What Is a Fill-or-Kill Order?
Most order types give the market some room to work with — a little time, a little flexibility on price. A fill-or-kill order gives it neither.
The short answer
A fill-or-kill order instructs the market to execute the entire requested quantity immediately, in one transaction, or cancel the whole order on the spot. There’s no partial fill and no resting on the order book waiting for the rest to come through. Because typical retail order sizes are small compared to the depth available in most markets, this all-or-nothing, right-now combination rarely comes up outside of larger, more specialized trades.
Two conditions, both required
The name describes exactly what has to happen: fill, or kill. “Fill” means the full share or contract quantity specified has to trade, not some fraction of it. “Kill” means if that full quantity can’t be matched the instant the order reaches the market, the entire order is withdrawn rather than left open to try again later. Both conditions apply together — an order that fills 90 percent immediately still gets killed, because partial completion doesn’t satisfy the terms.
How it differs from similar order types
It’s easy to confuse a fill-or-kill order with its close relatives, since all of them deal with speed and completeness. An all-or-none order also requires the full quantity to fill, but it’s willing to wait — it can sit unfilled for a while rather than canceling instantly. An immediate-or-cancel order takes the opposite approach on completeness: it accepts a partial fill right away and simply cancels whatever portion doesn’t execute. A fill-or-kill order is the strictest of the group, combining the urgency of one with the completeness requirement of the other.
Why these are uncommon for everyday trades
For someone buying a modest number of shares of a widely traded security, a regular market or limit order is almost always enough, since the available size at or near the current price typically covers the whole order without any special instruction. Fill-or-kill terms tend to matter more for larger orders, less liquid securities, or situations where partial execution would create an awkward position — for example, a strategy that only makes sense if every leg trades together. In markets with a wide bid-ask spread or thin trading volume, an all-or-nothing requirement is also more likely to result in the order being killed, simply because there isn’t enough available supply or demand at that instant to satisfy it.
What happens when it can’t be filled
If the full quantity isn’t available at an acceptable price the moment the order hits the market, nothing happens — no shares change hands, no partial position is created, and the order simply disappears rather than lingering. This can feel abrupt compared to other order types, but it’s the entire point: the instruction exists specifically to avoid ending up with an incomplete position. Anyone using this order type is trading certainty of size for a lower chance of any execution at all.
The takeaway
A fill-or-kill order is a precise tool for a fairly narrow situation — one where an incomplete trade would be worse than no trade at all. Its strict, immediate nature makes it uncommon in ordinary day-to-day investing, but understanding the mechanics helps make sense of order types more broadly, and of why exchanges offer so many variations on a theme that’s really about balancing speed, price, and completeness against each other.