, and the stop and target were calculated on the wrong assumption.
It shows up most in thin pairs, off-hours, and around news. On a major in London, a 1 lot order basically never partials. Push size up on an exotic at 3am and everything changes.
Consequences stack. The risk per trade is not what was planned. The position is smaller than expected, so the profit target pays less than the stop costs. Sometimes the unfilled part gets cancelled automatically, sometimes it sits as a pending order waiting to fill at the worst possible time.
Check the order history after trades, not just the open positions. The fill list tells the full story.
A habit that helps: set a hard rule for how much size goes into any single market order on thin instruments, and split bigger orders into chunks. Five separate 1 lot orders sometimes fill cleaner than one 5 lot order, though the slight delay between them has its own price.
On majors during busy hours, this barely matters. On crosses at odd hours it can be the difference between a clean entry and a messy one.
Does size ever get scaled down in thin conditions, or does it stay fixed no matter the hour?
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When half the order fills
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