A fill or kill order, usually shortened to FOK, is an order instruction that says: execute the whole order immediately, or cancel the whole thing.
That is the entire idea. No waiting around. No partial position. No leaving the remaining shares or contracts open for later. If the market cannot fill the full quantity at the required price right away, the order is killed.
This makes FOK different from the order types most beginners use. A normal limit order can sit in the market. An IOC order can take a partial fill. An AON order can wait for the full quantity. A fill or kill order is stricter than all of those: full size, immediate execution, or nothing.
FOK orders are not usually the first tool a new trader needs. They are more useful when exact size matters, when partial fills would create a messy position, or when a strategy only works if the full order can be completed at once.
Fill or Kill Order Meaning
A fill or kill order is a time-in-force instruction attached to a trade order. It tells the broker or trading venue that the order must be filled immediately and completely. If that cannot happen, the order is canceled automatically.
In most practical cases, a FOK order is paired with a limit price. For example:
- buy 10,000 shares;
- pay no more than $25.00 per share;
- fill the full 10,000 immediately;
- otherwise cancel the order.
If 10,000 shares are available at $25.00 or lower right away, the order fills. If only 6,000 shares are available, the order does not buy 6,000 and cancel the rest. It cancels everything.
FINRA describes fill or kill as an instruction to execute the entire order immediately or not at all. That is the cleanest way to remember it.
Quick Summary
| Question | Answer |
|---|---|
| What does FOK mean? | Fill or kill |
| What does it do? | Executes the full order immediately or cancels it |
| Does it allow partial fills? | Usually no |
| Does it stay open? | No, it is canceled if it cannot execute right away |
| Is it the same as IOC? | No. IOC can allow partial fills; FOK normally does not |
| Is it the same as AON? | No. AON requires full size but may remain open; FOK requires full size immediately |
| Best suited for | Exact-size trades, larger orders, fast strategies, and situations where partial fills are undesirable |
| Main risk | You may get no trade even when some liquidity is available |
The key tradeoff is simple: a FOK order gives up execution flexibility in exchange for certainty about size and timing.
How a Fill or Kill Order Works
Imagine you want to buy 50,000 shares of a stock, but only if you can get all 50,000 shares at $10.00 or better.
You place a FOK buy limit order:
- quantity: 50,000 shares;
- limit price: $10.00;
- time in force: fill or kill.
The order reaches the market and immediately checks available liquidity. If the full 50,000 shares can be bought at $10.00 or less, the trade happens. If the market can provide only 35,000 shares at that price, the order is canceled.
That may feel harsh, but that is the point. The trader is saying, “I do not want part of this trade. I either need the whole position now, or I would rather have no position.”
This matters because partial fills can change the trade. A strategy built around 50,000 shares may not behave the same with 7,000 shares. A hedge may not hedge enough. A large buyer may not want to show interest in the market unless the full block can be done. A short-term trader may not want an incomplete position sitting around while the price moves.
A Simple FOK Example
Suppose a trader wants to buy 5,000 shares of a stock at no more than $20.00.
The visible liquidity looks like this:
| Price | Shares available |
|---|---|
| $19.98 | 1,500 |
| $19.99 | 1,000 |
| $20.00 | 1,200 |
| $20.01 | 2,000 |
At $20.00 or better, only 3,700 shares are available. A regular limit order might buy those 3,700 shares and leave the rest waiting. An IOC order might buy the 3,700 and cancel the remaining 1,300. A FOK order cancels the entire request because the full 5,000 shares are not available at the required price immediately.
Now change the order to 3,000 shares at $20.00. In that case, the FOK order can fill because enough shares are available at the limit price or better.
This is why FOK is less about “getting a better price” and more about enforcing the exact execution condition.
Will the full order fill, or will it be killed?
Change the requested quantity, limit price, and liquidity profile. A FOK buy order only fills if enough shares are available at the limit price or better right now.
How Long Does a FOK Order Last?
A fill or kill order is designed to last only long enough for the market to check whether the full order can be executed immediately.
