How to read Polymarket odds (without getting fooled)

Polymarket prices contracts from 0¢ to 100¢ where 100¢ is the payout if the event resolves YES. If you come from a sportsbook background (fractional, decimal, American odds), the conversion is mechanical.

The three conversions

Let p be the Polymarket YES price in decimal form (e.g. 65¢ = 0.65).

• Implied probability = p = 65%
• Decimal odds (European) = 1 / p = 1 / 0.65 ≈ 1.54
• American odds(if p > 0.5): -100 / (1/p - 1) ≈ -186; (if p < 0.5): (1/p - 1) × 100
• Fractional odds (UK): (1 - p) / p = 0.35/0.65 ≈ 7/13

The catch: fees and spreads

The odds above assume you fill at exactly the quoted price. You don't. In a thin market, you pay 2-5% slippage, so your effective decimal odds on 65¢ YES are closer to 1.48 than 1.54. Plus ~$0.20 gas, which on a $10 bet is another 2%.

Net: a Polymarket quote of 65¢ implies ~65% probability, but you need ~68% true probability to break even after costs. Smaller bets need higher edge; larger bets need deeper liquidity.

How PolyGuru shows it

On every market we display: market price (implied prob), our calibrated probability, edge in pp, and net EV after fees on a $10 bet so you see the dollar impact directly.