A fill or kill order is an instruction to your broker to execute a trade immediately and in full at your specified price, or cancel the entire order on the spot. There is no middle ground. You either get every share you asked for in one transaction, or you get nothing. Traders use this order type to avoid the cost surprises that come with partial fills on large positions, where buying shares in scattered chunks can mean paying several different prices for what was supposed to be a single trade.
How a Fill or Kill Order Works
Two constraints fire at the same moment the order reaches the exchange. The first is all-or-none: if you order 5,000 shares, the matching engine will not fill 4,800 and leave you hunting for the rest. The second is immediacy: the engine evaluates your order once, right when it arrives, and if it cannot match the full quantity at your price or better, the order is dead on arrival.
That single-evaluation design is what makes the order type distinctive. It never sits in the order book waiting for more shares to appear. It never lingers through a price swing that could leave you with a fill at a worse level than you planned. You get a binary outcome every time: filled completely, or killed completely, usually within seconds.
Fill or Kill vs. IOC vs. All-or-None
Traders often confuse fill or kill with two similar order types, and the differences matter.
An immediate-or-cancel (IOC) order also demands instant execution, but it accepts partial fills. If you want 5,000 shares and only 3,200 are available, an IOC fills those 3,200 and cancels the rest. A fill or kill order in the same situation fills nothing.
An all-or-none (AON) order also refuses partial fills, but it stays active on the book until it can be filled completely or you cancel it. It can sit there for hours or days. A fill or kill order that cannot complete immediately is killed within milliseconds.
Think of fill or kill as both constraints combined: the complete-quantity requirement of AON plus the instant-execution demand of IOC.1American Association of Individual Investors. What Is a Fill-or-Kill Order? That double constraint makes it the strictest standard order type available to most investors.
When a Fill or Kill Order Makes Sense
Fill or kill orders show up most often in institutional trading, where position sizes run into hundreds of thousands of shares. When a fund manager needs to buy 500,000 shares at $100, getting filled on only 300,000 creates a real problem. The remaining 200,000 shares still have to be purchased, likely at a higher price now that the market has moved. On a half-million-share order, even a $0.50 per-share difference adds up to $250,000 in extra cost.
Individual traders use them less often, but they fit a few situations. If you are building a hedged position where the math only works at a specific share count, a partial fill leaves you exposed. If you are trading a thinly traded stock and want to avoid tipping the market by leaving a visible order on the book, a fill or kill either gets the job done instantly or disappears without a trace.
The decision rule is simple. Ask whether you would rather have some shares at a good price or no shares at all. If partial fills are acceptable, an IOC order gives you better odds of getting at least something done.
Placing the Order
On most brokerage platforms, placing one works the same as placing any other order, with one extra selection. You enter the ticker, choose buy or sell, and specify the quantity. In the time-in-force dropdown, sometimes labeled “duration” or “order type,” you select Fill or Kill. You then set a limit price capping what you will pay per share on a buy or the minimum you will accept on a sell. The order type is typically paired with a limit price, though some platforms allow it with market orders as well.2U.S. Securities and Exchange Commission. Fill-Or-Kill Order
Review the confirmation screen before transmitting.3Fidelity. Brokerage Handbook: Confirming, Reviewing, and Canceling Orders Once confirmed, the order goes to the exchange and is evaluated almost instantly. You will get a notification telling you whether it filled or was killed.
You also need sufficient buying power before the order will transmit. In a cash account, the full purchase amount must be available. In a margin account, Regulation T requires at least 50 percent of the purchase price as initial margin.4eCFR. 12 CFR 220.12 – Supplement: Margin Requirements Some brokers set their own requirements above that floor, so check if you are unsure.
When the Order Gets Killed
The kill side triggers whenever the matching engine cannot find enough shares at your price in that one evaluation. The usual cause is simple: not enough shares are offered at your limit. If you want 10,000 shares at $42 and only 9,500 are available, the order dies. No partial fill, no second attempt.
This happens most in low-liquidity stocks. Small-cap names or stocks with wide bid-ask spreads often lack the depth to absorb a large all-or-nothing order in one shot. Even in liquid markets, a fast price move during transmission can push available shares past your limit before the engine evaluates.
A killed order has no market impact. It never entered the order book, so other traders never saw it. It does not count as an executed trade for purposes like pattern day trader rules or wash sale calculations because no transaction occurred.5FINRA. Day Trading Your buying power is released immediately, and you can place another order right away at a different price or size.
Risks and Drawbacks
The biggest practical risk is simply not getting filled. Because the order evaluates once and accepts nothing less than the full quantity, your odds of execution drop sharply compared to a standard limit order or even an IOC. In thin markets, these orders get killed frequently. Repeatedly placing fill or kill orders that die while the price drifts away from you can cost more than accepting a partial fill would have.
The order type also misses liquidity that is not visible at the moment of evaluation. Shares sitting in non-displayed order types or dark pools might have filled a standard order that stayed on the book for a few seconds, but a fill or kill never gets that chance. It checks displayed liquidity, finds the quantity short, and dies.
There is a behavioral trap too. Traders who get a string of killed orders sometimes respond by widening the limit price to improve fill odds, which defeats the purpose of price control. If you find yourself constantly nudging the price up just to execute, an IOC order will at least capture the available shares while canceling the rest.
Where Fill or Kill Isn’t Available
Fill or kill orders are generally unavailable during pre-market and after-hours trading. Most brokerages restrict extended-hours trading to basic limit orders and reject fill or kill, all-or-none, and similar conditional types outside regular market hours.6E*TRADE from Morgan Stanley. Extended Hours Trading Agreement Extended sessions already have lower liquidity, and an all-or-nothing order would almost never fill in those conditions.
Dark pools are more nuanced. Some exchanges and alternative trading systems let fill or kill orders probe non-displayed liquidity, and smart order routers may check dark pools for price-improving fills before routing to the visible book. But orders that only check displayed size can get killed even when hidden liquidity existed that would have completed the fill. If dark pool access matters to your strategy, check how your broker routes fill or kill orders.
Cancellations and Spoofing
Routine kills are legal and unremarkable. Order cancellations are a normal part of market structure, and the SEC does not treat a killed fill or kill order any differently from any other cancelled order.
Regulators do watch for spoofing: placing orders you never intend to execute in order to create a false impression of supply or demand. Repeatedly placing and cancelling large orders to manipulate prices can bring SEC enforcement.7U.S. Securities and Exchange Commission. SEC Charges Trader for Scheme to Manipulate Exchange’s Closing That rule targets deliberate manipulation, not traders whose fill or kill orders happen to get killed because the market lacks liquidity. Placed in good faith, a cancellation is just the order doing exactly what you told it to do.