Probability Calculator

Calculate the probability of A and B, A or B, not A, exactly one, neither, and of an event happening at least once in n tries.

How do I calculate the probability of two events?

Enter the probability of A and optionally B as a decimal, percentage or fraction. For independent events the calculator gives P(A and B) = P(A) × P(B), P(A or B) = P(A) + P(B) − P(A)P(B), the complements, exactly one and neither, plus the chance that A happens at least once in a number of tries.

Example

The chance of rolling a six is 1/6 (16.67%). The chance of at least one six in six rolls is 1 − (5/6)⁶ ≈ 66.5% — not 100%.

Independent vs dependent events

These formulas assume one event does not affect the other, like two coin flips. Drawing cards without replacement is dependent and needs conditional probability.

Frequently asked questions

What formats can I use?

0.25, 25% or 1/4 all work.

What are odds?

Odds 1 : 5 mean one success for every five failures (a 1/6 probability).

Is it free?

Yes.