Fill-or-Kill: Fill the Whole Order Now or Cancel

Key Takeaways

• A fill or kill order FOK executes the whole order instantly or cancels all of it.
• It permits no partial fills, unlike an immediate-or-cancel order.
• It acts immediately, unlike an all-or-none order that can keep working.
• Traders use FOK for large orders where a partial position is useless.

What a Fill or Kill Order FOK Is

A fill or kill order FOK is a time-in-force instruction that combines two demands: the full quantity must trade, and it must trade right away. FINRA describes it plainly, noting that with this approach "your firm will execute your entire order immediately or not at all."

If both conditions cannot be met at the moment the order arrives, the entire order is canceled. There is no resting, no partial fill, and no second attempt. The order either completes in one shot or vanishes.

The Intuition

Some positions only make sense if you can get all of them at once. A trader assembling a precise hedge or a block tied to another leg cannot use half a fill. A partial position would leave the strategy unbalanced and force a scramble to complete or unwind it.

The FOK instruction removes that risk. It tells the market to give you everything immediately or nothing at all. You never end up holding an awkward fraction of your intended size, and you never leave an order sitting in the book where it can move against you.

How It Works

An FOK is attached to a limit or market order and tested against the liquidity available the instant it reaches the venue. If the full quantity can fill at an acceptable price right then, it executes. If not, it is canceled entirely.

FOK: fill the entire order immediately, or cancel all of it
IOC: fill what you can immediately, cancel only the remainder
AON: fill the entire order, but it may keep working over time

Two contrasts define FOK. Against an immediate-or-cancel order, the difference is partial fills: IOC keeps whatever trades instantly, while FOK rejects anything less than the full size. Against an all-or-none order, the difference is timing: AON also demands the full quantity but can keep trying across the session, while FOK insists on immediate completion. FOK is the only one that requires both the whole order and instant execution.

Worked Example

A trader needs exactly 10,000 shares to complete a paired trade and has no use for a partial fill. The current offer price is 50.00.

The trader sends a buy fill or kill order FOK for 10,000 shares at 50.00. At that instant the book shows only 6,000 shares offered at 50.00 or better. Because the full 10,000 cannot fill immediately, the entire FOK order is canceled and the trader owns nothing.

Now compare the same size sent as an immediate-or-cancel order. The IOC would have bought the 6,000 available shares and canceled the remaining 4,000, leaving the trader with a partial position. The FOK refused that outcome by design. For this trader, owning 6,000 of a needed 10,000 was worse than owning none, which is exactly why FOK was the right tool.

Common Mistakes

1. Using FOK in thin markets. When liquidity is scarce, the full size rarely fills at once, so FOK orders are canceled repeatedly and nothing gets done.
2. Confusing FOK with immediate-or-cancel. IOC accepts partial fills; FOK does not. Choosing the wrong one decides whether you get a partial position or zero shares.
3. Confusing FOK with all-or-none. AON also wants the full size but can keep working over time, while FOK demands immediate completion.
4. Forgetting the limit price still binds. An FOK limit order will not fill above your buy limit or below your sell limit, so a tight limit makes cancellation more likely.
5. Expecting partial progress. An FOK never leaves anything working. If it cancels, you must resend, and the market may have moved.

Frequently Asked Questions

What is a fill or kill order FOK in simple terms? A fill or kill order FOK buys or sells the entire amount instantly, or it cancels the whole order. There are no partial fills and no waiting.

How does a fill or kill order FOK affect investment decisions? It protects you from ending up with a partial position when only the full size is useful. In the worked example, an FOK for 10,000 shares canceled entirely because only 6,000 were available.

What is a real-world example of a fill or kill order FOK? A trader needing exactly 10,000 shares for a paired trade sends an FOK, and because only 6,000 can fill at once, the entire order is canceled and no shares are bought.

How can investors use a fill or kill order FOK effectively? Reserve it for situations where a partial fill has no value, expect cancellation in thin markets, and set a realistic limit price so the full size has a chance to execute.

How is a fill or kill order FOK different from an immediate-or-cancel order? An FOK requires the entire order to fill at once or it cancels everything. An immediate-or-cancel order takes whatever fills instantly and cancels only the unfilled remainder.

Sources

1. FINRA. Trading Terms: Time Parameters and Qualifiers on Stock Orders. https://www.finra.org/investors/insights/time-parameters-qualifiers-stock-orders
2. SEC Investor.gov. Types of Orders. https://www.investor.gov/introduction-investing/investing-basics/how-stock-markets-work/types-orders
3. Nasdaq Glossary. Fill or Kill Order (FOK). https://www.nasdaq.com/glossary/f/fill-or-kill-order
4. SEC Office of Investor Education and Advocacy. Trading Basics. https://www.sec.gov/files/trading101basics.pdf

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.