What Is a Fill or Kill (FOK) Order in Trading? Execution Explained

A Fill or Kill (FOK) order requires the entire order to be executed immediately under its specified price conditions. If the full quantity cannot be filled straight away, the whole order is cancelled. This prevents partial fills, but it also increases the chance that no trade takes place.

FOK instructions can be useful when an exact position size matters and a trader does not want part of an order left unfilled. However, availability and execution rules vary between brokers, exchanges and trading venues.

What Is a Fill or Kill (FOK) Order?

So what does a fill or kill order mean in practice? It's an execution instruction that requires an order to be completed immediately and in full. If this cannot happen, the order is cancelled without a partial execution.

FOK is commonly associated with limit orders, although the exact way it is implemented depends on the trading venue. Some venues treat FOK as an order type, while others apply it as an execution or time-in-force condition.

The key point is that an FOK order does not wait for additional liquidity to appear. The trading system evaluates the order when it arrives and determines whether the full quantity can be executed under the permitted price conditions.

For example, suppose you submit an FOK limit order to buy 100 contracts. If only 99 contracts are available at your limit price or better, the order is not partially filled for 99 contracts. Instead, the full order is cancelled.

This makes the Fill or Kill order meaning straightforward: execute the complete quantity immediately, or do not execute the order at all.

How Fill or Kill Orders Work in Practice

With a fill or kill order explained above, here's how it works in practice: when an FOK order reaches a trading venue or execution system, it checks whether enough executable liquidity is available to complete the entire order.

For a buy limit order, eligible liquidity may include prices at the limit price or lower. For a sell limit order, it may include prices at the limit price or higher. The precise matching process depends on the venue and its execution rules.

If enough eligible liquidity is available, the order can be filled in full. If there is not enough, the entire order is cancelled.

FOK orders are more likely to be cancelled when the available executable liquidity is small relative to the order size. This can happen during quieter trading periods or when trading less liquid instruments, although the effect varies by market, product and venue.

Consider a simplified example involving a FTSE 100 CFD. Assume the trading platform supports FOK orders and displays the relevant executable liquidity:

• Target order: Buy 50 CFD contracts at a limit price of 8,200.
• Order instruction: Fill or Kill (FOK).

Scenario A: Full Execution

Suppose at least 60 contracts are available at 8,200 or better.

Because the available executable quantity is greater than the 50 contracts required, the full order can be filled immediately. Nothing remains outstanding.

Scenario B: Full Cancellation

Suppose only 35 contracts are available at 8,200 or better, while another 20 contracts are offered at 8,201.

Because 8,201 is above the trader's buy limit of 8,200, those additional contracts cannot be used to complete the order. The system therefore cancels the full 50-contract order, with no partial fill.

For CFDs, the exact execution process can differ from an exchange order book because CFDs are generally traded through a provider rather than as ownership of the underlying asset. Traders should therefore check which order instructions their broker supports and how those instructions are executed.

Fill or Kill vs Immediate or Cancel vs All or None

Fill or Kill orders are often compared with Immediate or Cancel (IOC) and All or None (AON) instructions. The main differences concern whether partial fills are allowed and whether an order can remain active after it is submitted.

• Fill or Kill (FOK): The full quantity must be executed immediately. If it cannot be filled in full, the entire order is cancelled.
• Immediate or Cancel (IOC): The order attempts to execute immediately, but partial fills are allowed. Any quantity that cannot be filled straight away is cancelled.
• All or None (AON): The order must be filled in full rather than partially. Unlike FOK, AON does not necessarily impose an immediate-execution requirement by itself. Whether the order can remain active depends on the venue and any time-in-force conditions attached to it.

AON rules deserve particular attention because they are not identical across all markets. Some brokers and venues allow an AON order to remain active until it can be filled completely or is cancelled, while others restrict how AON can be combined with time-in-force instructions.

Why Traders Use Fill or Kill Orders and the Risks Involved

Whether an FOK instruction is suitable depends on the required position size, available liquidity and the trader's execution priorities.

Potential Advantages

• Exact position sizing: An FOK order prevents a single order from leaving the trader with only part of the intended position.
• Reduced fill fragmentation: Avoiding multiple partial executions can make position management simpler.
• Potential fee control: Where a broker's fee structure charges per execution, per transaction or applies minimum charges, avoiding fragmented fills may help control trading costs. This depends on the broker's pricing model.
• Defined price conditions: When FOK is used with a limit order, the order cannot execute at a price worse than the specified limit while still requiring the full quantity to be available.

Execution Risks and Limitations

• Greater chance of cancellation: An order may be cancelled even when most of the requested quantity is available. If 98 contracts are available but the FOK order requires 100, no part of the order will execute.
• Missed market entries: The market may move after an FOK order is cancelled, meaning the trader may not get another opportunity to enter at the intended price.
• Liquidity sensitivity: Large FOK orders can be harder to execute in markets where available liquidity is limited.
• Venue differences: FOK functionality is not identical across every broker, exchange or product, so traders should check the relevant execution rules before using it.

When trading CFDs, execution controls do not remove the underlying market risk. CFDs are leveraged products, so both gains and losses can be magnified.

For UK-regulated CFD providers, the percentage of retail investor accounts that lose money is provider-specific rather than a single industry-wide figure. FCA rules require relevant firms to publish an up-to-date percentage based on their own retail client accounts and to recalculate it every three months using the preceding 12-month period.

An FOK instruction can control whether an individual order is filled in full, but it cannot prevent losses if the market moves against an open position.

Conclusion

A Fill or Kill order gives traders strict control over execution quantity. The order must be completed immediately and in full under its permitted price conditions, or it is cancelled without a partial fill.

This can be useful when an exact position size is important, but the trade-off is a greater chance that the order will not execute when liquidity is insufficient. FOK orders should therefore be considered alongside alternatives such as IOC and AON instructions, as well as the specific rules of the broker or trading venue.

To see how FOK fits alongside market, limit and stop instructions, read our broader guide to order types in trading.

Trading CFDs and other leveraged products involves significant risk. Leverage can increase both potential gains and losses, and an execution instruction such as FOK does not protect a trader from adverse market movements.