What is a Fill or Kill Order?

A fill or kill order (FOK) is a trade instruction that must be executed immediately and completely, or not at all. If the market cannot fill the entire order right away, the whole order is cancelled, with no partial fills, as defined in the SEC's investor glossary.

If you are learning order types, this page gives you the practical picture: how a fill or kill order works, when traders reach for it, and how it compares with similar instructions. As with everything in investing education, this is informational, not financial advice.

Who this is for and how it works

This page is for beginners who already know what a buy or sell order is and want to understand execution qualifiers, plus anyone who trades position sizes large enough that partial fills hurt.

When you send a FOK order, your broker checks whether the full quantity can be executed immediately, usually at your specified limit price or better. Two outcomes are possible:

• Fill: the entire order executes at once, at your price or better.
• Kill: any part of the order cannot be filled immediately, so the exchange cancels all of it.

There is no middle ground and no waiting around on the order book. That all-or-nothing, right-now behavior is the entire point. Compare that with a plain limit order, which can sit open for hours and fill in pieces: 200 shares now, 300 later, and so on.

Key benefits

The advantages are focused:

• No partial positions. You either get the full size or stay flat, which keeps position management clean.
• No stale orders. Nothing lingers on the book to execute later at a moment you no longer want.
• Price control. Combined with a limit price, you cap what you pay or floor what you receive.
• Clean strategy execution. Trades that only work as a complete block, such as hedges, arrive whole or not at all.

Proof, examples, and objections

Why would anyone demand this strictness? Partial fills can be a real problem for large trades. Schwab's overview of order types and conditions notes that an all-quantity qualifier can prevent, say, a 100-share execution on a 5,000-share order - but the same strictness means the order may not execute at all. Getting a tiny slice of a big position leaves a trader exposed: they hold an awkward partial position and may move the price against themselves while finishing the job.

A concrete example: suppose you want 2,000 shares of a mid-cap stock at $25.00, and only 800 shares are available at that price right now. A limit order buys 800 and waits. An FOK order cancels entirely, and you keep your cash.

The honest objections mirror the benefits:

• Low fill probability. Demanding full size immediately means many FOK orders simply die, especially in thinly traded stocks.
• Missed moves. While your order gets killed repeatedly, the market can run away from your price.
• Limited retail need. For small orders in liquid stocks, partial fills are rare anyway, so the qualifier adds little.
• Availability varies. Not every broker or market supports FOK on every product, and some venues prohibit these qualifiers on certain order flows, so check your broker's rules first.