Odds to Probability Calculator
Convert betting odds — “A to B” for or against — into a clean probability percentage, decimal and implied fraction.
How it works
Odds A:B against → P = B ÷ (A + B) · Odds A:B for → P = A ÷ (A + B) · Decimal odds = 1 ÷ P
Odds compare the two ways an event can go, while probability expresses one side as a share of all outcomes. For odds of A to B against, there are A ways to lose and B ways to win, so the win probability is B ÷ (A + B). Reading the label matters: “3 to 1 against” is a 25% chance, but “3 to 1 in favor” is a 75% chance — the same numbers, opposite meaning.
Worked example
Odds of 3 to 1 against mean 3 ways to fail and 1 way to succeed, for 4 outcomes total. The probability of success is 1 ÷ (3 + 1) = 25%. The matching decimal (payout) odds are 1 ÷ 0.25 = 4.00.
Sources: Odds — Wikipedia
Frequently asked questions
What is the difference between odds and probability?
Probability is the chance out of all outcomes (0 to 100%). Odds compare favorable to unfavorable outcomes as a ratio. 3-to-1 against equals a 25% probability.
How do I read “3 to 1 against”?
It means three unfavorable outcomes for every one favorable outcome — four total — so the event happens 1 in 4 times, a 25% chance.
What are decimal odds?
Decimal odds are 1 ÷ probability and represent the total payout per unit staked, including the stake. A 25% chance is 4.00 decimal odds.
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This calculator is for educational purposes. Double-check important results before acting on them.