Free tool
Odds converter
Type into any box. American, decimal and fractional odds, implied probability and the price of a $1 prediction-market contract all update together. Nothing leaves your browser.
Remove the vig: fair odds from both sides
How the conversions work
- Prediction-market price: a contract pays $1 if it happens, so its price in cents is the market’s probability. 62¢ is about 62%.
- American odds: +150 means a $100 bet wins $150 (probability 100 ÷ 250 = 40%); −150 means you bet $150 to win $100 (150 ÷ 250 = 60%).
- Decimal odds: the total return per $1, stake included. 2.50 is 1 ÷ 2.50 = 40%.
- Fractional odds: 3/2 means win 3 for every 2 staked: 2 ÷ 5 = 40%.
Both sides of a sportsbook line add up to more than 100%; the extra is the book’s margin (the vig). On a prediction market, both sides adding up to less than $1 across two venues is a possible arbitrage, if it still pays after fees.
Fees decide whether a price gap pays. ArbTrader scans Kalshi, Polymarket US, Gemini and Polymarket for spreads that still pay after every venue’s fee, at the size actually for sale, and pings you when one opens.
Not financial advice. ArbTrader isn’t affiliated with any venue named here.
Common questions
How do I convert American odds to probability?
For positive odds, divide 100 by (odds plus 100): +150 is 100 divided by 250, or 40%. For negative odds, divide the odds by (odds plus 100), ignoring the minus sign: -150 is 150 divided by 250, or 60%.
What does a prediction market price mean?
A contract pays $1 if the event happens, so its price is the market's probability. A 62 cent contract means the market puts it at about 62%.
How do I remove the vig from odds?
Convert both sides to probabilities, add them up, and divide each side by the total. The results add up to 100% and are the fair odds without the margin.