Polymarket uses a Central Limit Order Book (CLOB) to match buyers and sellers. Understanding how the order book works is the difference between leaving money on the table and getting the best possible price on every trade.
What is an order book?
The order book is a list of all open buy and sell orders for a market. It has two sides:
- Bids (buy orders): Traders willing to buy at a certain price. Listed from highest to lowest.
- Asks (sell orders): Traders willing to sell at a certain price. Listed from lowest to highest.
- Best bid: The highest price someone is willing to pay right now.
- Best ask: The lowest price someone is willing to sell for right now.
- Spread: The gap between best bid and best ask. Tighter spread = more liquid market.
How to read the order book
Here's what a typical order book looks like:
Bids (buy side):
- 48¢ — 500 shares
- 47¢ — 1,200 shares
- 46¢ — 800 shares
- 45¢ — 2,000 shares
Asks (sell side):
- 52¢ — 300 shares
- 53¢ — 600 shares
- 54¢ — 1,500 shares
- 55¢ — 400 shares
Spread: 52¢ − 48¢ = 4¢ (the spread)
If you buy with a market order, you'd pay 52¢ (the best ask). If you sell with a market order, you'd get 48¢ (the best bid). The 4¢ difference is what you lose to the spread.
Limit orders vs market orders
Market orders:
- Fill immediately at the best available price
- You're a "taker"—removing liquidity from the book
- Pay taker fees
- Risk slippage on large orders (eating through multiple price levels)
Limit orders:
- Fill only at your specified price or better
- You're a "maker"—adding liquidity to the book
- Pay zero or reduced maker fees
- May not fill if the market doesn't reach your price
- Always preferred for getting better prices
Rule of thumb: Always use limit orders unless you need to enter or exit immediately. The savings on fees and spread add up significantly over time.
Understanding and using the spread
The spread tells you about market quality:
- Tight spread (1-2¢): Liquid market. Lots of competition among market makers. Your limit orders will fill quickly.
- Medium spread (3-5¢): Decent liquidity. Use limit orders to avoid paying the full spread.
- Wide spread (10¢+): Thin market. Be very careful with market orders. Place limit orders in the middle of the spread and wait.
Order book depth
Depth shows how many shares are available at each price level. Deep order books mean:
- Large trades can be filled without moving the price much
- The market is hard to manipulate
- Prices are more reliable as probability signals
Thin order books (few shares at each level) mean a single large order can move the price significantly.
Tips for better execution
- Place limit orders inside the spread: If the bid is 48¢ and ask is 52¢, place your buy at 49¢ or 50¢. You'll often get filled and save 2-3¢ vs. a market order.
- Be patient: Limit orders take time to fill. If the market is active, you may get filled in minutes. In quiet markets, it might take hours.
- Watch for iceberg orders: Some traders hide their full order size. The book might show 100 shares at 50¢ but 5,000 shares will fill at that price.
- Don't chase: If you miss a price, don't panic-buy at a worse price. Place another limit order and wait.
- Size your orders based on depth: If only 200 shares are available at 52¢, don't try to buy 1,000 at market—you'll eat through multiple levels and overpay.
Make smarter trades with Alphascope
Alphascope helps you identify which markets to trade—then use your order book skills to execute:
- Predictions → Find mispriced markets where AI estimates diverge from market prices.
- News → Time your entries when news creates temporary mispricings.
- Arbitrage → Cross-platform price gaps you can capture with well-placed limit orders.
FAQ
What is the CLOB on Polymarket?
CLOB stands for Central Limit Order Book. It's the system that matches buy and sell orders on Polymarket, similar to how stock exchanges work. All orders are visible and fill based on price-time priority.
Why are my Polymarket trades getting bad prices?
You're probably using market orders on thin order books. Switch to limit orders and place them inside the spread. This gives you better fills and lower fees.
Do limit orders expire on Polymarket?
You can set limit orders with or without expiration. Good-til-cancelled (GTC) orders remain open until filled or you cancel them. You can also set time-based expirations.