Frequently Asked Questions
Everything you need to know about the Probability Calculator.
What's the difference between AND and OR probability?
P(A AND B) is the chance both events happen; P(A OR B) is the chance at least one happens.
How do I calculate P(A OR B)?
Using P(A) + P(B) − P(A AND B), which avoids double-counting the overlap.
What is conditional probability?
The probability of one event occurring given that another has already occurred, written P(A|B).
What is Bayes' theorem used for?
Reversing a conditional probability — finding P(A|B) when you know P(B|A), P(A), and P(B).
What does "independent events" mean here?
If A and B are independent, knowing one occurred doesn't change the probability of the other, so P(A AND B) simply equals P(A) × P(B).
What if my inputs are inconsistent?
The calculator checks that your joint probability doesn't exceed either individual probability, and flags impossible combinations with an explanation.
Does it show a diagram?
Yes, a Venn diagram visualizes the overlap between your two events.
Does it show the formulas used?
Yes, every probability rule is shown with your specific numbers substituted in.
Is my data stored?
No, everything runs client-side in your browser.
Is it free?
Yes, completely free with no sign-up or limits.