Probability Calculator
Find the probability of two independent events happening together, either one happening, or something happening at least once over several tries.
Both A and B
15%
- A or B (or both)
- 65%
- Exactly one of them
- 50%
- Neither
- 35%
With independent chances of 50% and 30%, both happen 15% of the time and at least one 65%.
How it's worked out
- Both
50% × 30%15% - Either
50% + 30% − both65%
The rules
P(A and B) = P(A) × P(B) · P(A or B) = P(A) + P(B) − P(A and B) · P(at least once in n) = 1 − (1 − p)ⁿ
Worked example
With chances of 50% and 30%, both happen 15% of the time, at least one 65%, exactly one 50% and neither 35%.
Questions people ask
How do I find the probability of A and B?
For independent events, multiply: a 50% and a 30% chance give 0.5 × 0.3 = 15%.
How do I find the probability of A or B?
Add them and subtract the chance of both: 50% + 30% − 15% = 65%.
What is the chance of something happening at least once?
Work out the chance it never happens and subtract from 1. A 10% chance tried 10 times: 1 − 0.9¹⁰ = 65.1%.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem