Probability Calculator
Enter the probability of event A (and optionally event B) to see combined outcomes.
| P(A) | 50% |
| P(not A) | 50% |
| P(B) | 30% |
| P(A and B) | 15% |
| P(A or B) | 65% |
Basic Probability Rules
A probability is always between 0 (impossible) and 1 (certain), often written as a percentage. The probability that an event does not happen, P(not A), is simply 1 − P(A). Combining two events depends on how they relate to each other.
Independent Events
Two events are independent when one happening has no effect on the other (like two separate coin flips). For independent events, P(A and B) = P(A) × P(B), and P(A or B) = P(A) + P(B) − P(A and B).
Mutually Exclusive Events
Two events are mutually exclusive when they can never happen at the same time (like rolling a 2 and a 5 on a single die roll). In that case P(A and B) = 0, and P(A or B) is simply P(A) + P(B).