Use this Probability Calculator to calculate basic probability, complements, independent events, unions, and conditional probability. Inputs can be entered as decimals, percentages, or fractions.

Calculation Methods

Basic probability:

P(A) = favorable outcomes ÷ total outcomes

Complement:

P(not A) = 1 − P(A)

Independent events:

P(A and B) = P(A) × P(B)

Union of two events:

P(A or B) = P(A) + P(B) − P(A and B)

Conditional probability:

P(A|B) = P(A and B) ÷ P(B)

Example

If an event has 3 favorable outcomes from 12 equally likely outcomes:

P(A) = 3 ÷ 12 = 0.25

As a percentage:

0.25 × 100 = 25%

The complement is:

1 − 0.25 = 0.75, or 75%

Important Note

Probability values range from 0 to 1. Use the independent-event multiplication rule only when one event does not affect the probability of the other. Dependent events require conditional probability or another appropriate model.

Source

• OpenStax Introductory Statistics — Basic Rules of Probability

Disclaimer: This Probability Calculator uses standard probability formulas for educational and general estimation purposes. Results depend on accurate input values and correct event assumptions. Calculations may become misleading if dependent events are treated as independent, if events overlap unexpectedly, or if probability values outside the valid 0 to 1 range are used. For real-world risk, finance, health, legal, or safety decisions, use appropriate data and expert guidance.