What is Polymarket Order Book Data?
Understanding order book data in Polymarket prediction markets — what it is, why it matters, and how to access it.
Order Books in Prediction Markets
An order book is a real-time ledger of all outstanding buy (bid) and sell (ask) orders for a prediction market contract. On Polymarket, every BTC, ETH, and SOL Up/Down market has two order books — one for the "Up" outcome and one for the "Down" outcome.
Anatomy of a Polymarket Order Book
| Component | Description | Example |
|---|---|---|
| Bids | Prices buyers are willing to pay | $0.52, $0.51, $0.50... |
| Asks | Prices sellers are willing to accept | $0.54, $0.55, $0.56... |
| Spread | Gap between best bid and best ask | $0.02 |
| Depth | Total size at each price level | 500 shares at $0.52 |
| Midpoint | Average of best bid and ask | $0.53 |
Why Order Book Data Matters
- Price discovery: The order book shows where supply meets demand — the real market price
- Liquidity assessment: Deep books mean easier execution; thin books mean higher slippage
- Sentiment signals: Heavy bid-side depth suggests bullish sentiment; heavy ask-side suggests bearish
- Strategy development: Backtesting with order book data produces more realistic results than price-only backtests
Historical vs Live Order Books
Polymarket shows you the current order book, but once a market resolves, that data disappears. PolyHistorical captures snapshots every 300ms, preserving the complete order book history for every market — including resolved ones.
Accessing Order Book Data
curl -H "X-API-Key: YOUR_KEY" \
"https://api.polyhistorical.com/v1/markets/SLUG/snapshots?include_orderbook=true"Each snapshot includes full bid/ask depth, prices, volume, liquidity, and coin price at that moment.