Probability Calculator
Calculate the probability of two events occurring together or separately, and the binomial chance of exactly k successes in n trials. Free, instant and easy to read.
Two events
Binomial: exactly k successes
About Probability Calculator
The probability calculator handles two classic setups. For two events A and B, enter P(A) and P(B), as percentages or decimals, and it returns P(A and B), P(A or B) and P(not A), under your choice of independent or mutually exclusive events.
The distinction drives the math: independent events multiply, so P(A and B) = P(A) × P(B), while mutually exclusive events cannot co-occur, making P(A and B) zero and P(A or B) a simple sum. The general addition rule P(A or B) = P(A) + P(B) − P(A and B) ties it together.
A separate binomial section computes the chance of exactly k successes in n independent trials with success probability p, the “exactly 3 heads in 10 flips” kind of question, free and instant.
How to use Probability Calculator
- Enter P(A) and P(B) as percentages (e.g. 50) or decimals between 0 and 1.
- Choose whether the events are independent or mutually exclusive.
- Read P(A and B), P(A or B) and P(not A).
- For binomial problems, enter the trials n, successes k and success probability, then read P(exactly k).