Probability Calculator
Enter the probability of two independent events to see the chance of both, either, neither, or exactly one.
Reviewed by the OmniCalc teamMethod verified 2026-07-01
A number from 0 to 1
A number from 0 to 1
25%
Both events 25%- Either A or B
- 75%
- Neither
- 25%
- Exactly one
- 50%
- Not A
- 50%
- Not B
- 50%
- Both A and B
- 25%
Show steps
- Both events (A and B), independent: P(A) × P(B) = 50% × 50% = 25%.
- Either event (A or B): P(A) + P(B) − P(A and B) = 50% + 50% − 25% = 75%.
- Neither event: (1 − P(A)) × (1 − P(B)) = 50% × 50% = 25%.
- Exactly one of them: P(A or B) − P(A and B) = 75% − 25% = 50%.
How to use the probability calculator
- 1Enter P(A) and P(B) as decimals from 0 to 1.
- 2Read the four combined results: both, either, neither and exactly one.
- 3Open Show steps to see each rule applied to your numbers.
The four rules at a glance
Both: P(A)·P(B). Either: P(A) + P(B) − P(A)·P(B). Neither: (1 − P(A))·(1 − P(B)). Exactly one is “either” minus “both.” All assume the events are independent.
Frequently asked questions
What does “independent” mean here?
Two events are independent when one happening tells you nothing about the other — a coin flip and a dice roll, say. This calculator multiplies for “both,” which is only valid for independent events.
How do I enter a probability?
As a number between 0 and 1: a 50% chance is 0.5, a 1-in-4 chance is 0.25. To go from a percentage, divide by 100.
Why isn't P(A or B) just P(A) + P(B)?
Adding them would double-count the overlap where both occur. The rule P(A or B) = P(A) + P(B) − P(A and B) subtracts that overlap once, so the total never exceeds 100%.