Odds to Probability Calculator
Enter odds written as a to b in favour and get the probability as a percentage and a decimal — see exactly how 1 to 4 becomes a 20 % chance.
Percentage and decimal at once
Enter the odds for and against and the calculator returns the probability as a percentage (out of 100) and as a decimal (between 0 and 1) together.
Odds, not a ratio
Odds of "a to b" means a favourable chances and b unfavourable ones — the probability is a ÷ (a + b), not a ÷ b.
What does the odds to probability calculator do?
From a to b odds to a probability
The odds to probability calculator converts odds, written as "a to b in favour", into a probability. Odds describe how many favourable chances (a) there are against how many unfavourable ones (b); probability instead asks what share of all chances are favourable. The tool takes the two numbers and returns the probability both as a percentage out of 100 and as a decimal between 0 and 1. It is the number behind reading bookmaker lines, framing test-result likelihoods, and translating any "a to b" statement into a plain chance.
Enter the favourable chances (a) and the chances against (b) to get the probability as a percentage and a decimal instantly.
To turn odds into a probability, divide the favourable chances by the total number of chances — favourable plus unfavourable.
P = a ÷ (a + b)The total (a + b) is every possible chance, and a is the number that count as a success, so the ratio is the probability. Multiply by 100 to read it as a percentage. Suppose the odds are 1 to 4 in favour: the total is 1 + 4 = 5 chances, the favourable share is 1 ÷ 5 = 0.2, and 0.2 × 100 = 20 %. So odds of 1 to 4 in favour correspond to a probability of 0.2, or a 20 % chance of the event happening.
The conversion is exact, but a couple of points are worth keeping in mind.
Odds in favour, and the direction matters
This calculator reads the odds as "a to b in favour", so a is the favourable side and b is against. If a source quotes odds against (the more common gambling convention), swap the two numbers first. The tool computes the mathematical probability from the numbers you enter and is for educational use — it does not account for bookmaker margins, real-world bias, or anything beyond the ratio itself.