Convert American, decimal, and fractional betting odds to implied probability with EV analysis, Kelly criterion, edge visualization, and vig breakdown.
The implied probability calculator converts betting odds in American, decimal, or fractional format into the probability embedded in the line. That makes it easier to compare a sportsbook price with your own estimate of the true chance of an outcome.
It also shows expected value, edge, Kelly sizing, and the bookmaker margin implied by the market. The goal is not just to translate the odds, but to show whether the price looks favorable.
Use it whenever you need to compare different books, different odds formats, or your own forecast against the market.
Odds formats are easy to compare incorrectly when you are doing the math by hand. This calculator keeps the conversion and the interpretation together so you can see the break-even rate, the edge, and the expected return in one place.
It is useful for evaluating a single line quickly or for comparing several books side by side before placing a bet.
American (+): Implied = 100/(odds+100). American (−): Implied = |odds|/(|odds|+100). Decimal: Implied = 1/odds. Fractional (a/b): Implied = b/(a+b). EV = (trueProb × profit) − ((1−trueProb) × bet).
Result: Implied Probability = 28.57%, EV = $8.75, Edge = +6.43%
Odds +250 imply a 28.57% chance. If you believe the true probability is 35%, you have a 6.43% edge and positive expected value of $8.75 per $100 bet.
Each odds format encodes the same break-even probability. American odds show profit relative to a 100-unit base, decimal odds show total return per unit staked, and fractional odds show profit relative to stake.
When the implied probabilities from both sides add to more than 100%, the difference is the market margin. That overround is why the listed line is not the same as a fair probability.
The calculator becomes most useful when you already have a probability estimate of your own. If your estimate is higher than the implied probability, the line may be favorable; if it is lower, the bet is usually overpriced for your view of the outcome.
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It's the probability embedded in the odds. If odds pay 2-to-1 (decimal 3.0), the implied probability is 33.3%. It represents the break-even win rate needed to profit long-term.
A value bet occurs when your estimated true probability of an outcome exceeds the implied probability from the odds. If you believe a team wins 45% of the time but odds imply only 35%, that's a value bet.
A formula for optimal bet sizing: f* = (bp − q) / b, where b = decimal odds − 1, p = true probability, q = 1 − p. It maximizes long-term bankroll growth but can be aggressive — most bettors use half-Kelly.
The excess over 100% is the vigorish (vig) — the bookmaker's margin. On a typical American football spread, implied probabilities might total 104-105%, meaning ~4-5% goes to the house.
Decimal odds are simplest for calculation (payout = bet × odds). American odds are standard in the US. Fractional odds are traditional in the UK. All encode the same information.
Even small edges (2-5%) are profitable long-term with proper bankroll management. But you need hundreds of bets for the edge to manifest — variance is high in the short run.