Fill-or-Kill (FOK)
Fill-or-Kill (FOK) is an instruction attached to an order that tells the market: fill this whole order right now, or don't fill it at all. There is no in-between. It's a "Time in Force" setting, meaning it controls how long an order is allowed to stay active before it's either executed or withdrawn.
When you place an FOK order, the exchange or broker checks immediately whether there is enough buying or selling interest at your price to cover the entire quantity you requested. If yes, the whole order executes instantly. If no — even if only part of it could be filled — the entire order is cancelled on the spot. Nothing gets left resting on the order book, and you never end up holding a fraction of the position you asked for.
The nuance that trips people up is the difference between FOK and its close cousin, Immediate-or-Cancel (IOC). Both demand instant action and both cancel any unfilled portion, but IOC will happily take a partial fill and cancel only the leftover. FOK refuses partial fills outright — it's all-or-nothing. So an order for 1,000 shares that can only find 600 shares of matching interest would leave you owning 600 shares under IOC, but would leave you with nothing at all under FOK.
Because FOK demands an immediate, complete match, it's most useful in less liquid instruments or for larger orders where getting "some" of the position isn't useful — for example, if the trade only makes sense as a complete block.
Day traders use FOK when a partial fill would be worse than no fill at all — such as executing one leg of a multi-part or hedged trade, where being half-filled leaves unwanted, unhedged exposure.
A trader wants to buy 5,000 shares of a thinly traded stock at $12.50 using an FOK order. The order book only shows 3,200 shares available at that price. Because the full 5,000 can't be matched instantly, the entire order is cancelled and the trader ends up with zero shares — rather than the 3,200 they'd have gotten with an IOC order.
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