What is probability?
A number from 0 to 1 (or 0% to 100%) saying how likely something is. Zero means impossible, one means certain, and a half means an even chance like a coin flip.
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Probability Calculator
The chance of an event — favourable outcomes divided by all equally likely outcomes.
P = favourable outcomes / total outcomes
Frequently asked questions
How do I work it out?
For equally likely outcomes, divide the number of outcomes you want by the total number of possible outcomes. Rolling a 5 on a die is one favourable outcome out of six, so 1/6.
What does 'equally likely' mean and why does it matter?
It means each outcome has the same chance, like faces on a fair die. The simple favourable-over-total formula only works when that holds; a loaded die would need different maths.
What is the probability of the event NOT happening?
One minus the probability that it does. If there is a 0.25 chance of rain, there is a 0.75 chance of no rain. This calculator shows both.
Can probability be more than 1?
No. If your favourable count ever exceeds the total, something has gone wrong with the setup, which is why the calculator flags it.