A Fill-or-Kill (FOK) order is an order type that demands complete, immediate execution at the specified price — if the full quantity cannot be filled in a single attempt, the entire order is cancelled instantly. Unlike most order types that allow partial fills or time to accumulate liquidity, FOK is binary: either every share or contract fills right now, or nothing does.
Key Takeaways
- FOK requires 100% of the order to fill immediately at the limit price or better — any shortfall triggers instant cancellation of the entire order.
- FOK differs from Immediate-or-Cancel (IOC) in one critical way: IOC accepts whatever quantity is available and cancels the rest; FOK accepts nothing less than the full order.
- A failed FOK order is not a rejection — it’s a real-time signal that insufficient liquidity exists at your price right now, and you can retry or adjust.
How Fill-or-Kill Works
FOK is one of 5 core time-in-force instructions recognized by most US exchanges, alongside GTC, Day, IOC, and AON. When a FOK order reaches the exchange or broker’s order management system, it checks whether the full specified quantity can be matched immediately at the limit price or better. If yes, the order executes in full. If even one share or contract is unavailable at that price at that instant, the entire order is cancelled — no waiting, no partial execution.
The distinction between FOK and All-or-None (AON) is often confused. AON also requires a complete fill, but it allows the order to sit in the book and wait until enough liquidity accumulates. FOK gives the market exactly one instantaneous attempt. Think of FOK as AON with a zero-second time limit.
Order type comparison:
| Order Type | Partial Fill Allowed? | Can Wait for Liquidity? |
|---|---|---|
| FOK | No | No |
| IOC | Yes | No |
| AON | No | Yes |
| Day Limit | Yes | Yes (until close) |
Practical Example
A trader wants to enter a long strangle on AAPL before earnings: buy 10 contracts of the 175 call at $3.20 and 10 contracts of the 165 put at $2.80 simultaneously, for a total net debit of $6.00 per share ($6,000 total).
If each leg is submitted as a standard limit order, the call might fill immediately while the put sits unfilled for 30 seconds. The result: naked long call exposure heading into an earnings print — exactly the risk the strangle was designed to avoid.
Submitting each leg as a FOK order (or better, a combo order with FOK applied to the spread) eliminates this legging risk entirely. Either both legs fill at the target net debit of $6.00, or nothing executes. With AAPL trading over 100,000 options contracts per day, FOK succeeds at or near the mid-price during regular market hours. However, in the final 5 minutes before the close when bid-ask spreads widen significantly, the same FOK order might fail 3–4 consecutive times before the trader adjusts the limit price by $0.05–$0.10.
In options markets, partial fills on spread legs can create naked exposure — FOK eliminates this risk entirely, which is why it’s the preferred order type for multi-leg strategies where position integrity matters more than guaranteed execution.
A Fill-or-Kill order tells your broker to execute your entire order immediately at your price or cancel it completely. There are no partial fills. Traders use it when buying only part of a position would be worse than buying nothing at all.
When FOK Is the Right Tool — and When It’s Overkill
Use FOK when:
- You’re trading multi-leg options strategies. Filling one leg of a vertical spread or strangle without the other transforms a defined-risk strategy into an undefined-risk position. FOK on each leg or on the combo order prevents this.
- Your order size is large relative to available liquidity. An institution buying 50,000 shares of a mid-cap stock may face meaningful slippage if only 30,000 shares execute — the partial fill creates directional exposure the desk didn’t budget for.
- An algorithm requires exact position sizing. Risk models that size positions based on account equity break down if a FOK failure still results in a partial fill — the model expects 0 shares or exactly N shares, never something in between.
FOK is overkill when:
- You’re buying 100 shares of AAPL or SPY — liquidity is so deep that a standard limit order fills completely in milliseconds.
- You’re trading a single-leg position where a partial fill carries no structural risk.
- You’re using a market order where immediacy is already guaranteed.
Interactive Brokers supports FOK across equities, options, futures, and forex — one of the few retail platforms with full FOK support across all asset classes. Most retail traders, however, will find that standard limit orders handle the vast majority of their execution needs.
FOK and dark pools: Dark pool operators like IEX and Liquidnet accept FOK orders. A failed FOK probing a dark pool may inadvertently signal to other participants that a large buyer or seller is active, which can move prices against the trader on subsequent attempts.
How JournalPlus Tracks Fill-or-Kill Orders
JournalPlus logs order type alongside each trade entry, so traders can review whether FOK, IOC, or limit orders produced better execution outcomes over time. When a FOK order fails and the trader retries at a different price, each attempt is recorded separately, giving a clear picture of execution slippage across multi-leg strategies and large block trades.
