Probability
Single and combined event probabilities.
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The formula
independent: P(A∩B) = P(A)·P(B); P(A∪B) = P(A)+P(B)−P(A∩B)
For independent events, “and” multiplies and “or” adds — but the “or” rule must subtract the overlap, otherwise the cases where both happen get counted twice. Probabilities always sit between 0 (impossible) and 1 (certain).
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Common questions this calculator answers — select one to load its values.
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Frequently asked questions
Does this assume independence?+
Yes — the "and" rule shown multiplies probabilities, which is valid only when the two events do not affect each other.
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