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Probability calculator guide

How to use the Probability Calculator

The Probability Calculator finds P(A and B), P(A or B), complements, and independent-event intersections from percentage inputs.

Open the probability calculator

Quick start

  1. Enter P(A) and P(B) as percentages.
  2. Leave P(A and B) blank for independent events, or enter it when known.
  3. Press Calculate probability.
  4. Review the union, intersection, complements, and formula steps.

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).

Examples from the calculator

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%

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