For fractional format, enter it with a slash, like "5/1" or "3/2". For American format, the "+" sign before a positive number is optional.
Result
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Facts and limits of this method
| Decimal formula | probability = (1 / odds) × 100% |
|---|---|
| Fractional a/b formula | probability = (b / (a + b)) × 100% |
| American (+) formula | probability = (100 / (odds + 100)) × 100% |
| American (−) formula | probability = (|odds| / (|odds| + 100)) × 100% |
When it misleads you
- The result is the "implied" probability baked into the odds, not the objective probability of the event — bookmaker odds include a margin (overround), so the implied probabilities of all outcomes of an event usually add up to more than 100%.
- Decimal odds must be greater than 1 — odds equal to or below 1 don't make sense (they would mean a guaranteed loss with no possible win).
- For the fractional format, the first number is the net profit per unit staked, the second is the stake itself; for example, 5/1 means winning 5 units for every 1 unit staked.
- Rounding is applied only to the displayed result — internal calculations use the full precision of JavaScript floating-point numbers.
How it is calculated
For decimal odds, probability is computed as 100% divided by the odds themselves — the higher the odds, the lower the implied probability.
For fractional odds a/b (such as 5/1), probability is computed as b / (a + b), multiplied by 100%.
For positive American odds (such as +150, an underdog bet), probability is computed as 100 / (odds + 100), multiplied by 100%.
For negative American odds (such as −200, a favorite bet), probability is computed as the absolute value of the odds divided by (absolute value of the odds + 100), multiplied by 100%.
Questions and answers
Why do the probabilities of all outcomes usually add up to more than 100%?
Because odds include the bookmaker's margin (overround) — their built-in profit; this causes the implied probabilities of all possible outcomes to sum to more than 100%, rather than exactly 100%.
What do positive and negative American odds mean?
Positive odds (such as +150) show how much you'd win on a 100-unit stake — used for less likely outcomes; negative odds (such as −200) show how much you'd need to stake to win 100 units — used for more likely outcomes (favorites).
How do I enter fractional "evens" odds (1/1)?
Enter it as "1/1" — this corresponds to an implied probability of 50%.
Is my data sent anywhere?
No, the whole calculation runs in your browser.