What Is Fill or Kill?
A Fill or Kill (FOK) order must be executed immediately in its entirety or not at all — there are no partial fills. If the order book doesn't have enough liquidity at the specified price (or better) to fill the entire order, it is canceled instantly. This prevents partial fills that leave a trader with an undesirable fraction of their intended position.
How Fill or Kill Works
FOK orders are used when partial fills would undermine the trade's purpose. For example, if a hedging strategy requires exactly 10 BTC short to match a 10 BTC long, a partial fill of 3 BTC would leave the position partially unhedged. FOK ensures the hedge is either fully in place or not at all.
Why It Matters for Traders
FOK orders are most useful for traders implementing precise strategies where position sizes must be exact — hedging, arbitrage, and spread trades. For regular directional trading, the strictness of FOK can be a disadvantage because it may reject orders that could have been partially filled profitably. Most exchanges support FOK through their advanced order type options.
