What is a Fill-or-Kill Order? Definition, Formula, and Example
A fill-or-kill (FOK) order instructs a broker to execute the full order size immediately at the specified price or better, or cancel it entirely with no partial fill.
What Is a Fill-or-Kill Order?
A fill-or-kill (FOK) order is an instruction that gives the market exactly one chance to fill the entire order, right now, at the limit price or better — or the order is canceled outright with zero shares executed. There's no resting on the book, no partial execution, and no second attempt. FOK combines two separate order conditions that can also be used independently: "all-or-none" (the full quantity or nothing) and "immediate-or-cancel" (execute now or cancel, no resting). An FOK order requires both simultaneously.
How It Works
When an FOK order hits the market, the exchange or ECN checks whether the full requested size is available at the limit price or better across the visible (and in some venues, hidden) order book at that instant. If yes, the entire order fills in one transaction. If the available size at that price is even one share short, the entire order is rejected and canceled — nothing fills, not even the portion that could have matched. This is what distinguishes FOK from an immediate-or-cancel (IOC) order, which allows a partial fill of whatever quantity is available and only cancels the unfilled remainder.
Worked Example
Say a trader wants to buy 5,000 shares of a thinly traded micro-cap at $2.05 or better, submitted as FOK. At the instant the order reaches the book, the ask side shows 3,200 shares available at $2.05 and better. Because 3,200 is less than the requested 5,000, the entire order is killed — the trader gets 0 shares, not 3,200. Had the same order been submitted as IOC instead, the trader would receive 3,200 shares immediately and the remaining 1,800 would cancel. Had it been submitted as a plain limit order, the 3,200 would fill immediately and the remaining 1,800 would rest on the book waiting for more sellers at $2.05.
When Traders Use Fill-or-Kill Orders
FOK orders matter most when a partial fill is worse than no fill at all. Multi-leg options strategies — an iron condor or a spread where each leg's risk depends on the others being filled together — use FOK-style logic to avoid "leg risk," the danger of getting one leg filled and being stuck holding a naked, unintended position if the other legs don't execute. Institutional block traders use FOK when a small partial fill would signal their intent to the market (revealing a large buyer is present) without accomplishing the actual objective of establishing the full position. Algorithmic execution systems also use FOK as a building block within smart order routing logic when testing for available liquidity without leaving a resting order that could be picked off.
Limitations and Common Misconceptions
The strict all-or-nothing, right-now-or-never requirement makes FOK orders fail constantly on illiquid names — if the full size isn't sitting on the book at that exact instant, the order dies, regardless of how close it came. This makes FOK a poor choice for patiently building or exiting a position over time; a good-til-canceled limit order or working the order in smaller clips will get filled far more often, just not instantly or all at once. FOK support is also inconsistent across retail brokerage platforms — many only expose it through API/algo order routing rather than the standard order ticket, since the order type is used far more by institutional and program trading desks than by discretionary retail traders. Traders sometimes confuse FOK with AON (all-or-none without the immediacy requirement) or IOC (immediacy without the full-size requirement) — mixing these up produces very different fill behavior than intended.