Implied Prob (YES)
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Implied Prob (NO)
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Overround / Vig
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True Prob (YES)
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True Prob (NO)
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When Should You Use This?
Use this to understand the true probability implied by Polymarket prices. Share prices include the market's vig (overround). This calculator removes the vig to show you the market's actual probability estimate, helping you identify mispriced outcomes.
How It Works
1
Enter Share Prices
Input the current YES and NO prices from Polymarket in cents.
2
Read the Overround
The overround shows how much the market charges as vig. An overround of 4% means the combined prices are 104 cents. Negative overround means potential arbitrage.
3
Compare True Probability
If your own probability estimate differs significantly from the true probability shown, there may be a trading opportunity.
Frequently Asked Questions
Overround (or vig) is the amount by which total implied probabilities exceed 100%. If YES is 62 cents and NO is 42 cents, the total is 104 cents, giving a 4% overround. This is the market's built-in cost.
Lower is better for traders. Liquid Polymarket markets typically have 1-4% overround. Less liquid markets can have 5-15%. Compare to sportsbooks which often have 5-10%.
If your research suggests the true probability is 70% but the market prices YES at 55 cents (true prob ~53%), the market may be underpricing YES. Buy if the gap exceeds your expected costs.
Market makers post bid and ask prices with a spread. The ask prices (what you pay to buy) typically sum to more than 100 cents. The mid prices (average of bid and ask) are closer to fair value.