Fill or Kill (FOK)
In simple terms
A Fill or Kill order is like saying "buy this item for me right now in full, or forget about it." If the exchange can't give you the entire amount you want immediately, the whole order gets cancelled instead of giving you just part of it.
Definition
An order that must be executed in its entirety immediately, or it's cancelled.
In depth
A Fill or Kill (FOK) order is an instruction to a trading venue that mandates immediate execution of the entire order quantity at the specified price or better, with any unfilled portion automatically cancelled. When submitted, the matching engine checks available liquidity across the order book; if sufficient counterparty orders exist to fill the complete FOK request within a single market sweep, the transaction executes atomically. If the full quantity cannot be matched instantly—whether due to insufficient liquidity, price slippage, or market conditions—the entire order is rejected rather than partially filled, ensuring an all-or-nothing execution model.
How does Fill or Kill (FOK) work?
Fill or Kill is a time-in-force instruction attached to an order, not an order type of its own. On receipt the exchange checks whether the entire quantity can be executed immediately at your specified price or better against resting liquidity. If it can, the whole order fills at once. If it cannot — even if most of it could — the exchange cancels the entire order and nothing executes. There is no partial fill and no resting on the book. The order either completes in full immediately or ceases to exist.
An example
Illustrative figures only. Someone submits a FOK buy for 100 units at $10. The book holds 60 units at $10 and 40 more at $10.05. Because only 60 can fill at $10 or better, the whole order is cancelled and zero units are bought. Had 120 units been available at $10, all 100 would have filled immediately in a single execution.
Figures are illustrative only.
What beginners get wrong
- FOK is easily confused with Immediate or Cancel, which does allow a partial fill; FOK allows none at all.
- In thin markets a FOK order can be cancelled repeatedly, and traders sometimes miss that nothing was ever executed.
- Sending a large FOK order into a shallow book almost guarantees cancellation, because the full size rarely sits at one price.
- Not every venue supports FOK on every product, and unsupported flags may be silently downgraded to a different instruction.
Related terms
Part of
What do the different crypto order types do? — the subject page for order types, with all 9 of its definitions in one place.
Educational only — not financial advice.