What Is a Prediction Market Order Book?
A clear, plain-English guide to reading the list of buy and sell interest in a prediction market
By Top Prediction Markets EditorialReviewed September 7, 20264 min read
Answer first
A prediction market order book is a live list of standing buy (bids) and sell (asks) orders for a contract. It shows the best prices, how many contracts are available at those prices, and the spread between buyers and sellers. Reading the order book helps you understand immediate execution prices, available liquidity, and short-term price pressure in Yes/No markets.
What is a prediction market order book and how is it different from the trade history?
A prediction market order book is the running list of everyone’s active buy and sell orders for a particular contract. It shows what price buyers are willing to pay, what price sellers are willing to accept, and the quantity offered at each price. In short, an order book is the market’s memory of standing orders: what could be traded right now.
That contrasts with the trade history, which is a tape of completed trades. The history tells you what just happened; the order book tells you what will determine immediate execution prices. If you want to know where trading could happen at this moment, you look at the order book rather than the last trade.
How are prices shown for Yes/No contracts and what does the spread tell me?
Yes/No event contracts are quoted as a price between $0 and $1 (often shown as cents). A Yes contract—an event contract that pays $1 if the event happens—might trade at 62¢, which the market interprets as an implied probability of 62%. If someone bids $0.62 for a Yes, they are offering $0.62 per contract to buy Yes; if someone asks $0.65, they are offering to sell Yes at $0.65.
The spread is the difference between the best buy (highest bid) and the best sell (lowest ask). A narrow spread generally means trading right now is cheaper and liquidity is better; a wide spread raises your immediate cost to trade. The order book also reveals depth—the quantities sitting at nearby prices—which affects how much impact a new order will have on the quoted price.
How do bids, asks, limit orders, market orders, and matching work in a prediction market order book?
Buyers place bids and sellers place asks. Limit orders sit on the book at the price you choose and wait to be matched; market orders take liquidity immediately by matching with standing orders on the book. When a new order’s price crosses the opposite side (for example, a buy at or above the best ask, or a sell at or below the best bid), the exchange matches orders and a trade occurs.
If prices don’t cross, a limit order remains on the book until it is matched or cancelled. The order book typically shows multiple price levels; each level lists the total number of contracts offered at that exact price. Traders look at several levels to judge how big an order would move the price and how much liquidity is immediately available.
If I place a market or limit order, what will happen to my execution and costs?
Suppose a Yes/No contract currently trades around 62¢ and the book shows best bid: 61¢ for 40 contracts (buyers waiting at 61¢) and best ask: 63¢ for 30 contracts (sellers waiting at 63¢). If you place a market buy for one Yes contract, you will pay the best ask: 63¢. Buying that single contract costs $0.63; if the event occurs and the contract pays $1, your gain before fees is $0.37. If the event does not occur, the contract expires at $0 and your loss is $0.63.
If instead you place a limit buy at 62¢ for one contract, your order will sit on the book and only execute if a seller accepts 62¢ or lower. If sellers stay at 63¢ and nobody lowers their asks, your 62¢ order will remain unfilled. Size matters: if you want to buy 50 contracts immediately, you will clear the offers at 63¢ (30 contracts) and then buy the next available asks at higher prices. The average price you pay will be higher than 63¢; that’s the effect of limited depth shown in the book.
What common mistakes do people make when reading an order book?
Treating the last trade price as the best available price is a frequent error. The last trade shows what just happened, not what will happen next—the best bid and best ask on the order book determine immediate execution prices. Always check the top-of-book prices rather than relying solely on the tape.
Ignoring the spread is another trap: a wide spread makes immediate execution expensive, even if the last trade looks favorable. Also don’t confuse size on the book with guaranteed liquidity. Quantities displayed are standing orders that can be canceled; visible depth gives a sense of liquidity, but large opposing traders or rapid cancellations can change the book quickly.
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Frequently asked questions
What exactly is the spread in an order book?
The spread is the gap between the best ask (lowest sell price) and the best bid (highest buy price). It measures how costly it is to execute a trade immediately.
How do I use the order book to place a trade?
Use the best ask to buy immediately with a market order, or post a limit buy at a price on the bid side and wait for someone to sell into it. The order book shows the prices and sizes you will interact with.
Can I see every participant’s orders in the order book?
Many platforms show only aggregated public orders (price levels and totals). Some platforms allow anonymous or hidden orders. Check the platform’s display and order visibility settings.
Are prediction market order books the same as order books on other exchanges?
They work on the same basic principles: bids, asks, depth and matching. Differences are mostly in contract payout ($0–$1), contract unit sizing, and platform rules.
Are prediction market order books legal to use in my country?
Rules vary by location and platform. See our dedicated guide on whether prediction markets are legal in the US.
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