What Is a Fill or Kill (FOK) Order?

Executing large trades in fast-moving markets requires precision. The definition of a Fill or Kill (FOK) order is an order that must be executed immediately in its entirety or canceled in full; there is no allowance for partial fills. One wrong move—or a partial fill at the wrong time—can derail an entire strategy. That’s where the Fill or Kill (FOK) order comes in, giving traders strict control over execution outcomes, and FOK orders are characterized as extreme orders due to their strict execution requirements.

In this guide, you’ll learn how FOK orders work, when they’re most effective, and how they compare with other conditional order types like Immediate or Cancel (IOC) and Good Till Canceled (GTC).

What Is a Fill or Kill (FOK) Order?

A Fill or Kill (FOK) order is a time-in-force instruction that requires an order to be executed immediately and in full at a specified price or better—or canceled entirely. A fill or kill order is designed for situations where immediate execution or cancellation is essential, especially in fast, liquid markets. There is no partial execution.

The “fill” means 100% of the requested quantity must be executed, so the whole order or entire position must be completed immediately. The “kill” means that if full execution isn’t possible instantly, the order is canceled with no position opened. Cancellation occurs instantly if the order cannot be completed, and the rest of the order is discarded. You either get exactly what you want, or nothing at all.

FOK orders are commonly used by institutional traders, high-frequency traders, and active retail traders handling large positions in markets such as stocks, futures, and FX. FOK orders are used primarily by active traders or institutional investors managing large volumes of securities to avoid price disruption. When trading large size, partial fills can distort average entry prices and complicate risk management.

Conceptually, a FOK order combines features of two other order types: All or None (AON) and Immediate or Cancel (IOC). Like AON, it requires full quantity execution. Like IOC, it demands immediate evaluation. This hybrid nature makes FOK one of the strictest order types available, and FOK orders are characterized as extreme orders due to their strict execution requirements.

The main goal of a FOK order is to eliminate uncertainty. The way the order works is to ensure the entire position is filled or canceled, which is especially important for large trades in securities. Partial fills can create unintended exposure, unbalanced hedges, or operational inefficiencies. For many strategies, receiving 70% of a position is worse than receiving nothing.

Although FOK orders are less common than standard market or limit orders, they are essential when price sensitivity and execution certainty matter more than execution probability. FOK orders require high liquidity to succeed, and in low-liquidity markets, these orders are often killed, leading to no transaction. Exchanges may cancel unfilled FOK orders within seconds, so it is advisable to use liquid assets when placing such orders. Most exchanges limit the period an incoming FOK order can remain unactioned before triggering automatic cancellation, commonly terminating within seconds.

How Does a Fill or Kill (FOK) Order Work?

To place a fill or kill (FOK) order, you must define the trade specifications directly with your broker, including the asset, quantity, and maximum price. You also need to select a broker that supports fill or kill orders.

When placing a FOK order, the trader selects the instrument, direction (buy or sell), quantity, and limit price, then chooses FOK as the time-in-force condition. After logging into your broker's website or trading interface, locate the 'New Order' option to initiate a fill or kill order.

Assume an investor wants to buy 10,000 shares at $10. The investor would enter these details, determining the exact quantity and price, and select FOK as the order type. Before submitting, it is important to review all order parameters to ensure they match your intended trade. Once you have confirmed the order details, you can proceed with submitting the fill or kill order to the market.

Once submitted, the order is routed to the market or liquidity pool. The matching engine immediately checks whether the entire order can be filled at the specified price or better. The order may be filled by hitting the first bid or offer available; some exchanges execute FOK orders by filling what is available at the first bid price before canceling any remaining shares. FOK orders are often placed as limit orders to specify the maximum price.

If 100% of the requested size is available, the order executes in full. If even a small portion is unavailable, the entire order is canceled instantly. There is no waiting, no partial fill, no re-evaluation, and no need to manually cancel the order, as the cancellation is automatic.

FOK orders are assessed only once upon arrival. This makes them useful for avoiding slippage and controlling market impact, particularly when entering or exiting large positions during volatile conditions.

While implementation details may vary slightly across exchanges or brokers, the core principle remains the same: the process of determining whether the order can be filled is handled instantly by the broker and exchange, ensuring immediate, all-or-nothing execution.

How Long Does a Fill or Kill (FOK) Order Last?

A FOK order lasts only for the instant it is evaluated—typically milliseconds on electronic trading platforms.

This contrasts with Day orders, which remain active until the market closes, and GTC orders, which can stay open for days or weeks. A FOK order either executes immediately or expires almost instantly.

There is no open period and no chance for conditions to improve. This ultra-short lifespan provides clarity: you are either fully in the trade or completely out.

In fast-paced markets like FX or index CFDs, the entire process happens faster than human reaction time, requiring no manual intervention.

Types of Orders

Understanding alternative order types helps traders choose the right execution tool.

All or None (AON) orders require full quantity execution but do not require immediacy. They can remain active until filled completely or expired. AON prioritizes quantity integrity over timing.

Immediate or Cancel (IOC) orders require immediate execution, but allow partial fills. Any unfilled portion is canceled instantly. IOC is useful when getting some exposure is better than none.

Good Till Canceled (GTC) orders remain active until filled or manually canceled, often for weeks. They suit longer-term strategies where timing is flexible.

