Why this tool is useful
Check a transparent Bayes update for teaching, diagnostics or evidence review without hiding any assumed probability.
Everyday Calculators · runs locally
Bayes Theorem Calculator
Calculate a posterior probability from an explicit prior, likelihood and evidence probability.
Ready. Your working data stays in this browser.
How it works
It applies P(A|B) = P(B|A) × P(A) ÷ P(B), converting the entered percentages consistently.
Practical example
A 10% prior, 80% likelihood and 20% evidence probability produce a 40% posterior.
Validation
The calculation is deterministic and explains validation errors instead of silently changing invalid input.
Privacy boundary
Input and output remain in this tab. Copy and download happen through browser APIs without a server upload.
Result passport
Every run reports its tool mode, timestamp and input/output size so transformed data can be audited.