In plain English, that usually means seconds or less. If the required liquidity is not available right away, the order is canceled. It does not sit on the book like a day order or a good-till-canceled order.
This is why FOK belongs inside the broader category of time in force orders. Time in force tells the platform how long an order should remain active. FOK uses the strictest version: now, in full, or cancel.
One important detail: exact handling can vary by broker, exchange, asset class, and platform. Investor.gov notes that available order types and trading instructions may differ between brokerage firms. So if FOK behavior matters to your strategy, check how your broker defines it before relying on it.
FOK vs IOC Order
FOK and IOC orders are easy to confuse because both are immediate instructions. The difference is what happens when only part of the order can be filled.
An IOC order means immediate or cancel. It tries to fill as much as possible right away, then cancels the unfilled remainder.
A FOK order means fill or kill. It must fill the whole order right away, or cancel the whole thing.
| Scenario | IOC result | FOK result |
|---|---|---|
| You want 10,000 shares, and 10,000 are available | Full fill | Full fill |
| You want 10,000 shares, and 6,000 are available | Buys/sells 6,000 and cancels 4,000 | Cancels the whole order |
| You want exact position size | May leave you with a partial position | Avoids partial position |
| You just want whatever is available now | Often more suitable | Usually too strict |
Think of IOC as “take what you can get right now.” Think of FOK as “all of it right now, or forget it.”
For many active traders, IOC is more flexible. For strategies where a partial fill creates a problem, FOK is cleaner.
FOK vs AON Order
AON means all or none. Like FOK, it avoids partial fills. But it does not always require immediate execution.
An AON order can remain active while the broker or market waits for enough liquidity to complete the full order. A FOK order does not wait. If the full quantity is not available immediately, it cancels.
| Question | FOK | AON |
|---|---|---|
| Must the order fill in full? | Yes | Yes |
| Can it partially fill? | Usually no | No |
| Must it fill immediately? | Yes | Not necessarily |
| Can it remain open? | No | Often yes, depending on broker rules |
| Main use | Immediate exact-size execution | Exact-size execution without the same urgency |
Nasdaq’s glossary describes FOK order handling as exchange-dependent, which is a useful caution. In real trading, labels can sound universal while routing rules and venue definitions differ.
FOK, IOC, AON, or regular limit?
Set the execution priorities. The router translates them into the time-in-force instruction that usually fits the trade logic.
FOK vs Regular Limit Order
A regular limit order says: buy or sell at this price or better.
A FOK limit order says: buy or sell the entire quantity at this price or better immediately, or cancel.
That extra instruction changes everything.
Suppose you place a regular buy limit for 20,000 shares at $15.00. If 8,000 shares are available, you may get 8,000 shares now and leave the rest open, depending on the market and broker settings.
With a FOK limit order, that same 8,000-share availability is not enough. The order disappears unless the full 20,000 can be executed immediately.
If you are still learning the basics of order tickets, it helps to understand the ordinary market order and limit order first. FOK is not a separate trading thesis. It is a stricter execution instruction.
When a Fill or Kill Order Can Make Sense
FOK is useful when the full size of the trade matters more than simply getting some execution.
One example is a trader who needs an exact position size for a short-term strategy. If the strategy is built around 25,000 shares, getting 4,000 shares may not be helpful. It may create a position too small to matter, but still large enough to require attention.
Another example is a hedging trade. If a hedge only works at a certain size, a partial fill can leave the portfolio exposed. In that case, no trade may be cleaner than an incomplete hedge.
A third example is a large order in a market where the trader does not want the order to sit publicly and reveal interest. A FOK instruction can test whether immediate liquidity exists without leaving the rest of the order waiting.
FOK can also fit fast strategies where the opportunity disappears quickly. If the trade only works at a specific price and size right now, the trader may prefer instant cancellation over a lingering order.
When FOK Is Probably the Wrong Tool
FOK is often too strict for everyday trading.
