Order Book
An order book is a list of buy and sell orders organized by price, used by exchanges and prediction markets to match trades. It’s the same mechanism that powers stock and cryptocurrency exchanges, applied to betting markets.
How It Works
Each market has two sides:
Buy side (bid): Orders to buy at a specific price or below. Sell side (ask/offer): Orders to sell at a specific price or above.
A typical order book on a Kalshi contract about the Lakers winning a game might look like:
| Price (cents) | Size | Side |
|---|---|---|
| 53 | 200 contracts | Sell |
| 52 | 350 contracts | Sell |
| 51 | 500 contracts | Sell |
| —SPREAD— | ||
| 50 | 600 contracts | Buy |
| 49 | 1,000 contracts | Buy |
| 48 | 750 contracts | Buy |
The best bid is 50¢ (highest price someone is willing to buy at). The best offer is 51¢ (lowest price someone is willing to sell at). The gap between them (51 - 50 = 1¢) is the bid/ask spread.
To buy 100 contracts at market price, you’d hit the 51¢ offer for 100 contracts. To sell 100 contracts at market price, you’d hit the 50¢ bid.
To buy more than what’s available at the best price, you’d “walk the book” — hitting 51¢ for 500 contracts, then 52¢ for the remaining size, paying a higher average price.
Why It Matters
Order books are how exchange-style platforms create their markets, and understanding them is essential for trading on these platforms effectively.
Order book depth signals liquidity. A market with thousands of contracts at every price level offers good liquidity — you can trade meaningful size without moving the price much. A market with only a few contracts at each level has thin liquidity — even small orders can move the price significantly.
The spread reveals market efficiency. A tight spread (1-2 cents on a typical Kalshi market) indicates a well-traded, efficient market. Wide spreads (10+ cents) indicate either an illiquid market or significant uncertainty about the true price.
Order book mechanics differ from bookmaker lines. At a sportsbook, you take whatever odds the book offers. On an order book, you can post your own price and wait for someone to match it (a “limit order”) or accept an existing price (a “market order”). This adds strategic options — patient traders can often get better prices by posting limit orders rather than crossing the spread.
Visible order books reveal more than bookmaker lines. You can see exactly how much volume is offered at each price level, giving you a clearer picture of liquidity than you’d ever get from a sportsbook. This is one of the structural advantages of trading on exchanges and prediction markets.
Practical Implications
Be cautious with large orders on thin books. A $10,000 order on a market with only $5,000 of liquidity at the top of the book will cause significant price movement, costing you several cents on average price.
Use limit orders strategically. Posting an order inside the spread (e.g., 50.5¢ when the spread is 50/51) often gets filled at a better price than just hitting the existing offer.
Watch the spread for signal. A widening spread can indicate uncertainty, news, or imbalanced flow. A tightening spread suggests increased confidence in the consensus price.
Arbitrage calculations need to use the right side of the book. When estimating arbitrage between an order book platform and a sportsbook, use the price you’d actually pay (the offer for buys, the bid for sells), not a midpoint estimate.
For more on how exchanges and prediction markets use order books, see Betting Exchanges Explained and Prediction Markets Explained.