How to read a Polymarket order book

An order book is the live list of every outstanding buy and sell order in a market, grouped by price. Each row is a price and the shares available at it. Bids are what buyers will pay. Asks are what holders will sell for.

Polymarket runs a central limit order book (CLOB), not an automated market maker (AMM). Prices come from traders posting orders, not from a formula, so the book shows how much liquidity sits at each price, where the large orders are, and how far your trade will move the price. To read the same book from code, 0xinsider.com/learn/polymarket-api walks through the CLOB call that returns it.

Traders who read the book get better fills. Traders who skip it pay for liquidity they did not check.

Bids are sorted highest first. A top bid of 64¢ means someone will pay 64¢ a share right now, and the bids below it at 63¢ and 62¢ add demand at lower prices.

Asks are sorted lowest first. A lowest ask of 66¢ is the cheapest price you can buy at immediately. The asks above it, 67¢, 68¢ and beyond, only fill if buying pushes through the lower levels first.

The gap between the highest bid and the lowest ask is the spread: here 2¢. Buy at the ask and sell at the bid right away and you lose 2¢ a share. A tight spread means many participants are quoting prices. A wide spread means thin liquidity and a higher cost to get in.

The depth chart draws the book as cumulative liquidity. The left side, usually green, adds up all bids from the best bid down. The right side, usually red, adds up all asks from the best ask up. The height at a price is how much capital it takes to move the market there.

A steep wall of bids at one price is support: the price is unlikely to fall through it without heavy selling. A wall of asks is resistance: the price struggles to rise past it until buying absorbs those orders. Walls shift fast, so treat them as snapshots.

Compare the two sides. In a healthy market they are about equally deep. A much deeper bid side means more traders want to buy than sell, and a deeper ask side means selling pressure. This reading works best in liquid markets, where the depth is many participants and not a few large orders from one.

The spread sets your round-trip cost, so check it before any trade. A 1¢ spread is minimal friction. A 5¢ spread costs 5¢ a share to take part and needs a larger edge to overcome. The chart above shows the cost at each width.

Markets with daily volume above $100K tend to have tight spreads. Niche markets below $10K in volume often run 3¢ to 10¢.

A spread also tells you how much participants trust the price. In an actively debated market, market makers compete to quote tight. In quiet periods, or when the outcome is highly uncertain, they widen their quotes to charge for the risk of being wrong. A spread that suddenly widens in a tight market usually means new information is arriving.

Use limit orders when the spread is wide. With a bid at 63¢ and an ask at 67¢, a limit buy at 64¢ or 65¢ saves 2¢ to 3¢ a share against a market order. On 1,000 shares that is $20 to $30, repeated on every trade.

The spread is the first check on whether a price is fair. The guide at 0xinsider.com/learn/how-to-tell-if-a-polymarket-price-is-fair adds the others: the overround across outcomes, how the price moved, and who holds each side.

Market makers provide most of the liquidity. They post bids and asks on both sides, for example 64¢ and 66¢, and earn the spread for giving other traders an immediate fill. They update quotes every few seconds and manage their net position to avoid heavy directional exposure.

Professional market makers quote by algorithm, adjusting for the depth at each level, the size of incoming orders, and correlated markets. When they detect a large informed buyer, they widen their spreads or pull their quotes. This is why liquidity sometimes vanishes right before a major price move.

You can read that behavior. Thin liquidity on one side often means market makers pulled their orders. Deep orders refreshed at a tight spread mean a stable market and reliable fills, and orders placed then fill better than orders pushed through a thin, volatile moment.

Spend 30 seconds on the book before any trade. Check the spread, read the depth on both sides, and look for large resting orders that could act as support or resistance.

Size orders against the liquidity on offer. If the book shows 2,000 shares at the best ask and you want 5,000, a single order eats through several price levels. Buy 1,000 now and place limit orders for the other 4,000 at prices you accept. Professionals call this slicing, or working an order.

Expect the book to thin before scheduled events. Before an economic report or an election result, market makers cut exposure and spreads widen. Afterward liquidity returns and spreads tighten. Place orders before the book thins or wait until it refills: the window right before or during the event has the widest spreads and the most slippage.

A stacked wall is a large number of shares at one price. A bid wall of 50,000 shares at 60¢ says a large buyer sees value there, and the price will not fall below 60¢ unless the wall is absorbed or withdrawn. Walls can be spoofed: placed to fake support and pulled before they execute. Compare the wall's size with the market's average volume before you trust it.

An iceberg is the opposite: an order that shows only a small slice and refills as each slice fills. You spot one by repeated fills at the same price from an order that never leaves the book. It points to a large trader who does not want to show full size.

A vacuum is one side of the book suddenly emptying. If all the asks above 70¢ disappear, even a small buy can push the price much higher. Vacuums often come right before rapid moves, and entering during one is risky because you may not be able to exit at a reasonable price. Stay out unless you have a strong view on direction.

Read these patterns next to the large trades on 0xinsider.com/trades. The book tells you the mechanical state of a market. The large trades tell you who is driving it and with how much conviction.