Quick start
- Enter P(A) and P(B) as percentages.
- Leave P(A and B) blank for independent events, or enter it when known.
- Press Calculate probability.
- Review the union, intersection, complements, and formula steps.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Find the chance of A or B happening using the union rule.
- Calculate complements such as not A or not B.
- Assume independent events when no intersection is entered.
- Check classroom probability examples and quick planning estimates.
Independent events
When events are independent, one event does not change the chance of the other. The calculator uses P(A) times P(B) to estimate the intersection.
If you already know the overlap, type it into the intersection field instead of leaving it blank.
Union and complement
P(A or B) includes event A, event B, or both. The calculator uses P(A) + P(B) - P(A and B).
The complement of A is the chance that A does not happen, which is 100% - P(A).
Worked examples for Probability Calculator
P(A or B)=55%
P(A or B)=75%
P(not A)=60%
FAQ in plain language
What does P(A or B) mean?
P(A or B) is the probability that event A happens, event B happens, or both happen.
What formula does the calculator use for union?
It uses P(A or B) = P(A) + P(B) - P(A and B), so the overlapping part is not counted twice.
What do the main Probability Calculator inputs mean?
The main inputs are the numbers, operation, mode, or known values the calculator needs. Keep units consistent, enter percentages the way the page label shows, and use the examples as a quick check before trusting the answer.
How should I read the Probability Calculator answer?
Read the headline answer, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.
What should I double-check before trusting the Probability Calculator?
Check units, signs, rounding, and the selected mode before copying the answer. If the number feels weird, rerun one of the examples first, then put your own values back in slowly.
What happens if I leave P(A and B) blank?
The calculator assumes the events are independent and uses P(A and B) = P(A) x P(B).
What is a complement?
The complement of A is not A. Its probability is 1 - P(A), or 100% minus P(A) when using percentages.
Sources
Use these if you want to compare the formula, inputs, or limits with a trusted outside explanation.
Related tools
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Keep exploring
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Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.