Math & Education Utility
Probability Calculator
Calculate an event probability from favorable and total equally likely outcomes.
Calculate simple probability
Method
For a finite set of equally likely outcomes, simple event probability is the number of favorable outcomes divided by the total number of outcomes. The complement is one minus that probability. The tool reports both decimal and percentage forms.
Worked example
With 3 favorable outcomes among 10 total outcomes, the event probability is 0.3000 or 30.00%, and the complement is 70.00%.
When this is useful
This calculator is useful for introductory probability exercises, classroom examples, simple games of chance analysis, and quick checks where all listed outcomes have the same probability.
Privacy
The counts and calculation stay in the browser. No probability inputs are sent to an external service.
Limitations
The favorable-over-total formula assumes equally likely, mutually exclusive outcomes. It does not handle weighted outcomes, conditional probability, dependence, continuous distributions, or uncertainty in estimated probabilities.
FAQ
What does the complement mean?
The complement is the probability that the event does not occur. For a 30% event, the complement is 70%.
Can I use unequal or weighted outcomes?
Not in this version. The favorable-over-total method assumes the counted outcomes are equally likely.
Related tools