Probability Calculator
Calculate probabilities of two events — both, either, neither and conditional — plus the chance of at least one success in repeated trials.
How to use
- 1Enter the probability of event A and event B (as % or decimal).
- 2Enter the number of trials to see the chance of at least one success.
How it's calculated
For independent events: P(A and B) = P(A) × P(B); P(A or B) = P(A) + P(B) − P(A and B); P(neither) = (1 − P(A))(1 − P(B)). At least once in n trials = 1 − (1 − p)ⁿ.
Frequently asked questions
What does independent mean?
One event doesn't change the chance of the other — like two coin flips. Drawing cards without replacement is not independent.
What are the chances of rolling at least one six in 4 rolls?
Each roll misses with probability 5/6, so it's 1 − (5/6)⁴ ≈ 51.8% — slightly better than even.
Percent or decimal?
Either — values above 1 are treated as percentages, so 25 and 0.25 both mean 25%.