Compute Probability Calculator
Compute probability calculator works out P(not A) and P(not B) from Probability of event A, Probability of event B and Probability of A and B together. The default figures on this page give P(not A) of 0.5; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
1 step
- P(not A) = 1 - 0.5 = 0.5.
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
4 sources
What the compute probability calculator calculates
Computes the probability of single and combined events: the complement, the union and intersection of two events, neither and exactly one, conditional probability and Bayes' theorem, the chance of at least one occurrence over repeated trials, and the mean and variance of a discrete distribution.
How to use this compute probability calculator
Enter Probability of event A, Probability of event B, Probability of A and B together and Probability of A given B. P(not A) updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind P(not A), P(not B) and P(A and B).
The method follows Kolmogorov axioms and the addition rule: P(A or B) = P(A) + P(B) - P(A and B), OpenStax, Introductory Statistics 2e, chapter 3 "Probability Topics", Bayes' theorem: P(A|B) = P(B|A) P(A) / P(B) and NIST/SEMATECH e-Handbook of Statistical Methods, 1.3.6.2 Related Distributions.
Two independent events with probabilities 0.5 and 0.4 example
With Probability of event A: 0.5, Probability of event B: 0.4, Treat A and B as independent: true, the calculator returns P(A and B): 0.2, P(A or B): 0.7, P(neither A nor B): 0.3. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects P(not A), P(not B) and P(A and B). The inputs you can vary are Probability of event A, Probability of event B, Probability of A and B together, Probability of A given B, Number of independent repetitions and Outcome values.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to compute probability formula?
Computes the probability of single and combined events: the complement, the union and intersection of two events, neither and exactly one, conditional probability and Bayes' theorem. Open “How this is worked out” beside the result to see each step for your own figures.
How to use TI-84 to calculate probability?
It needs Probability of event A, Probability of event B, Probability of A and B together, Probability of A given B, Number of independent repetitions and Outcome values. Nothing else is required, and the figures stay in your browser.
How do you calculate the probability of something happening multiple times?
The result gives P(not A), P(not B), P(A and B), P(A or B), P(neither A nor B) and P(exactly one of A, B), each labelled with its unit so the figure can be read on its own.
How to calculate probability with a calculator?
The page opens with worked default figures: P(not A) of 0.5. Replace them with your own and every figure updates as you type.
What information do I need for the compute probability calculator?
You need Probability of event A, Probability of event B, Probability of A and B together, Probability of A given B, Number of independent repetitions, Outcome values, Outcome probabilities and Question asked. The page opens with default figures that give P(not A) of 0.5, so you can see the scale before entering your own.
What does this compute probability calculator show?
It shows P(not A), P(not B), P(A and B), P(A or B), P(neither A nor B), P(exactly one of A, B), P(A given B) and P(B given A), with the working and any warnings for your entries.