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Probability Calculator

Use this free probability calculator to find P(A and B), P(A or B), complements, independent-event intersections, steps, copy, and history.

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Formula steps Examples included Copy results Private history
Assuming independent eventsP(A or B) = 55%
P(A and B)
10%
Not A
60%
Not B
75%

Steps

  1. Convert entered percentages to decimals.
  2. Use P(A and B) = P(A) x P(B).
  3. Use P(A or B) = P(A) + P(B) - P(A and B).

How to use the probability calculator

  1. Enter P(A) and P(B) as percentages.
  2. Leave the intersection blank for independent-event multiplication, or enter P(A and B).
  3. Press Calculate probability to see union, intersection, complements, and steps.
  4. Use examples, recent answers, or copy the result while checking probability problems.

Common uses

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.

Examples

Independent events P(A)=40%, P(B)=25%

P(A or B)=55%

Known overlap P(A)=60%, P(B)=30%, P(A and B)=15%

P(A or B)=75%

Complement P(A)=40%

P(not A)=60%

Frequently asked questions

Quick answers about formulas, inputs, examples, result copying, and private in-browser history.

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 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.

Can probabilities be more than 100%?

No. Each probability must be between 0% and 100%, and the final union cannot be more than 100%.

Is my probability history private?

Yes. Recent probability answers stay only in the current browser tab while you use the page. They are not sent to a server.

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