Fill or kill (FOK) is an order that must execute fully and immediately at the current price, or be cancelled with no fill. It's an all-or-nothing mechanism that ensures you get the complete order or nothing.
Fill or kill (FOK) is an order that must execute fully and immediately at the current price, or be cancelled with no fill. It's an all-or-nothing mechanism that ensures you get the complete order or nothing.
Fill or kill, commonly abbreviated as FOK, is one of the most straightforward order types in trading. At its core, an FOK order says: execute this entire order right now at the best available price, or cancel it completely. There is no middle ground. Either you get all of your shares at the specified price or better, or the order disappears and you get zero shares. This all-or-nothing structure makes FOK orders useful for traders who need certainty about whether their trade happened and how much capital they deployed. It removes ambiguity: you will either own the full position you wanted or own nothing at all.
The term originates from traditional financial markets, where traders have long needed to distinguish between orders that are flexible (willing to wait or accept partial fills) and orders that are rigid (all or nothing, right now). Prediction markets like Polymarket inherited this language and logic because both traditional markets and prediction markets share the same core problem: liquidity. When you submit a large order, there may not be enough shares available at your target price to fill the entire amount. An FOK order acknowledges this friction upfront. It says, "I would rather walk away than get a partial fill." In prediction markets, where events resolve months out and conviction levels vary, this clarity matters. A trader might FOK a large bet because they believe the window for good pricing is closing, or because their trading plan depends on deploying a specific dollar amount into a single market all at once. An FOK order is a way of saying your thesis is indivisible: you want this bet at this price and size, or you want nothing.
On Polymarket, traders encounter FOK orders primarily through the orderbook and limit orders interface. When you place a limit order to buy or sell a certain quantity of shares at a specific price, you typically have the option to specify whether that order is a standard limit order (willing to wait, willing to accept a partial fill if it arrives gradually) or a fill-or-kill order (execute now at this price or cancel). Some traders use FOK orders defensively: they want to buy a large block of Yes shares in a politics market, but only if they can do it all at once at the current mid-price. If the orderbook doesn't have enough depth at that price to fill the entire order, the FOK order cancels and they can reassess their strategy. Other traders use FOK orders aggressively: they spot an inefficiency in the market, place an FOK order, and if it doesn't fill immediately, they know the opportunity has passed and they move on to the next trade. This quick on-off dynamic helps traders avoid the regret of sitting with half a position that never completed.
One common misconception is that an FOK order guarantees a lower price than a patient limit order. It does not. An FOK order competes for execution at the moment it lands, against whatever liquidity is available right then. If the orderbook is thin at your target price, an FOK order is more likely to fail to fill than a limit order that waits. Another frequent confusion arises from thinking of FOK as a speed play. It is not about being the fastest; it is about being all or nothing. A market maker placing a bulk order for hedging purposes might use FOK to ensure they either capture a large defensive position or skip the trade entirely, without accidentally ending up with an awkward partial position. Similarly, a speculator in a weather market might FOK a large bet right before a news drop, because they believe this is the only moment when pricing is favorable, and a partial fill is worse than no fill.
Related to FOK are several other order types that offer different flexibility trade-offs. An immediate-or-cancel (IOC) order is similar but more permissive: it executes whatever portion it can right now and cancels the remainder, allowing partial fills. A good-for-day (GFD) order will wait for the entire order to fill during a full trading session, resting on the orderbook. A post-only order is placed with the goal of adding liquidity, not taking it. Understanding the spectrum of these order types helps traders choose the right tool for their situation. In prediction markets, where you are betting on real-world outcomes and liquidity can be spotty, choosing between FOK (all or nothing, now), IOC (what fills now, plus cancellation), and a patient limit order (wait all day if needed) is a key tactical decision that affects both your fill rate and your ultimate position size.
Suppose you believe the probability of the Federal Reserve cutting rates next month is much higher than the current market odds of 35%, so you place a fill-or-kill order to buy 1,000 shares of Yes at 38 cents. If the orderbook has at least 1,000 shares available at 38 cents or lower, your entire order executes immediately. If fewer than 1,000 shares exist at that price, the FOK order cancels and you own zero shares, allowing you to instead wait for a better entry or deploy your capital elsewhere.