About the Probability Calculator
Enter the probability of one or two independent events to get the chance they both happen, that at least one happens, that neither happens, and the chance an event occurs at least once over several tries.
How it’s calculated
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 tries) = 1 − (1 − P)ⁿ.
Example calculation
With P(A) = 50% and P(B) = 25%, both happen 12.5% of the time and at least one happens 62.5% of the time.
Frequently asked questions
What does 'independent' mean?
That one event doesn't change the odds of the other — like separate coin flips. If events influence each other, these formulas don't apply directly.
How is 'at least once' calculated?
It's easier to find the chance of never succeeding — (1 − P) every try — and subtract from 1. Even small per-try odds add up over many tries.
This tool provides estimates for general informational and educational purposes only. Results are based on the values you enter and standard formulas, and may not reflect your specific circumstances.