Fill or Kill Order Definition

A fill or kill (FOK) order tells a trading venue to execute the entire order immediately at the specified price. If the full size is not available right away, the order is canceled on the spot. FOK is a “time in force” instruction that sets very strict conditions on both timing and quantity.

How it works

With FOK, you usually set a limit price and a total quantity. The system checks the order book. If there is enough volume at your price to satisfy the whole order instantly, the trade goes through. If not, nothing happens and the order disappears. Some markets enforce this immediacy within seconds of the order reaching the book.)

Why traders use it

Traders pick FOK when they want the full position right away and do not want partial fills. This can be handy for time-sensitive strategies or when coordinating across multiple venues. In crypto, a trader might send the same full-size FOK order to several exchanges and then keep whichever one fills in full, canceling the rest.

Pros and cons

Pros. You control execution quality by avoiding partial delivery and the hassles that come with stitching small fills together. You also prevent lingering orders that could be picked off in a moving market.

Cons. You may walk away with no trade at all, even if most of your size was available. That can mean missed opportunities, especially in thin books or fast markets.

How FOK compares to similar order types

• Immediate-or-Cancel (IOC): Tries to fill what it can right away, then cancels any remainder. FOK is stricter because it requires the entire order to fill immediately, or nothing happens.
• All-or-None (AON): Requires a full fill but does not require it to happen immediately. FOK requires both immediate and complete execution.

Typical use cases

• Buying or selling a large block without tipping the market or accepting slippage from piecemeal fills
• Time-critical trades where partial positions create risk or operational complexity.

These cases show up in both traditional markets and crypto exchanges.

Example

Say you want 1,000,000 shares at 15.00. You send a limit FOK at 15.00 for the full amount. If the book cannot satisfy the entire million shares at 15.00 or better right away, the whole order cancels. If it can, you get filled instantly at 15.00 or better.

Notes on venue behavior

Details can vary by exchange or platform, including how quickly “immediate” is enforced. Some venues allow FOK only as limit orders, and the feature is not as common as other time-in-force settings. Always check your broker or exchange documentation.