Guide
The Polymarket order book in a 5-minute window
Updated 19 October 2026 · Checked against our recorded data
Each 5-minute market has two outcome tokens, Up and Down, each with its own order book. Knowing how they relate is the first step to reading the data correctly.
Two books that mirror each other
An Up share and a Down share together always pay exactly $1, so their prices are linked: a bid for Up at 0.46 is the same as an offer to sell Down at 0.54. Our book file records both sides so you never have to derive one from the other.
| elapsed_ms | outcome | bid1_px | ask1_px | bid1_sz | ask1_sz |
|---|---|---|---|---|---|
| 8 | UP | 0.46 | 0.47 | 256.62 | 100.00 |
| 8 | DOWN | 0.53 | 0.54 | 100.00 | 256.62 |
Tick size and minimum order
Prices move in steps of 0.001 and the minimum order is 5 shares (per Polymarket's market settings, October 2026). Most activity still clusters on whole cents.
How activity changes through the window
Across the 12 BTC windows in our sample hour, this is how book updates, trades and the typical Up spread change minute by minute:
| minute of window | Up book updates | trades (both sides) | median Up spread |
|---|---|---|---|
| 1 | 2,524 | 4,297 | 0.010 |
| 2 | 2,571 | 4,005 | 0.010 |
| 3 | 2,317 | 4,092 | 0.010 |
| 4 | 2,587 | 3,903 | 0.010 |
| 5 | 1,387 | 1,607 | 0.010 |
Once the outcome looks settled late in a window, prices pin near 0 or 1 and spreads change character. Strategies that look great at 90 cents often fail on what is really available there.
Reading it from the files
bid1_px / ask1_pxare the best prices;bid1_sz / ask1_szthe shares available there.- Each row is a change in the top two levels, timestamped by the exchange (
exch_ts_ms) and by us (recv_ts_ms). - To know the book at any moment, take the last row at or before that
elapsed_ms, as shown in the backtesting guide.
Test it on real data
Every number here can be checked in our free BTC sample: one hour, 12 windows, every book update and every trade. Full history for all seven coins from £1.99 a month.