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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Illustration for Probability Calculator showing calculate event union, intersection, complements, and independent-event probability. — Probability Calculator artwork matches the live tool workflow: calculate event union, intersection, complements, and independent-event probability. Use it with the calculator, examples, and result notes. View in the smoke-kawaii gallery

Inputs to check Example numbers Copyable answer Tab-only history

P(A) percentP(B) percentP(A and B) optional

Assuming independent eventsP(A or B) = 55%

P(A and B): 10%
Not A: 60%
Not B: 75%

Steps

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

How to use the Probability Calculator

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

What people use it for

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.

Quick 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%

Need the guide or a nearby tool?

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How to use the Probability Calculator See slower examples, input meanings, formula notes, result limits, and mistakes to avoid.
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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 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.

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.