Probability tree calculator
Enter the probability of the first event and the conditional probabilities at the second stage. The tool shows four mutually exclusive paths, their products and the sum leading to event B.
Your numbers stay in this browser. No account or upload to the server is needed.
How do I use this page?
- Name events A and B, then enter P(A).
- Enter P(B|A) and P(B|not A), each from 0 to 1.
- Read the path products and P(B); download the diagram or table if needed.
Method and worked example
The first level contains A and not A with probabilities P(A) and 1−P(A). At the second level each branch splits into B and not B.
Multiply probabilities along a complete path. The four terminal outcomes are mutually exclusive and their probabilities sum to 1.
The law of total probability gives P(B)=P(A)P(B|A)+P(not A)P(B|not A).
For P(A)=0.4, P(B|A)=0.75 and P(B|not A)=0.2, P(B)=0.4×0.75+0.6×0.2=0.42.