How to read a Polymarket order book before you trade

Every Polymarket outcome trades as a share that pays one dollar if the outcome happens and nothing if it does not. A Yes share at 62 cents implies a 62 percent chance; the matching No share trades near 38 cents. Buying Yes and buying No at the same time locks in a dollar minus the spread, which is why the two prices track each other closely.

Because the payout is fixed, the price you pay is the entire trade. There is no later moment where the position becomes cheaper to have opened. Every cent of slippage at entry comes straight out of the edge.

The best bid is the highest price someone will pay for a share; the best ask is the lowest price someone will sell one for. The gap between them is the spread. Cross it with a market order and you pay the ask; the fair value sits somewhere in the middle, so you begin the trade behind by roughly half the spread.

Tight spreads mean an active market where the cost of entry is small. Wide spreads mean the market is thin, uncertain, or both, and a signal that looks like a few points of edge can be eaten entirely by the cost of getting in.

Each price level shows how many shares are resting there. That is depth. A thin book has a few hundred dollars at each level, so a moderate order walks through several levels and fills at an average price well above the best ask. A deep book absorbs the same order at one price.

Depth is also where the book can deceive. A large resting order, a wall, can be pulled the moment the market moves toward it. Treat displayed depth as an offer that can be withdrawn, not as a promise, and size your order to what the book shows at the levels you are willing to pay.

• Read the ask side for buys and the bid side for exits before you open anything.
• Add up the shares available at each level until you reach your size; that average is your real entry.
• Compare that real entry to the last traded price. A large gap means the last trade tells you little about your fill.
• Check the No side too. If Yes plus No is far above a dollar, the book is thin and the mid is soft.

A market order takes whatever is on the book and guarantees a fill at the cost of the spread and any depth you walk through. A limit order names your price and waits. In a thin market the limit order is how you avoid paying the book's asking price for your own urgency.

The trade-off is time. A limit order below the ask may never fill, and a market that moves in your favor moves away from your order. Whether patience pays depends on how far the market is from resolution and how much the signal depends on getting in before others notice.

When you study profitable traders, their entry prices tell the story. A position built at 35 cents in a market now trading at 55 was an edge whether or not it resolves Yes; a position built at 55 after the move is a bet on the crowd being right. The same wallet, the same market, and a very different trade.

Aligned records the average entry price behind every alignment next to the current price, and the tape shows the fills as tracked traders execute them. The gap between those two numbers is the first thing to read: it is how much of the move the signal caught, and how much is left for anyone arriving now.