Fill or Kill

Fill or Kill (FOK) is a type of trading order that must be executed in its entirety immediately; if the full quantity cannot be filled at the specified price or better the moment the order reaches the market, the order is automatically cancelled. It belongs to the category of time‑in‑force instructions and is used when a trader requires certainty that either the entire trade occurs or none at all.

FOK orders matter because they protect traders from unwanted partial fills that could leave them with an unintended position or expose them to adverse price movement. For example, an investor wanting to buy 50,000 shares of a stock at $20.00 will place a FOK order; if fewer than 50,000 shares are available at that price, the order is cancelled entirely, ensuring the investor does not end up with only a fraction of the desired stake.