A Fill or Kill (FOK) order is a trading instruction that must be executed immediately and in full; if the entire order cannot be filled at once, it is automatically cancelled. This makes FOK orders useful for traders who cannot accept partial fills and need a complete execution at a specific moment. Although similar to an All or Nothing (AON) order, the key difference is timing. AON orders also require full execution, but they do not demand immediate fulfillment, while FOK orders must be filled instantly or not at all.

Traders often use FOK orders when timing is critical or when they need to acquire a specific quantity of an asset across multiple markets without the risk of receiving incomplete amounts. For example, if a trader needs a fixed number of tokens to activate a masternode right away, they can place multiple FOK buy orders across different exchanges. The order will only go through if the full amount is available instantly, ensuring that the trader gets exactly what they need without waiting for partial orders to accumulate.

FOK orders provide precision and protect traders from fragmented execution, making them valuable in situations where speed, quantity and certainty are essential. They are commonly used in volatile markets, large trades or time-sensitive strategies where partial fills would create risk or disrupt the trading plan.