Implied probability calculator
Enter a Polymarket or Kalshi contract price and this tool converts it into implied probability and expected value at a given true probability.
How to read it
A YES contract trading at $0.42 on Polymarket maps to a 42% implied probability: if the market is pricing efficiently, that is the crowd's consensus that the event will happen. Subtract trading fees and the liquidity gap to estimate what you would actually pay for exposure.
Expected value (EV) is positive when your own probability estimate exceeds the market's implied probability minus fees. EV is not a guarantee; it quantifies edge.
Kelly fraction
The tool optionally shows a Kelly-fraction bet size recommendation. Kelly is a geometric-growth optimizer and can be aggressive — most professional traders use a fraction of full Kelly (often 0.25 to 0.5×).