Reading Polymarket odds as probabilities: a five-minute field guide
A 62¢ YES is not a price — it's the market saying 62%. How to read prediction market odds, where the vig hides, and why the last cent matters.
A prediction market contract pays $1 if the event happens and $0 if it doesn’t. That single fact does most of the work: the price of YES is the market’s probability estimate. A 62¢ YES means the crowd — weighted by money, not by votes — puts the odds at roughly 62%.
“Roughly” is where trading lives.
Where the price stops being the probability
Three things pull the quoted price away from the true probability:
| Distortion | What it looks like | Typical size |
|---|---|---|
| Spread | YES 61 / 63 — the mid is your estimate, not the ask | 1–3¢ on liquid markets |
| Longshot bias | 3¢ contracts trading where 1¢ is fair | worst below 10¢ and above 90¢ |
| Stale quotes | News is out, book hasn’t moved yet | seconds to minutes |
The third row is the interesting one. Odds don’t move by themselves — someone has to read the headline, decide what it means for the contract, and reprice the book. On a Tuesday-afternoon CPI release the sharps are watching and the window is a few hundred milliseconds. On a Saturday sports market, a lineup change can sit unpriced for minutes.
The practical checklist
Before you take any prediction market price at face value:
- Use the mid, not the last trade. Last trade might be an hour old.
- Check depth. A 62¢ quote backed by $40 of book depth is an opinion, not a market.
- Ask what the market hasn’t read yet. If you know something the book doesn’t reflect, that gap is the trade — and it’s measured in cents.
- Respect resolution rules. “Fed cuts before September” can resolve differently than you assume. The contract text, not the headline, pays out.
None of this requires automation. What automation changes is row three of the table: a bot that parses the wire in milliseconds simply gets to the stale quote before you do. That’s the entire business model of every fast trader on these venues — human or not.
If you’d rather be on the fast side of that race, that’s what we build.