If you are buying a small number of shares for a long-term position, a normal limit order is usually easier to understand and manage. If you are happy to take a partial fill, IOC may be more appropriate. If you want the whole order but are willing to wait, AON may fit better, assuming your broker supports it.
FOK can also create frustration in thin markets. You may see some shares available at your price, but not enough for the full quantity. The order cancels, even though a partial trade was possible.
The mistake is using FOK because it sounds professional. It is not automatically better. It simply enforces a narrow condition.
How much execution probability are you giving up?
FOK can be the right choice when partial execution is harmful. This model shows the cost of making the order full-size, immediate, and price-limited.
Advantages of a Fill or Kill Order
The biggest advantage is clarity. You know almost immediately whether the full trade happened or did not happen.
FOK also prevents unwanted partial fills. That can be valuable when position size matters, when a hedge needs to match another exposure, or when an incomplete order would create operational clutter.
It can also reduce the risk of a forgotten remainder. With some order types, the unfilled part may stay open, depending on the settings. With FOK, there is no leftover order waiting in the market.
Finally, FOK can help a trader avoid chasing. The order either meets the condition or disappears. That can be useful for disciplined execution, as long as the trader does not immediately re-enter worse and worse FOK orders out of frustration.
Risks and Limitations
The main risk is simple: you may get no trade.
That is not a bug. That is the design. But it can be a problem if you actually needed some exposure and the market was willing to give you a partial fill.
Another limitation is availability. Not every broker offers FOK on every asset, account type, or trading interface. Some platforms support it for equities but not for certain derivatives. Some support similar behavior under different labels.
There is also a liquidity problem. FOK works best when the market is deep enough to satisfy the full order right away. In thin markets, it can cancel repeatedly.
Finally, the exact interpretation may vary by venue. That is why the Nasdaq caution about exchange-dependent handling matters. If the trade is important, do not assume every platform uses the term in exactly the same way.
How to Place a FOK Order
The exact workflow depends on the broker, but the order ticket usually follows the same logic.
First choose the instrument and side: buy or sell.
Then enter the quantity. With FOK, this number matters because the full quantity must be available immediately.
Next choose the price instruction. In many cases this will be a limit price. A buy limit sets the maximum price you are willing to pay. A sell limit sets the minimum price you are willing to accept.
Then choose time in force and select Fill or Kill, FOK, or the platform’s equivalent label.
Before submitting, check four things:
- the quantity;
- the limit price;
- whether you truly need full execution;
- whether the asset has enough liquidity for that size.
After submission, the result should be quick. Either the order fills in full, or it is canceled.
Practical Checklist Before Using FOK
Ask yourself:
- Do I need the entire quantity, or would a partial fill be acceptable?
- Is the market liquid enough for this order size?
- Is my limit price realistic?
- Am I using FOK because the strategy needs it, or because it sounds safer?
- Would IOC be better if I can accept partial execution?
- Would AON be better if I need full size but can wait?
- Does my broker support FOK for this instrument?
- Do I understand how the platform defines “immediate”?
If you cannot answer those questions, FOK is probably not the order type to start with.
Common Beginner Mistakes
The first mistake is thinking FOK guarantees execution. It does not. It guarantees all-or-nothing handling if the order can be executed under the conditions you set.
The second mistake is using FOK in markets without enough liquidity. The order may cancel again and again because the full quantity is not available at the limit price.
The third mistake is confusing FOK with IOC. IOC can leave you with a partial fill. FOK usually avoids that.
The fourth mistake is confusing FOK with AON. AON can wait. FOK does not.
The fifth mistake is ignoring price. A FOK order with an unrealistic limit is just a fast cancellation request.
Bottom Line
A fill or kill order is useful when “some of the trade” is not good enough.
It tells the broker or trading venue to execute the full order immediately at the required terms, or cancel it. That makes it clean, strict, and sometimes very useful. It also makes it unforgiving.
For most beginners, FOK is not an everyday order type. Learn market orders, limit orders, stop orders, and basic time-in-force settings first. Then use FOK when the logic of the trade genuinely requires full size right away.
