Convert decimal odds into implied probability, fractional odds, and American odds instantly. Optionally add a second outcome to calculate bookmaker margin.
Add for margin calculation in a 2-way market
Outcome 1 (decimal: 2.50)
Implied Prob
40.00%
Fair Odds
2.50
Fractional
3/2
American
+150
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Implied probability is the probability of an outcome as implied by the bookmaker's odds. For decimal odds of 2.50, the implied probability is 1/2.50 = 40%. It tells you how often the bookmaker expects this outcome to occur, including their built-in margin.
Value exists when your own estimated probability for an outcome is higher than the implied probability in the odds. If you believe a team has a 50% chance of winning but the implied probability is 40%, there is potential value in that bet. Compare your model's estimates against the implied probability to find discrepancies.
Bookmaker margin (overround) is the built-in profit edge. In a true 50/50 market, fair odds would be 2.00 each, but a bookmaker might offer 1.90, making the margin around 5.3%. Over time, this margin is the primary reason most bettors lose. Choosing bookmakers with lower margins gives you a better chance long-term.
Implied probability includes the bookmaker's margin, so the sum of implied probabilities across all outcomes in a market exceeds 100%. True probability (also called fair probability) is the implied probability scaled so all outcomes sum to exactly 100%, removing the margin. This calculator shows fair odds based on the decimal odds you enter.
Decimal to fractional: subtract 1 and express as a simplified fraction (e.g. 2.50 - 1 = 1.5 = 3/2). Decimal to American: if odds are 2.00 or above, multiply (decimal - 1) by 100 and add a + sign. If below 2.00, divide -100 by (decimal - 1). This calculator performs all these conversions automatically as you type.
