Polymarket order-book imbalance compares the displayed resting bid size with the displayed resting ask size for one outcome, within a clearly defined depth window. It describes the book at a particular moment—not a forecast, proof of trader intent, or guarantee that displayed liquidity will still be available when you trade.
What Polymarket’s order book shows
Each Polymarket outcome has an outcome token traded on the central limit order book (CLOB). Its price reflects what traders are willing to pay; the book lists resting bids and asks by price level. Each level has a price and a size. The official Prices and Order Books documentation describes the book response and its fields, including the outcome asset ID, condition ID, timestamp, tick size, minimum order size, and book hash.
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For a given outcome, a bid is a resting offer to buy and an ask is a resting offer to sell. The best bid is the highest bid; the best ask is the lowest ask. The gap between them is the spread. Polymarket’s documentation identifies the lowest ask as the best execution price for a buy and the highest bid as the best execution price for a sell. A depth total combines sizes across multiple price levels, so it does not mean all that size is available at the best price.
Polymarket’s FAQ says outcome-share prices range from 0.00 to 1.00 USDC. It describes prices as determined by what users are willing to buy and sell, and says a YES/NO pair is fully collateralized by 1.00 USDC. Those price bounds provide context for the market; they do not turn an imbalance reading into a probability forecast. See What is Polymarket?
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How to calculate imbalance transparently
Choose the depth window before comparing sides. You might sum the first five price levels on each side, or sum all levels within a fixed price distance of the best price. Apply the same definition to bids and asks, and use the same outcome and units for both sums.
- Record the outcome, the book timestamp, and the window definition—for example, the first five levels on each side.
- Sum the sizes of the bid levels inside that window. Call this bid depth.
- Sum the sizes of the ask levels inside the same window. Call this ask depth.
- Calculate
(bid depth − ask depth) / (bid depth + ask depth).
This is a descriptive calculation derived from Polymarket’s documented price-size levels, not an official Polymarket statistic. Its value ranges from −1 to +1 when the total depth is nonzero. A positive result means more displayed size on the bid side in the selected window; a negative result means more on the ask side; a result near zero means the two summed sizes are similar. If both sides have zero size in the window, the calculation is undefined.
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For example, if the first five bid levels total 120 shares and the first five ask levels total 80, the calculation is (120 − 80) / (120 + 80) = 0.20. That says the selected snapshot has more displayed bid size in those five levels. It does not say the price is likely to rise. A different window, such as a fixed price band around the best prices, could produce a different result. Always report the window and timestamp alongside the number.
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What to check alongside the imbalance
- Best prices and spread: A large depth imbalance can coexist with a wide spread or a thin best level. Check the best bid, best ask, and spread before interpreting the summed sizes.
- Outcome and side: Compare bids and asks for the same outcome token. Do not combine YES and NO books or treat their imbalances as interchangeable without explaining the relationship.
- Changes over time: A single book is a snapshot. Polymarket’s book response includes a timestamp and hash; comparing hashes can help determine whether the book changed between reads. A change does not, by itself, explain why it changed.
- Last trade and price updates: Compare the displayed book with recent trades and price changes. A last trade records an execution; resting orders describe displayed interest that may change before execution.
- Tick size and minimum order size: These fields help indicate what orders can practically be submitted. They do not establish that a particular displayed level will remain available.
How to read book data and keep it current
For a one-outcome REST snapshot, Polymarket documents the CLOB /book endpoint; /books supports batch retrieval. The current documentation says bids are listed in ascending-price order and asks in descending-price order, with the best bid and ask as the last entries of their respective arrays. Check the documentation when implementing a reader, since interface details can change: Prices and Order Books.
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For ongoing updates, the Real-Time Data documentation describes a market stream subscribed by outcome asset ID. Standard event types include book, price change, last trade price, and tick-size change. Enabling customFeatureEnabled adds event types that include best bid/ask and market lifecycle updates. A feed is useful for seeing changes, but it does not make displayed depth a promise of future liquidity.
Identifier naming depends on the market version. Polymarket’s current quickstart calls the identifier assetId: use the token ID for a CTF market or the position ID for a Polymarket Protocol V2 market. The quickstart also says market orders fill against available liquidity and cancel any unfilled amount rather than leaving it open. That execution behavior is another reason not to treat a depth total as guaranteed execution at one price. See Place Your First Order.
What imbalance can—and cannot—tell you
Imbalance can summarize how displayed size is distributed between bids and asks for a specific outcome, window, and moment. It can help describe liquidity conditions when read with price quality and recent changes. It cannot establish trader intent, predict the next price move, or show whether the displayed size will remain available. Polymarket’s technical documentation explains the data and interfaces; it does not report predictive performance for an imbalance statistic.
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