What Is a Prediction Market Order Book?

What it means

In simple terms, a prediction market order book is a running list of everyone’s active buy and sell orders for a particular contract. The order book shows what price buyers are willing to pay and what price sellers are willing to accept, plus the quantity offered at each price.

Here's the basic idea: an order book is the market’s memory of standing orders. It is not a history of past trades (that’s a separate tape). The order book is what you use if you want to see where trading could happen right now.

Why it matters

The key thing to know is that an order book tells you about immediate liquidity and likely short-term price moves.

• It reveals the spread (difference between the best buy and best sell) and how costly it is to trade right now.
• It shows depth (how many contracts sit at nearby prices), which affects whether a small order moves the price a little or a lot.

How it works

1. Bids and asks: Buyers place bids (orders to buy at a stated price). Sellers place asks (orders to sell at a stated price). The highest bid and the lowest ask are called the best bid and best ask.

2. Yes/No contracts: A Yes contract — an event contract that pays $1 if the event happens — is quoted as a price between $0 and $1 (often shown as cents). 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.

3. Order types: 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.

4. Matching: When a new order’s price crosses the opposite side (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, the limit order remains on the book until matched or cancelled.

5. Depth and layers: 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.

A simple example

A simple example helps make this concrete. Imagine a Yes/No contract where a Yes contract currently trades around 62¢.

The order book shows:

• Best bid: 61¢ for 40 contracts (buyers waiting at 61¢)
• Best ask: 63¢ for 30 contracts (sellers waiting at 63¢)

If you want to buy one Yes contract immediately and place a market order, you will pay the best ask: 63¢. Buying one contract costs $0.63. If the event happens and the contract pays $1, your gain before fees is $0.37. If the event does not happen, the contract expires at $0 and your loss is $0.63.

If instead you place a limit buy order at 62¢ for one contract, your order will sit on the book and will only execute if a seller accepts 62¢ or lower. If sellers stay at 63¢ and no one lowers their ask, your 62¢ order will remain unfilled.

Now consider size: if you want to buy 50 contracts right away, 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.

Common mistakes

Treating last trade price as the best available price

Last trade shows what just happened, not what will happen next. The best ask and best bid in the order book are what determine immediate execution prices.

Ignoring the spread

A wide spread increases your cost to trade immediately. Small retail orders can be cheap to execute in tight markets but expensive in wide-spread, thin markets.

Confusing 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 cancellations can change the book quickly.

Related concepts

• What Are Prediction Markets?
• What Are Yes/No Contracts?
• How to Read Prediction Market Prices
• What Is Implied Probability?