Basic probability compares the outcomes you want with all possible outcomes. The same result can be displayed as a fraction, decimal or percentage.
The basic formula
Probability equals favorable outcomes divided by total outcomes. One favorable result in six equally likely outcomes is 1/6, or about 16.67%.
Check the assumptions
The simple formula assumes the outcomes are counted correctly and are equally likely. Real-world events may need a different model or more data.
Use probability for comparison
A probability is not a promise that an event will happen in a single trial. It describes the expected share over repeated comparable trials.
- Keep favorable outcomes no greater than total outcomes.
- Use the same event definition each time.
- Convert the result to the format your audience understands.
Frequently asked questions
Can probability be greater than 100%?
No. A valid probability is between 0% and 100%, inclusive.
Does a 20% chance mean it will happen once in five tries?
Not exactly. It is a long-run expectation, not a guarantee for a small number of trials.