Calculator tool
How this calculator works
Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.
Basic Event Probability
For equally likely outcomes, probability is:
If a fair die has 6 faces and only one face is a 4, then the chance of rolling a 4 is .
The Complement
The complement means the event does not happen:
If the chance of rain is 30%, the chance of no rain is 70%.
Independent Events
Two events are independent only when one does not change the other. Two separate die rolls can be treated that way. Drawing two cards from one deck without replacement cannot, because the first card changes what remains.
Input Checks
Favorable outcomes cannot be greater than total outcomes. If they are, the inputs do not describe a real probability situation.
Frequently asked questions
What does the complement mean?
It is the probability that event A does not happen. If , then the complement is 0.70.
When can I multiply probabilities?
Only when the events are independent. If one event changes the chance of the other, multiplication alone is not enough.
Why is favorable greater than total invalid?
Because favorable outcomes must be part of all possible outcomes. You cannot have more successful outcomes than total outcomes.
Why subtract the overlap for A or B?
Because adding and counts cases where both happen twice, so the overlap must be removed once.
