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An order book lists buy orders (bids) and sell orders (asks). With a best bid of 64¢ and a best ask of 65¢ the spread is 1¢ and the midpoint, the price Polymarket shows, is 64.5¢. A market buy of $5,000 into the example book below fills 7,652 shares at an average of 65.35¢, not 65¢: that difference is slippage, and taker fees come on top.
| Bid | An order to buy. The highest bid is what you get if you sell right now. |
| Ask | An order to sell. The lowest ask is what you pay if you buy right now. |
| Spread | Best ask minus best bid. 1¢ is tight; a wide spread is what you give up to trade at once. |
| Midpoint | Halfway between the best bid and ask: the probability Polymarket shows. With a spread over 10¢ it shows the last trade instead. |
| Depth | How many shares wait at each price. Thin depth means a big order moves the price (slippage). |
| Maker / taker | A limit order that rests on the book is a maker order and pays no fee; an order that fills against the book pays the taker fee. |
| YES bid = NO ask | YES and NO pay $1 together, so a bid for YES at p is an ask for NO at 1 - p. |
| Price | Shares | Side |
|---|---|---|
| 67¢ | 20,000 | ask |
| 66¢ | 10,000 | ask |
| 65¢ | 5,000 | ask |
| 64¢ | 12,000 | bid |
| 63¢ | 6,000 | bid |
| Spend | Shares | Average price | vs best ask |
|---|---|---|---|
| $100 | 154 | 65.00¢ | +0.00¢ |
| $1,000 | 1,538 | 65.00¢ | +0.00¢ |
| $5,000 | 7,652 | 65.35¢ | +0.35¢ |
| $10,000 | 15,224 | 65.69¢ | +0.69¢ |
Before fees. A taker pays shares x rate x p x (1 - p) on top: Polymarket fees. Your own numbers: spread and slippage calculator. A live book with real depth: how to read a Polymarket order book.