Probability Calculator
Calculate the probability of single and combined events — AND, OR, and the complement — from individual event probabilities.
P(A and B)
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P(A or B) —
P(not A) —
P(neither) —
Assumes A and B are independent events.
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How it works
P(A and B) = P(A)·P(B); P(A or B) = P(A)+P(B)−P(A and B); P(not A) = 1−P(A)Frequently Asked Questions
How do I calculate combined probability?
For independent events, 'both happen' multiplies the probabilities and 'either happens' adds them minus the overlap. Two fair coins both landing heads is 0.5 × 0.5 = 0.25; at least one heads is 0.75.
What is the difference between AND and OR probability?
AND (both events) is always less likely than either alone and uses multiplication; OR (at least one) is more likely and uses addition minus the shared case. Mixing them up is the classic probability mistake.
What does independent mean here?
One event's outcome doesn't affect the other's — like separate coin flips. These formulas assume independence; dependent events (drawing cards without replacing) need conditional probability instead.