Order Type | Execution Timing | Partial Fills | Best Use Case
FOK | Immediate | No | Exact-size, large orders
IOC | Immediate | Yes | Fast markets, flexible size
AON | Can wait | No | Block trades
GTC | Can wait | Yes | Swing or position trades

What Order Types Are Available on IUX?

IUX supports advanced order types designed for active traders, including FOK and IOC orders.

Available order types include market orders, limit orders, FOK, IOC, Day, and GTC orders. These tools allow traders to tailor execution based on liquidity, volatility, and position size requirements.

When comparing fill or kill vs immediate or cancel, the key difference is how partial fills are treated. FOK requires full execution or nothing. IOC allows partial execution and cancels only the remainder. The handling of FOK orders can vary across other exchanges, so traders may prefer standardized order types like IOC or AON for reliability across multiple trading venues.

FOK orders can be especially beneficial when purchasing large amounts of stock held in multiple unlinked markets, as they allow for simultaneous completion of the entire order without manual cancellation or risk of partial execution.

Pros of FOK on IUX include precise position sizing, strong protection against partial fills, and reduced execution uncertainty. The trade-off is a higher chance of non-execution in thin or fast-moving markets.

IOC offers greater execution probability and flexibility but may result in smaller-than-planned positions that require follow-up trades.

Understanding Kill Orders

A kill order is a specific type of conditional order that plays a crucial role in the execution of fill or kill (FOK) orders. In essence, a kill order is an instruction to cancel the entire order if it cannot be executed immediately and in its entirety at the set price. This mechanism ensures that traders and investors avoid the pitfalls of partial fills, which can introduce unwanted risk or disrupt carefully planned strategies.

Within the context of a FOK order, the kill order is automatically triggered if the entire order cannot be filled at the specified price the moment it hits the market. There is no room for partial execution—if the full quantity isn’t available, the unfilled balance is instantly canceled. This all-or-nothing approach is especially valuable when dealing with large quantity trades, where even a small unfilled balance can impact the overall position and risk profile.

In corporate finance, kill orders are often used to maintain strict control over large transactions. For example, a trader might submit a FOK order to purchase a significant block of stock at a set price. If the market cannot provide the entire order immediately, the kill order ensures that the transaction is canceled, protecting the trader from partial exposure and potential price slippage.

Consider a practical example: An investor wants to buy 15,000 shares of a stock at $75 per share using a FOK order. If only 12,000 shares are available at that price, the kill order is activated, and the entire order is canceled—no shares are purchased, and the investor avoids holding a partial position that may not fit their strategy. This is in stark contrast to a good till canceled (GTC) order, which would remain open until filled or manually canceled, potentially resulting in multiple partial fills over time.

Kill orders are also a key component in complex strategies where precise execution is critical. By combining kill orders with limit or market orders, traders can design conditional orders that optimize execution and minimize risk. For instance, a trader might use a FOK order with a limit price to ensure that a large position is executed only if the market can meet the entire order at the desired price or better. If not, the kill order cancels the transaction, maintaining the integrity of the strategy.

Ultimately, understanding how kill orders function within fill or kill FOK orders empowers traders and investors to manage risk more effectively. By ensuring that an entire order is either executed immediately or not at all, kill orders help maintain control, reduce the chance of unwanted partial execution, and support the implementation of sophisticated trading strategies in today’s fast-paced markets.

Example of Using a Fill or Kill (FOK) Order

Scenario: Failed FOK Execution

A buyer places a FOK order to buy 250,000 shares of a stock at $42.50. The order book shows only 200,000 shares available at or below that price. Because the full quantity cannot be filled, the buyer's order is not completed and is immediately canceled with zero execution.

Scenario: Successful FOK Execution

If the order book shows 270,000 shares available at or below $42.50, the entire 250,000-share order executes instantly. The buyer receives the full position, possibly across multiple price levels within the limit.

An example of a FOK order is when an investor places an order to buy 10,000 shares at $10, and if only 8,000 shares are available at that price, the entire order is canceled. This ensures the order is only completed if the full quantity can be filled at the specified price.

Had the trader used an IOC order in the first scenario, they would have received a partial fill of 200,000 shares. This difference highlights why FOK is chosen when exact position size is critical.

Conclusion

Mastering Fill or Kill (FOK) and Immediate or Cancel (IOC) orders is essential for traders executing large or time-sensitive positions. FOK prioritizes certainty and exact sizing, while IOC prioritizes speed and execution probability.

Choosing between fill or kill vs immediate or cancel depends on whether position completeness or participation matters more for your strategy. Understanding how these orders behave helps control slippage, manage risk, and improve execution quality.

Before using these orders with significant capital, practice in demo environments or small sizes to understand how they perform under different market conditions.

FAQs

Q: What is slippage?
A: Slippage is the difference between the expected price and the actual execution price, commonly occurring in volatile or low-liquidity markets.

Q: What is Buy/Sell?
A: A Buy order opens or increases a long position, while a Sell order reduces or closes a position, or opens a short position depending on the product.

Q: What is a pip?
A: A pip is the standard unit of price movement in forex trading, typically the fourth decimal place for most currency pairs and the second for JPY pairs.