🎲 Odds & Probability Calculator
Convert odds to a probability, or a probability to odds — with decimal (betting) odds shown too.
Probability
75%
Odds (for : against)
3 : 1
Decimal odds
1.333
Odds of A:B "in favour" mean A chances to win for every B to lose, so the probability is A ÷ (A + B). A 3:1 favourite therefore has a 75% chance. Decimal (European) odds are 1 ÷ probability — the total payout per unit staked. Fractional and American odds are just other ways of writing the same ratio. 🔒 In your browser.
How the odds & probability calculator works
Odds and probability describe the same chance in different ways. Odds of A:B "in favour" mean A chances of the event happening for every B chances of it not — so the probability is A ÷ (A + B). A 3:1 favourite has a 3 ÷ 4 = 75% chance. The tool converts either direction: enter odds to get the probability and decimal odds, or enter a probability percentage to get the simplified odds and the decimal odds (1 ÷ probability, the total return per unit staked).
Be clear whether odds are quoted "for" (chances to win first) or "against" (chances to lose first) — 3:1 on and 3:1 against are very different. Decimal (European) odds include your stake in the payout; fractional (UK) and American (moneyline) odds are just other notations for the same underlying ratio.
Frequently asked questions
How do I convert odds to probability?
For odds of A:B in favour, probability = A ÷ (A + B). So 3:1 gives 3 ÷ 4 = 75%, and 1:4 gives 1 ÷ 5 = 20%. Enter the two numbers and the tool returns the percentage and decimal odds.
How do I convert probability to odds?
Odds "for : against" are probability : (1 − probability). A 75% chance is 0.75 : 0.25 = 3:1. This calculator simplifies the ratio to whole numbers and also shows the decimal odds.
What are decimal odds?
Decimal (European) odds are 1 ÷ probability — the total amount returned per unit staked, stake included. A 25% chance is decimal odds of 4.0, meaning a 1-unit bet returns 4 units (a 3-unit profit). They map directly to the implied probability.
What is the difference between "odds for" and "odds against"?
"Odds for" (or "on") lists chances to win first, e.g. 3:1 on = 75% chance. "Odds against" lists chances to lose first, e.g. 3:1 against = 25% chance. Always check which way a quote is stated, as they invert the probability.
What is implied probability in betting?
The probability that a set of odds corresponds to: 1 ÷ decimal odds, or A ÷ (A + B) for fractional odds. Bookmakers set odds so the implied probabilities sum to more than 100% (the "overround"), which is their margin.