Post-Only Order
In simple terms
A post-only order is a way to place a buy or sell order that adds new listings to the marketplace instead of immediately buying from existing ones. It's like adding your item to a store shelf rather than grabbing something already there—and exchanges often reward you with lower fees for doing this.
Definition
Ensures the order adds liquidity to the book rather than taking it, often with lower fees.
In depth
A post-only order is restricted to only adding liquidity to the order book by becoming a maker, never executing as a taker against existing orders. When submitted, the exchange's matching engine checks if the order would immediately cross the spread and execute against counterparties; if so, the entire order is rejected rather than partially filled. This mechanism incentivizes market makers by offering reduced or rebated fees, as their orders tighten spreads and improve price discovery for other traders.
How does Post-Only Order work?
You attach a post-only flag to a limit order. Before accepting it, the exchange checks whether the order would immediately match against something already resting in the book. If it would — meaning it would take liquidity — the exchange rejects or cancels it rather than filling. If it would not match, the order posts to the book and waits, adding liquidity. The purpose is fee control: makers who add liquidity are charged less than takers who remove it, and on some venues makers receive a rebate.
An example
Illustrative figures only. The best ask is $30.00. Someone submits a post-only buy limit at $30.05, which would cross and execute immediately, so the exchange rejects it. They resubmit at $29.95, below the ask. It posts to the book and waits. On a venue charging 0.25 percent maker and 0.50 percent taker, a $1,000 fill as maker costs $2.50 rather than $5.00.
Figures are illustrative only.
What beginners get wrong
- Rejection is the intended behaviour, not an error, and traders who resubmit repeatedly during a fast move often never get filled at all.
- Waiting on the passive side to save a fee can cost far more than the fee if price moves away before the order fills.
- Post-only cannot be applied to market orders, since a market order takes liquidity by definition.
- Some venues cancel silently rather than returning an error, so an automated strategy can believe it is in a position when it is not.
Related terms
Part of
What do the different crypto order types do? — the subject page for order types, with all 9 of its definitions in one place.
Educational only — not financial advice.