Fill or Kill
A fill or kill (FOK) order requires that the entire order be executed immediately and in full; if the order cannot be completely filled at once, it is cancelled outright with no partial execution permitted.
Fill or kill is a time-in-force instruction that combines two strict requirements: immediacy and completeness. A fill or kill order demands that the broker or exchange fill the entire specified quantity at the limit price (or better) the instant it reaches the market. If even a single share of the requested quantity cannot be filled immediately, the entire order is cancelled — hence the 'kill' component.
This order type is primarily used by institutional investors and sophisticated traders who need to transact large blocks of shares without accepting partial fills. In certain strategies — such as risk arbitrage or pairs trading — executing a partial position can create undesired exposure that is worse than not trading at all. The FOK instruction ensures the position is either fully established or not established at all.
Fill or kill orders are distinct from two related order types. An 'immediate or cancel' (IOC) order also requires immediate execution but accepts partial fills — whatever portion can be filled immediately is executed, and the remainder is cancelled. An 'all or none' (AON) order requires full execution but does not require it to happen immediately; it can persist on the order book but will only execute when the full quantity is available at once.
For individual retail investors, fill or kill orders are less commonly used because most retail trade sizes are small enough that full immediate execution is the norm. The instruction becomes relevant primarily in thinly traded securities or when attempting to execute large blocks quickly. Retail brokerage platforms may or may not support FOK orders depending on the platform's capabilities and the venue to which orders are routed.
Under Regulation NMS, U.S. broker-dealers have best execution obligations and must route orders in a manner consistent with obtaining the most favorable terms. FOK instructions constrain where and how an order can be routed, which may occasionally limit the ability to seek better prices across multiple venues. Investors should weigh the certainty provided by FOK against the potential cost of forgoing a more favorable execution that might have been available through a less restrictive order type.