In plain English
What Fill or kill means
Fill or kill is used when a trader wants either the entire order executed right away or none of it executed. Partial fills are not acceptable. The order is usually tied to immediate liquidity, so if the market cannot absorb the full size at the specified terms, the order disappears.
Why it matters
FOK is important when partial execution would create unwanted exposure, inventory, or hedging mismatch. It lets a trader express a strict execution condition, but it can also increase the chance that no trade happens at all, especially in thinner markets or with larger order sizes.
Example
Suppose a trader sends a FOK buy order for 100,000 EUR/USD at a quoted price. If only 60,000 is available immediately at that price, the whole order is canceled. If all 100,000 can be matched at once, the full trade executes; the numbers are simplified.
Quick answers
Common questions
Does FOK guarantee a fill?+
No. It guarantees only that the order will not remain working if the full quantity cannot be filled immediately. Whether it fills depends on available liquidity and venue rules.
Is FOK the same as IOC?+
No. IOC can execute part of an order and cancel the remainder. FOK requires the entire order to be executable immediately or cancels the whole order.
Sources