Instantly calculate event probabilities — AND, OR, and complement rules
Calculation Mode
Event Probability
Enter values as decimals (0.5), fractions (1/6), or percentages (50%).
Result
50%
Probability Formulas
Complement Rule
P(not A) = 1 − P(A)
The probability that an event does NOT occur.
AND — Independent Events
P(A and B) = P(A) × P(B)
Both events occur; valid only when A and B are independent.
AND — Dependent Events
P(A and B) = P(A) × P(B|A)
Use when the outcome of A affects the probability of B.
OR — General (Inclusion-Exclusion)
P(A or B) = P(A) + P(B) − P(A∩B)
At least one of A or B occurs; subtracts the overlap.
OR — Mutually Exclusive
P(A or B) = P(A) + P(B)
Events cannot both happen at the same time, so P(A∩B) = 0.
Odds In Favour
Odds = P(A) : P(not A)
Ratio of success to failure; e.g. "3 : 2" means 3 wins per 2 losses.
A probability calculator helps you find the likelihood of events occurring — whether you need to know the chance of a single outcome, both events happening together (AND), at least one event occurring (OR), or the complement (the chance something does NOT happen). Enter any value as a decimal, fraction, or percentage and get instant results with visual probability bars.
Multiply their individual probabilities: P(A and B) = P(A) × P(B). For example, rolling a 6 on a die (1/6) AND flipping heads on a coin (1/2) gives 1/12 ≈ 0.083. Select the 'Both Events' mode and choose 'Independent events'.
The complement P(not A) = 1 − P(A). It represents the probability that an event does NOT happen. If P(rain today) = 0.3, then P(no rain) = 0.7.
Use P(A or B) = P(A) + P(B) − P(A∩B) whenever events can overlap (i.e., they are not mutually exclusive). If drawing a card that is a Heart OR a Queen, the Queen of Hearts is counted in both, so it must be subtracted once.
Yes. The calculator accepts decimals (0.25), fractions (1/4), and percentages (25%) interchangeably for all inputs.