Fill or Kill (FOK)
Fill or Kill (FOK) is a time-in-force instruction that requires an order to be filled immediately and in full — otherwise the entire order is cancelled.
In plain terms: "Give me everything I want right now, or give me nothing."
In This Article
How Fill or Kill Works
FOK is a time-in-force setting (also called a duration or validity instruction). It tells the market or broker how long your order should remain active and under what conditions it should be cancelled.
- The order must be filled immediately
- The order must be filled in full (no partial fills)
- If either condition fails, the entire order is cancelled ("killed")
FOK is typically used with limit orders to specify both a price and a strict execution rule.
FOK vs IOC: Key Difference
Both FOK and Immediate or Cancel (IOC) require immediate execution, but they differ on partial fills:
| Instruction | Partial Fills? | Unfilled Portion |
|---|---|---|
| FOK | No | Entire order cancelled |
| IOC | Yes | Remainder cancelled |
Summary: IOC says "fill what you can, cancel the rest." FOK says "fill everything or cancel everything."
When to Use FOK
FOK is a specialist tool. Common use cases include:
- Hedging: You need to offset a specific exposure. A partial fill would leave you with unwanted residual risk.
- Large block trades: Institutional traders needing a full allocation at a particular price.
- Operational simplicity: Your workflow is easier if every trade is either fully executed or not executed at all.
Most retail traders rarely need FOK. Standard market, limit, and stop orders cover the majority of trading needs.
Example: FOK on a Share Order
You want to buy 10,000 shares of a stock at £2.00 using a limit price. The order book at £2.00 currently has only 6,000 shares available.
- If you submit the order as FOK, it will not partially fill 6,000 shares
- Because the full 10,000 is not available immediately, the order cancels (kill)
- If liquidity increases (more sellers appear), you could try again
If you had used IOC, you could have been filled for 6,000 and the remaining 4,000 cancelled.
Pros and Cons of FOK
- Pro: avoids partial fills that can create unwanted leftover exposure
- Pro: cleaner execution logic for hedging and operational workflows
- Con: higher chance of no fill in markets with limited liquidity
- Con: can miss opportunities if liquidity appears moments later
Practical takeaway
FOK is a "strict" execution rule. If your priority is participating, even partially, IOC may be more suitable.
Common Misconceptions
-
"FOK guarantees a good price."
No. It only controls execution conditions. Your price depends on whether you use market or limit pricing and current liquidity. -
"FOK is useful for every retail trader."
Most beginners rarely need it. It's more common in exchange trading, larger sizes, and hedging workflows. -
"FOK means the order stays open until it fills."
That's GTC. FOK is the opposite: immediate fill or cancel.
✅ Quick Checkpoint
Test yourself. Try answering before expanding the model answers.
1) What does FOK require?
2) How is FOK different from IOC?
3) When might FOK be preferred?
If you can explain these points clearly, you understand what FOK is actually designed for.
Frequently Asked Questions
Is FOK available on CFDs and FX platforms?
Can FOK be used with a limit order?
Does FOK reduce slippage?
What should beginners use instead of FOK?
Summary
Fill or Kill (FOK) is a time-in-force instruction that demands an order be filled immediately and in full — otherwise it is cancelled. It's a specialist tool used to avoid partial fills, often in exchange markets and hedging scenarios.
The key trade-off: guaranteed full size versus higher chance of no execution.