Post-Only Limit Order: What It Is and How It Works

A post-only order is a limit order that will be added to the order book only if it rests as maker liquidity; if it would execute immediately against resting orders, the matching engine cancels it or, on some venues, automatically reprices it one tick away to avoid taking. Traders use a post-only order to guarantee maker status, avoid taker fees, and, where applicable, earn maker rebates.

How a Post-Only Order Works

Exchanges distinguish between makers, who add liquidity, and takers, who remove it. Maker vs taker fees reflect that split: makers typically pay lower fees or receive rebates, while takers pay higher fees. This order type enforces the maker role by combining a limit price with an instruction that blocks immediate execution. If the limit price would trade right away against the opposite side, the order is rejected, canceled, or repriced depending on venue logic.

The interaction between limit price and the post-only instruction is straightforward: a buy must be priced below the best ask to rest; a sell must be above the best bid. For example, with a 99 bid and 100 ask, a post-only buy at 99.80 improves the bid and joins the book as maker, while a post-only buy at 100.00 would be canceled or slid down to avoid taking. Typical users include market makers, arbitrageurs seeking fee efficiency, and active traders who care about queue position and slippage control.

Compared with a standard limit, this instruction prevents you from accidentally arriving as a taker; compared with a market or stop-limit order, it prioritizes liquidity role control over immediate execution.

Key Features and Venue Handling

Many venues label the instruction maker-only or add-liquidity-only (ALO); all refer to the same behavior. Posting early protects queue priority under price-time matching, because displayed orders that arrive first at a given price are filled first. Some venues require display to qualify for maker rebates, so hidden or midpoint-pegged variations may not earn maker economics even if they add liquidity.

When a post-only order would lock or cross the book, handling differs by market. Some venues cancel the order outright; others automatically slide the price one tick to keep it on the maker side; a few adjust it further if tick-size or anti-cross rules require. These differences also show up in partial scenarios, such as self-trade prevention or price bands, where the unfillable portion may be canceled while the rest is repriced to post.

Interactions with time-in-force matter. GTC is the common pairing, because the order’s purpose is to rest and provide liquidity. IOC and FOK compatibility varies: some venues reject post-only orders with an immediate-or-cancel or fill-or-kill instruction because they cannot both rest and demand immediacy; others accept them but will cancel entirely if they cannot post as maker on arrival. Implementations differ across venues due to matching-engine design, fee schedules, tick-size and lock/cross rules, and how maker vs taker fees are applied.

How to place a post-only order is typically a short set of steps: select a limit order, enable the post-only toggle (often labeled maker-only or ALO), set your limit price and a time-in-force such as GTC, then submit. For example, with a 99/100 book, a post-only sell at 100.40 will rest and display; a post-only sell at 99.90 would be canceled or slid up to remain on the maker side.

Post-Only Versus Other Order Instructions

• Post-only vs standard limit: A standard limit can arrive as a taker if its price crosses the opposite side, while this instruction guarantees you will not remove liquidity by canceling or repricing on arrival.
• Post-only vs market order: A market order prioritizes immediate execution at the best available prices, accepting taker fees and potential slippage, while this order type prioritizes maker status and fee control over immediacy.
• Post-only vs IOC/FOK: IOC and FOK are time-in-force constraints about how long an order may live, whereas a post-only instruction governs liquidity role; some venues allow both, but post-only will still cancel if it cannot rest as maker.
• Post-only vs reduce-only: Reduce-only ensures an order can only decrease an existing position size; it does not control maker/taker status, while this instruction explicitly controls the liquidity role.
• Terminology: Maker-only and add-liquidity-only (ALO) are equivalent terms for a post-only order on many venues.

Practical Considerations and When to Use It

This order type helps when you aim to avoid taker fees, capture maker rebates, or protect queue priority at a target price level. It is useful around fair value for market makers quoting two-sided markets, for arbitrageurs managing edge-based entries, and for active traders who prefer controlled, displayed liquidity over immediate fills. It is common on centralized order books in spot markets, derivatives venues, and some equities platforms.

Be aware of limitations and edge cases. Hidden, midpoint, or pegged variants may not qualify for maker rebates even if they add liquidity. Venue logic can produce partial rejections, sliding, or price adjustments, and fee schedules may change maker vs taker economics or introduce thresholds that override expectations. Always review the venue’s rulebook, self-trade prevention settings, and fee schedule to understand whether it will cancel, reprice, or display, and whether it earns maker economics at your chosen price.