What is post-only order?

A limit order instruction that cancels the order rather than filling it if it would trade immediately against the book.

Traders use it to guarantee the maker fee or rebate instead of being charged the taker rate. If the market moves through your price first, nothing happens, which is the trade-off.

The flag sits on the order ticket of most spot and derivatives venues, sometimes labeled "post only" and sometimes "maker only". It exists because a limit order does not by itself guarantee you pay the maker rate. Price a limit buy above the best offer and it executes instantly as a taker, at the taker fee. Post-only removes that possibility by refusing the order instead.

How it works

An exchange charges two fee rates. A resting order that waits on the book adds liquidity and pays the maker rate. An order that executes against something already resting removes liquidity and pays the taker rate, which is higher on almost every venue. See maker taker fee.

When you submit a post-only order the matching engine checks it against the opposite side of the book before accepting it.

1. If the order would rest without matching, it is placed normally and will pay the maker rate when someone trades against it.
2. If it would cross and execute immediately, the venue rejects or cancels it outright. Some venues instead reprice it to sit just inside the spread, so read the venue's own order-type documentation to know which behavior applies.

The rejection is silent in the sense that you get no fill and no fee. Your order simply is not there, which matters if you assumed it was working.

Example

Illustrative numbers. The book shows a best bid of $100.00 and a best ask of $100.10. You want to buy and the venue charges 0.40% taker and 0.20% maker.

Submit an ordinary limit buy at $100.10 for $5,000 and it matches the resting ask immediately. You are the taker and pay $20.00 in fees.

Submit the same order post-only and it is cancelled, because it would have crossed. Resubmit at $100.00, joining the bid, and it rests. If a seller comes to you, you buy at $100.00 and pay the maker rate of $10.00. You saved $10.00 on fees and $0.10 per unit on price, and you accepted the risk that no seller arrives and the market leaves without you.

Why it matters when you buy

For a single purchase the saving is small and the risk of not filling is real, so post-only suits people who buy repeatedly and can wait. The size of the prize depends entirely on the gap between the two rates at your venue, which the fee comparison sets out per exchange. Whether resting at the bid is realistic depends on the spread and depth, which the liquidity pages measure.

Related terms

maker taker fee — why resting orders pay less; limit order — the order type this modifies; maker rebate — when the venue pays you; time in force — how long an order stays alive; order book — what the order rests on; taker — the side that pays more.

Questions

Does a limit order always pay the maker fee?

No. A limit order priced so that it crosses the spread executes immediately and is charged as a taker. Post-only is what turns "probably maker" into "maker or nothing".

What happens if my post-only order would cross?

Depending on the venue it is rejected, cancelled, or repriced to rest just inside the spread. Check the exchange's order documentation, because assuming a rejection when the venue reprices leaves you holding an order you did not intend.

Is it worth using for a one-off purchase?

Usually not. The fee difference on a small buy is a few dollars, while missing the fill entirely changes what you own. It earns its keep for people placing many orders where the rate difference compounds.