A fill or kill (FOK) order is a conditional trading instruction that requires a brokerage to execute a transaction immediately and in its entirety, or cancel it completely.
How a fill or kill order works
A fill or kill order combines two different market conditions: an all-or-none parameter and an immediate-or-cancel timeframe. When a trader submits this order type, the exchange evaluates the available liquidity for the specified asset at the exact requested price. If the total quantity is available, the trade executes. If the full amount is unavailable at that exact moment, the system cancels the order. There is no partial execution and no resting period on the order book.
Traders use these orders to ensure they receive a specific quantity of an asset at a specific price. Partial fills can result in unhedged positions if a trader needs a precise amount of stock or derivatives to execute a broader strategy. Fill or kill orders prevent situations where only a fraction of the desired order executes before the market price moves.
This order type is common in fast-moving global markets and high-volume derivatives trading. It is less common for retail investors who trade small quantities of highly liquid equities, as their orders usually fill instantly. Institutional investors moving large blocks of shares or contracts use fill or kill instructions to avoid signaling their intentions to the market, which happens if a large order sits on the exchange or executes in small fractions.
Example
Suppose you, as an Elephant, want to purchase 50,000 shares of the global agriculture firm Peanut Holdings at exactly $20 per share. You submit a fill or kill order to your broker. The market currently has 30,000 shares available at $20 and another 20,000 shares available at $20.05. Because your order specifies a strict price of $20 and requires the entire 50,000-share block to be filled at once, the exchange cannot complete the transaction. The broker immediately cancels your order, leaving you with zero shares and zero partial execution fees. If a seller offers 50,000 shares at $20 a moment later and you submit the exact same order, the trade executes instantly in its entirety.