Probability Calculator
Calculate basic probability, complements, independent events, unions, and conditional probability using decimals, percentages, or fractions.
Probability inputs can be entered as decimals, percentages, or fractions. Examples: 0.25, 25%, or 1/4.
Results
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
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.