Probability Calculator
Enter the probabilities of two independent events to find their combined and complementary probabilities.
Assumes A and B are independent: P(A) × P(B).
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Formula
About this calculator
Probability measures how likely an event is to happen, on a scale from 0 (impossible) to 1 (certain). This calculator works with two independent events, A and B, meaning the outcome of one does not affect the other — like rolling two separate dice or flipping two coins.
For independent events, the probability that both happen is the product P(A) × P(B). The probability that at least one happens uses the addition rule with the overlap removed: P(A) + P(B) − P(A and B). The complement, P(not A) = 1 − P(A), gives the chance that an event does not occur. If your events are dependent or mutually exclusive, these formulas change accordingly.
Frequently asked questions
Two events are independent when the occurrence of one does not change the probability of the other. For independent events, P(A and B) equals P(A) × P(B).
Adding P(A) and P(B) double-counts the outcomes where both occur. Subtracting P(A and B) removes that overlap so each outcome is counted once.
If A and B cannot both happen, then P(A and B) = 0, so P(A or B) simply becomes P(A) + P(B). This calculator assumes independence rather than mutual exclusivity.
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