Prediction Markets
Calculator
Convert any contract price into American odds, decimal odds, and implied probability: instantly. Then use the EV calculator to check if a trade has positive expected value.
Convert price โ odds โ probability
Enter any one value: the other three update instantly.
Is this trade worth making?
Enter your own probability estimate and the market price to see whether the trade has positive expected value: and how much edge you have.
Common price โ odds conversions
| Price (ยข) | Implied Prob. | American Odds | Decimal Odds |
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Found a positive EV trade? Size it with Kelly.
Kelly Criterion โ Optimal Bet Sizing
Once you know your edge, Kelly tells you the mathematically optimal fraction of your bankroll to bet. Includes half-Kelly and quarter-Kelly so you can dial in the risk level that suits you.
โ Fee comparisonHow much does $1,000 cost to trade?
Kalshi, Polymarket, Robinhood, ForecastEx, Drift Bet and more: side-by-side fee breakdown with a concrete $1,000 trade example for each platform.
The maths behind the conversions
Why a 65ยข contract = 65% chance
A YES share in a binary prediction market pays $1 if the event happens and $0 if it doesn't. YES + NO always sum to ~$1. So a YES share at $0.65 (65ยข) implies a 65% chance: anyone who thought the true probability were higher would buy YES and push the price up.
How the ยฑ format works
Favourite (โ): The number tells you how much you must stake to win $100. โ185 means stake $185 to profit $100.
Underdog (+): The number tells you what you win on a $100 stake. +200 means stake $100 to profit $200.
Formula: p โฅ 50%: odds = โp/(100โp) ร 100. p < 50%: odds = (100โp)/p ร 100.
Total return per unit staked
Decimal odds represent your total return including your stake. Decimal 1.54 means you receive $1.54 for every $1.00 staked (profit of $0.54). Formula: decimal = 100 รท implied_probability. Decimal odds below 2.0 represent favourites; above 2.0 are underdogs.
Edge = your estimate โ market price
If you think something has a 55% chance and the market prices it at 40ยข, you have 15 percentage points of edge. EV on a $100 stake = $100 ร (your_prob โ market_price) รท market_price. Positive EV doesn't guarantee profit on any single trade, but positive-EV trading is profitable in the long run.