Probability Calculator (Two Events)

How to use this calculator

Enter the probabilities of two independent events A and B (as percentages). All combined probabilities are calculated.

Formulas

P(A and B) = P(A) × P(B)
P(A or B) = P(A) + P(B) − P(A)P(B)
P(not A) = 1 − P(A)

Worked example

P(A) = 50%, P(B) = 30%:

• Both = 15%
• Either = 50 + 30 − 15 = 65%
• Neither = 35%

At least once in several tries

The chance of at least one six in four dice rolls is 1 − (5/6)⁴ = 51.8% — a classic gambling puzzle.

Frequently asked questions

How do I find the probability of two events both happening?

For independent events, multiply — P(A and B) = P(A) × P(B).

How do I find the probability of either event?

P(A or B) = P(A) + P(B) − P(A and B), so the overlap isn't counted twice.

What does independent mean?

One event doesn't change the chance of the other, like two separate coin flips.

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