Conditional Probability Calculator
Find P(A given B) from joint and marginal probabilities.
Probability given evidence
Conditional probability updates a chance in light of what you already know: the probability of A given B. It divides the joint probability of both by the probability of the condition.
If A and B occur together 20% of the time and B occurs 50% of the time, then P(A given B) is 0.4. This idea underpins Bayes’ theorem and evidence-based reasoning.
Fundamental and private
The calculation runs in your browser, so nothing you enter is uploaded.
Frequently Asked Questions
What is conditional probability?
The probability of A given that B has happened — the joint probability of A and B divided by the probability of B.
What do I enter?
The probability that both A and B occur, and the probability of B.
Why does it matter?
It is the heart of Bayesian reasoning and updating beliefs with new evidence.
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