Guide
How to convert betting odds (American, decimal, fractional, implied %)
To convert betting odds, turn any format into implied probability, then read it in whichever format you want. Decimal odds = 1 ÷ probability; American +150 means a $100 stake wins $150 (implied 40%); −150 means risk $150 to win $100 (implied 60%). Implied probability is the common language that lets you compare a sportsbook line to a prediction market. Here are the formulas and the fastest way to do it.
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The three formats
Decimal (e.g. 2.50) = total return per $1 staked, including stake. Fractional (e.g. 6/4) = profit-to-stake. American: positive (+150) = profit on a $100 stake; negative (−150) = stake needed to win $100. They all describe the same thing — convert via implied probability.
Convert to implied probability
Decimal → probability = 1 ÷ decimal (2.50 → 40%). American positive → 100 ÷ (odds + 100) (+150 → 40%). American negative → −odds ÷ (−odds + 100) (−150 → 60%). Implied probability is the number you actually compare across books and against a prediction market's price.
Watch the vig
Add up the implied probabilities of all outcomes in a market: at a sportsbook they sum to more than 100%, and the excess is the book's margin (the vig). To compare a fair probability, remove the vig first. A prediction market's clean YES/NO price already sits near the true probability.
FAQ
How do I convert American odds to probability?
For positive odds: 100 ÷ (odds + 100) — so +150 = 100/250 = 40%. For negative odds: (−odds) ÷ (−odds + 100) — so −150 = 150/250 = 60%. Our free odds converter does it instantly.
What are decimal odds?
Decimal odds show the total return per $1 staked including your stake, so 2.50 returns $2.50 ($1.50 profit). Implied probability = 1 ÷ decimal, so 2.50 = 40%.
Why do implied probabilities add up to more than 100%?
Because sportsbook odds include the bookmaker's margin (the vig). The amount over 100% is roughly the book's edge. Remove it to compare a fair probability against a prediction market.