Bayes Theorem Calculator
Calculate bayes theorem with explicit inputs and a worked example.
Formula
- Use the units shown beside each field.
- Results update automatically whenever you change an input.
- Displayed values use up to four decimal places, or scientific notation for very small or large numbers. Calculations use standard floating-point arithmetic; exact integer and fraction tools identify their own precision rules.
Example Calculation
- Prior P(A): 0.2
- P(B given A): 0.8
- P(B given not A): 0.1
Prior 0.2, likelihood 0.8 and alternative likelihood 0.1 gives posterior 2/3.
Frequently asked questions
How do I use this calculator?
Enter prior p(a), p(b given a), p(b given not a). The result updates automatically. Use Reset to restore the example values.
What method does it use?
P(A|B) = P(B|A)P(A) / [P(B|A)P(A)+P(B|not A)(1−P(A))]. Follow the units and definitions shown with the inputs to apply this relationship consistently.
Why might a rounded result differ?
The calculation keeps full numeric precision internally, then rounds the displayed result. Rounding intermediate steps by hand can produce a slightly different answer.
Are my inputs saved online?
No. This calculator processes your inputs in your browser. A recently used list stores only calculator names on this device, not your entered values.
Check the assumptions and units before using this result. Read about calculation methods.