A fill or kill (FOK) order is a time-in-force trading directive that requires a brokerage to execute a transaction immediately and completely, or else cancel the order entirely.
Mechanics and usage of fill or kill orders
A fill or kill order combines the conditions of an all-or-none order and an immediate-or-cancel order. It ensures that investors do not receive partial fills on their requested shares or contracts. If the market cannot supply the exact quantity at the requested price the moment the order reaches the exchange, the matching engine cancels the trade.
Active traders and institutions use these orders when buying or selling large quantities of assets. Receiving a partial fill leaves them exposed to price movements on the remaining unexecuted portion of their planned position. The execution timeframe is strictly immediate, meaning the system does not wait for additional shares to become available in the order book.
These orders are available on global financial exchanges across equities, commodities, and forex markets. They are common in options trading, where traders need specific numbers of contracts executed at an exact premium to construct their positions. Modern electronic trading systems process this matching automatically in milliseconds.
Example
Suppose an Elephant trading on the London Stock Exchange wants to purchase 100,000 shares of an agricultural firm at exactly 50 pence per share. To avoid buying a smaller fraction of the shares, the Elephant submits a fill or kill order for the 100,000 shares. When the order reaches the exchange, the order book only has 80,000 shares available at 50 pence. Because the system cannot fill the order completely, the exchange cancels the trade immediately. The Elephant buys zero shares. If the full 100,000 shares were available at that price, the exchange would execute the entire trade instantly.