Partial fill definition
A partial fill happens when only a portion of a limit order is executed, the share price surpasses the specified limit order target during the trade. In this instance shares will only be exchanged up to the price on the limit order, and the rest remain unfilled.
For example, an order for 500 shares at $50 may limit the fill to 20 shares as the share price tips up to $50.01 mid-trade, leaving the order short of 480 shares. Partial fills are common when using limit orders.
How to avoid partial fills
Partial fills can be avoided by choosing a market or stop order instead of a limit order. These order types make the entire fill once triggered regardless of price changes.
Alternatively, you can create a condition on the limit order such as ‘all or nothing (AON) or ‘fill-or-kill’ (FOK), stipulating that a trade can only be executed in full, not partially.