Probability Calculator

Enter the chance of two independent events as percentages. The calculator gives the probability of both, either, neither and exactly one, plus the chance of the first event happening at least once over several tries.

Type a number and the answer appears straight away.

Your answer

Both A and B happen15%

A or B (at least one)
65%
Exactly one of them
50%
Neither happens
35%
A does not happen
50%
A at least once in 5 tries
96.875%
A every time in 5 tries
3.125%

For independent events, multiply the chances to get both. For either, add them and subtract the chance of both.

The math behind it: Probability

P(A and B) = P(A) x P(B); P(A or B) = P(A) + P(B) - P(A and B)

With a 50% chance of A and a 30% chance of B, there is a 15% chance of both and a 65% chance of at least one. Over five independent tries, a 50% event happens at least once 96.875% of the time.

Common questions

How do I calculate probability?

Divide the number of outcomes you want by the total number of equally likely outcomes. The chance of rolling a 4 on a six-sided die is 1 / 6, or 16.67%.

How do I work out the probability of two events both happening?

If they are independent, multiply their probabilities. A 50% chance and a 30% chance together give 0.5 x 0.3 = 0.15, or 15%.

What is the chance of rolling at least one six in four rolls?

About 51.77%. It is easier to work out the chance of no six, which is (5/6) to the power of 4, and subtract that from 1.

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