Bayes Theorem Calculator

How to use Bayes Theorem Calculator

Posterior P(A|B) equals P(A)P(B|A) divided by P(A)P(B|A)+(1−P(A))P(B|not A). The inputs are your assumptions, not estimated prevalence or measured evidence.

1. Enter prior p(a), likelihood p(b|a), likelihood p(b|not a).
2. Choose the stated convention and check the units and supported model.
3. Calculate, inspect the result, then copy or download it.

Good to know

• Probabilities0–1; zero evidence denominator rejects. No clinical defaults, diagnosis, causal certainty or inferred likelihoods.
• Runs locally. Text/JSON fields are capped at30,000 characters; stricter page-specific and numeric-string limits also apply. Numeric inputs are bounded to magnitude1e15 and nonzero magnitude at least1e-100 unless an exact-string mode states otherwise. Approximate summaries use12 significant digits; CSV numeric cells retain the complete stored value and potentially formula-like string cells gain a protective apostrophe; unsupported, non-finite or detected underflow states return an error.