Polymarket Order Book: Prices, Spread, Depth

What is in a Polymarket order book?

An order book is a live list of orders that are waiting for a counterparty. Each entry has a price and a size, the number of shares on offer. Offers to buy are bids, offers to sell are asks, and together they make the book. Polymarket calls its design a central limit order book. Nobody at Polymarket sets the price: it emerges from the orders people leave there.

Each outcome has a book of its own. In Polymarket's reference, an order book summary is retrieved for a specific token ID, and each outcome has a token of its own, so the Yes book and the No book are read separately. How their prices are tied together comes up further down. New to Yes and No shares? Start with the guide to how Polymarket works.

What do best bid, best ask, spread and midpoint mean?

The top of each side matters most. The best bid is the highest price anyone currently offers to pay, and the best ask is the lowest price anyone currently offers to accept. The spread is how far apart those two prices are, and the midpoint sits halfway between them.

Polymarket's documentation links the spread to liquidity: tighter spreads mean more liquid markets. A buyer who acts at once pays the ask and a seller receives the bid, so a wider spread means a bigger gap between the two prices a trader faces.

What price does a market card show?

Polymarket's documentation says the displayed price is the midpoint of the bid-ask spread, with one exception: if the spread is wider than $0.10, the card falls back on the last traded price. A card therefore shows a summary of the book, or of the last trade, and not an offer to trade at that price. In the example above it would read 0.48 while a buyer pays 0.50.

What you can trade at right now comes from the two sides of the book: a market buy starts at the lowest ask and a market sell at the highest bid, and a larger order then works through further levels, as the slippage section below shows. A limit order lets you name a price instead, at the cost that it may wait or never fill. The displayed-price rule is Polymarket's description of its own display, and this guide cannot say what any other screen shows, so check which price a screen is showing before you read it as a probability. The odds converter turns any price into odds.

What is a tick?

Every price in a book sits on a grid. The step of that grid is the tick size, which Polymarket calls the minimum price increment, and an order price must be a multiple of the active minimum price increment. The documentation lists these steps:

Minimum price increments (tick sizes) and their step. Source: docs.polymarket.com/market-data/market-details, checked Oct 19, 2026.

Because every order price is a multiple of the tick, the gap between a bid and an ask is also a multiple of it, so the narrowest spread is one tick. That is arithmetic from the rule above, not a figure in the documentation. The grid is not the same everywhere: World Cup to advance, moneyline, spreads and totals markets use a tick of 0.0025, which Polymarket introduced to allow tighter spreads.

A market's tick can also change while it trades. The market stream has a tick size change event, which its reference describes as an update sent when the price approaches its limits; it does not say which limits it means. Polymarket tells integrators to check the active value on the market instead of assuming a fixed step, and the exchange rejects any order whose price is off the market's current tick grid.

Why are Yes and No prices linked?

Yes and No are the two halves of one pair of shares, so their prices are tied together, and Polymarket's documentation describes the tie in two places.

The first is how a new market finds its first price. A new market has none; the documentation says the first price emerges when someone bids for Yes and someone else bids for No at complementary prices, which add up to one dollar. Its example is a Yes bid at 0.60 and a No bid at 0.40, which add up to 1.00, and the two orders match: 1 pUSD of collateral creates a complete set (pUSD is Polymarket's collateral token), and each buyer receives the token they bid for. The reverse also exists, since a complete set can be merged back into 1 pUSD. More generally, the Order Lifecycle page says that depending on the orders and the assets available, a settlement can move existing tokens, create complete sets from pUSD, or merge complete sets back into pUSD.

The second is the market-making page, which states the idea in trading terms with one pair of prices: buying No at 0.48 is economically equivalent to selling Yes at 0.52. It gives that single example and does not say how any screen draws the No side of a book, so read the book of the outcome you trade.

What are depth and slippage?

Depth is how many shares are on offer at the best prices and at the levels behind them. The book itself has no cap on order size, although a large order can move the price a lot, and what caps an order in practice is your available balance minus what open orders already reserve. The maker rebate page links liquidity and price: more liquidity brings tighter spreads and less price impact.

A market order uses the book level by level. For a buy the documentation estimates the fill by walking the asks from the best price upward, adding up price times size until the amount to spend is used, and for a sell by walking the bids from the best price downward until enough shares are sold. The average price of a large order can therefore be worse than the best price, and the best price itself moves afterwards. The difference between the price you saw and the average price you got is usually called slippage.

Any estimate of a fill price is a snapshot of the current order book, so the book can move between your look and your order. A limit price caps how far the order walks: it will not trade beyond that price. Fees come on top of all of this and are covered in the fees guide.

What changes on a thin book or with a wide spread?

The documentation gives no typical depth or spread for any market, so this guide has no figures to quote. It also says nothing special about short-window markets such as the 5-minute crypto markets, other than that one order book system is shared across markets. What it does document applies to any thin book: a wide spread changes the price a card displays, as shown above, and a large order can move the price, as the example shows. The crypto Up/Down guide covers short-window markets, and the order types guide explains the short hold that can apply to an order sent into one.

PolyTerminal is an independent interface for Polymarket and is not operated or endorsed by Polymarket. Orders placed through PolyTerminal carry PolyTerminal’s Polymarket builder code, and PolyTerminal may charge a builder fee on top of Polymarket’s own fees. This page is educational and is not investment advice. Rules were compared with the official documentation on Oct 19, 2026; check the source before you act on a number.