Odds to probability calculator
Odds of 1 : 4 in favour mean one success for every 4 failures, a 20% chance. Enter both sides and pick the direction.
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How to use it
- Enter first number a, second number b and odds type.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Odds in favour a : b → probability = a ÷ (a + b). Odds against a : b → probability = b ÷ (a + b).
Example
Inputs: First number a = 1, Second number b = 4, Odds type = In favour. Result: Probability of the event: 20%; As a decimal: 0.2; Probability it does not happen: 80%.
Limits and caveats
- Handles ratio odds a : b only, not decimal or American betting formats.
Questions about this tool
Are odds the same as probability?
No. Odds compare successes with failures, probability compares successes with all cases. Odds of 1 : 1 mean a 50% chance.
What do odds of 3 : 1 against mean?
The event fails 3 times for every success on average, so the chance is 1 ÷ 4 = 25%.
Can I use decimals?
Yes, 2.5 : 1 works and gives the same result as 5 : 2.