How to read a Polymarket order book

Every price on Polymarket comes out of an order book: a live list of what people are willing to pay and accept for a share of an outcome. Read the book and you know more than the headline price tells you. This post covers what the numbers mean, using nothing but a terminal-sized example.

A share pays $1 if you are right

Every Polymarket market settles in shares that redeem for $1 if the outcome happens and $0 if it does not. Prices are quoted in cents, so a YES share trading at 33c implies the market puts roughly a 33% probability on the outcome. Buy at 33c and you are risking 33c to win $1; the trade is profitable in expectation only if you think the real probability is higher than 33%.

That is the whole trick of reading any prediction market price: the price is the crowd's probability estimate. Everything else in this post is about how firm that estimate is.

YES and NO are mirror images

A binary market has two outcome tokens, YES and NO, and each has its own order book. But they are the same market seen from opposite sides: a share of NO pays $1 exactly when YES pays $0, so NO's price is 100c minus YES's price. An order to buy YES at 32c is economically the same as an offer to sell NO at 68c, and the exchange treats them that way.

In practice this means you only ever need to read one book, then flip the sign. If YES's best bid is 32c, NO's best ask is 68c. Traders often work the NO side simply because the number they care about is easier to think in.

The two prices that matter

Here is a small YES book, laid out the way most terminals (and polymarket-tui) draw it:

BID (buyers)              ASK (sellers)
size      price           price     size
1,200     32c             34c       850
3,400     31c             35c       2,100
5,000     30c             36c       4,700

• Best bid, 32c: the most anyone is currently willing to pay for a YES share. Sell instantly and this is your price.
• Best ask, 34c: the least anyone will currently sell for. Buy instantly and this is your price.
• Spread, 2c: the gap between them, and the cost of impatience. A round trip at market prices loses the spread before the outcome moves at all.
• Mid, 33c: halfway between bid and ask. When someone quotes "the price" of a market, they usually mean the last trade or the mid.

A tight spread (1c on a liquid market) means the probability estimate is contested and sharp; both sides are willing to trade close to it. A wide spread (5c or more) means nobody is confident enough to quote tightly, and the headline price deserves less trust.

How much conviction sits behind a price

Size matters as much as price. The book above shows 1,200 shares bid at 32c and 5,000 more at 30c: buyers get more eager as the price falls. Depth tells you two things:

• Slippage: buy 3,000 shares "at market" and only 850 fill at 34c; the rest walk up the book to 35c and beyond. Your average price is worse than the quote you saw. On thin markets this effect dominates.
• Confidence: a market can print 33c on ten shares. A price backed by tens of thousands of dollars within a cent of the mid is a far stronger probability estimate than the same price on a nearly-empty book.

This is why watching the book beats watching the price. Two markets can both say 33%; one of them barely means it.

The grid prices move on

Orders must land on the market's tick size, normally 1c. When a market's price approaches 0 or 100, the exchange re-grids it to 0.1c ticks so longshots can still be priced (0.4c instead of a forced choice between 0c and 1c). Orders also have a minimum size, typically 5 shares.

One quirk worth knowing: Polymarket has no native market order. A "market" order is a limit order priced across the spread so it fills immediately against resting orders. That is also the honest way to think about it: you are always trading against the book you can see.

Reading a live book with polymarket-tui

polymarket-tui streams this whole picture into your terminal: YES and NO books tick live from the exchange with depth bars per level, the trade tape prints alongside, and the current tick size comes from the live book itself. Cursor onto any level and press b or s to start an order prefilled at that price; space flips to the NO book. New setups start in dry-run, where orders are built and signed but never posted, so you can practice reading and acting on a book with nothing at stake.

$ uv tool install polymarket-tui
$ polymarket-tui

No account is needed to browse markets, books, and charts. The install page has Homebrew and one-liner options.