Post Only, Reduce Only and IOC/FOK: Understanding Order Options

You understand price and quantity, but what are the checkboxes below them? Post Only, Reduce Only and GTC/IOC/FOK change an order's behavior. Here is what each does, illustrated with numbers.

An order specifies three things

Placing an order actually specifies three things: at what price, for what quantity, and how it should execute. The first two are intuitive; the third often confuses beginners.

The checkboxes and dropdowns near the bottom of the order form control that “how.” Names differ between exchanges, but they fall into three groups.

If market and limit orders are still unclear, start with the complete guide to order types. This article covers the options added on top of those basics.

① Post Only: Maker execution only

Post Only means “if this order would execute immediately, cancel it instead.” It declares that the order may rest in the book but may not consume someone else's quote.

The reason is fees. Exchanges charge different rates to orders that post liquidity, makers, and orders that take liquidity, takers. Maker fees are lower.

Although $6 may seem small, 10 daily trades make it $60 a day and $1,800 a month. With rapid turnover such as scalping, fees can completely change the performance record. Try the fee calculator to see their effect on P&L.

What happens when Post Only is enabled: If your limit crosses the current book, such as a buy above the best ask, the order is immediately canceled rather than accepted. This commonly explains why an order disappears after submission. It is intended behavior, not an error.

Enable it when you are not in a hurry and can wait at your chosen price. Disable it when immediate execution is required, especially for a stop-loss. If that order is canceled, losses can keep growing.

② Reduce Only: Preventing a position increase

Reduce Only means “use this order only to reduce a position; do not execute anything that would increase exposure.”

A futures sell order can mean either closing a long or opening a new short. The exchange sees a quantity and does not automatically distinguish your intent.

This matters because a scheduled stop can remain unchanged after a partial close. Suppose you hold long 1 with a stop sell for 1, then manually take profit on half. When the original stop for 1 triggers, it can close 0.5 and open a new short of 0.5. That is how you can wake up to an unexpected opposite position.

The rule is simple: Enable Reduce Only for stop-loss, take-profit and other closing orders; disable it only for entries. The same principle applies to automatic exits such as OCO orders and trailing stops.

Hedge mode behaves somewhat differently because long and short positions are managed separately. Orders must identify which position leg they refer to; omitting this can cause a closing order to be rejected or sent in an unintended direction.

③ Time in Force: An order's lifetime

The third option is the order's expiry behavior, usually selected from GTC, IOC or FOK in a dropdown.

GTC, Good Till Cancel: The order stays active until canceled. This is the default on most exchanges and the usual behavior of a limit order. It can remain in the book for days, so periodically clear out forgotten orders.

IOC, Immediate Or Cancel: Fill whatever can execute immediately, then discard the remainder. Partial fills are allowed.

FOK, Fill Or Kill: Unless the entire quantity can execute immediately, fill nothing and cancel everything. It is all or nothing.

The difference matters most when liquidity is thin. A large market order in a quiet altcoin walks up the book and worsens the average fill price: slippage. An IOC order with a price cap means “take only up to this price and abandon the rest,” limiting that slippage.

FOK suits cases where a partial fill would be problematic, such as two instruments that must be held in an exact ratio. Filling only half of one side disrupts the calculation. The tradeoff is the highest chance of no execution.

Why partial fills deserve attention

Partial fills under IOC or GTC can create issues involving fees and minimum order units.

A remainder below the minimum order quantity can leave you with a position you cannot close. Tick size and minimum order quantity explains why these thresholds exist. A practical response described here is to buy a little more to exceed the minimum, then close it all together.

Combining options for different situations

A patient entry: Limit + Post Only + GTC. Save fees and wait at your chosen price. If the market never reaches it, do not buy.

A stop-loss: Market, or a generously priced limit, + Reduce Only. Always disable Post Only. Executing the exit matters more than saving a few dollars.

A scheduled take-profit: Limit + Reduce Only + GTC. Adding Post Only may produce a maker fee, but remember that a fleeting price move can leave the order canceled instead of filled.

A large order in a thin market: Split limit + IOC orders into parts, rather than sweeping through the book in one order.

Closing the entire position: Enable Reduce Only and check the remaining quantity precisely. For automated trading, the article suggests hard-coding this flag for closing orders.

Four common mistakes

1. Enabling Post Only on a stop: Fast markets cancel the order while losses keep growing. This is the most expensive mistake.
2. Omitting Reduce Only on a close: An opposite position can open unnoticed, potentially incurring funding fees in the opposite direction.
3. Forgetting a GTC order: A limit placed days earlier executes at an unintended moment. Review open orders at least once a day.
4. Using FOK as the default: Repeated fill failures can look like an exchange malfunction when the option itself is the reason.

Three key points

① Post Only means maker-only execution. It saves fees but can cancel an urgent order.
② Reduce Only prevents exposure from increasing. Enable it on all closing orders.
③ GTC keeps waiting, IOC fills what is available, and FOK is all or nothing.

Caution

The fee rates and quantities are illustrative examples. Actual values vary by exchange, tier, instrument and time. Option names and behavior also differ, so check the order rules of the exchange you use. Your investment decisions and their consequences are your responsibility.