How the Polymarket Order Book Works

The direct answer

Think of an order book as a queue of conditional offers. Bids are offers to buy an outcome at stated prices; asks are offers to sell it at stated prices. A compatible buy and sell order can match. Until then, the order remains in the book.

Polymarket documents a central limit order book (CLOB): participants provide the prices, rather than Polymarket setting one fixed price. That means the most useful number depends on the action. A prospective buyer should inspect current asks; a prospective seller should inspect current bids.

Bids, asks and the spread

A narrower spread means the closest buyer and seller are closer in price. That can be a more convenient trading environment than a wide spread, but it says nothing by itself about whether the available size is enough for a particular order.

A small order-book example

Imagine a fictional Yes outcome with these resting offers. The market and values are invented solely to explain the mechanics.

Here, the spread is 4¢: 55¢ minus 51¢. The midpoint is 53¢. A buyer wanting 20 shares could potentially match at 55¢ if those 40 shares were still available. A buyer wanting 90 shares cannot assume all 90 would cost 55¢: after the first 40, the next visible ask is 58¢, and the book may change before submission.

Likewise, a seller offering 50 shares cannot assume all 50 will receive 51¢. The first 30 shares may have that available bid, while further shares would need another buyer or a lower compatible price. This is the basic mechanism behind execution differences and slippage. A larger order may need to reach more distant price levels if the closest visible offers are too small.

What depth changes

Depth matters when the order size is large relative to the offers near the best price. The first row alone answers only a small-order question. Reading several rows gives a better sense of the prices an order might reach if it consumes the closest available offers.

This does not make the book a promise. Orders can be cancelled, new orders can arrive, and another participant can match first. Polymarket’s market-data documentation treats the order book as a state that can change between reads; for a real decision, check the current book and then check the result after submitting an order.

Displayed price versus executable price

Polymarket documents the midpoint as the average of the best bid and best ask. In the fictional 51¢ bid and 55¢ ask example, that midpoint is 53¢. It describes the middle of the current gap, not an offer either side has made.

Polymarket also says a last traded price can be displayed when the spread is wider than 10¢. A last trade records something that matched earlier; it is not evidence that the same counterparty or price remains available now. How Polymarket Prices, Odds and Probabilities Work explains the distinction in the context of market-implied probability.

The common order-book mistake

The common mistake is to see one visible cents price and assume it applies to any size in either direction. It does not. A buyer and seller use opposite sides of the book, and the available amount at the first level can be smaller than the intended order.

Before relying on the book, confirm the exact market and outcome, distinguish bid from ask, inspect size beyond the first row if size matters, and separate a submitted order from a completed fill. For the execution choice itself, read Market Orders vs Limit Orders on Polymarket.

Bottom line

The Polymarket order book shows current buyer and seller offers for an outcome. Best bid is the strongest available buyer; best ask is the cheapest available seller; the spread is the difference; and depth shows how much is visible at each level. Use the side and size relevant to your action, not only the displayed midpoint, then verify what actually matched.