±Free
Bayes probability
Calculate P(A|B) from prior P(A), P(B|A) and P(B|not A).
Enter P(A), P(B|A) and P(B|not A). The calculator combines the two possible sources of B to find P(B), then divides P(A) × P(B|A) by that evidence probability.
How to use
- Enter your text, values, or image.
- Adjust the options and run the tool.
- Copy or download your result.
Worked example
Example input
P(A): 0.5 P(B | A): 0.8 P(B | not A): 0.2
Example result
{
"evidenceProbability": 0.5,
"posterior": 0.8
}Limitations and details
All inputs must lie from 0 to 1; zero-probability evidence makes the posterior undefined, and the tool does not estimate likelihoods.
Inputs and options
- P(A)
- Number · Minimum: 0 · Maximum: 1
- P(B | A)
- Number · Minimum: 0 · Maximum: 1
- P(B | not A)
- Number · Minimum: 0 · Maximum: 1
Frequently asked questions
Is this tool free and private?
Yes. All 300 tools are free in this phase. Processing happens in your browser; your input and files are not sent to our servers.