Using the Probability Calculator
Independence means learning that one event occurred does not change the probability of the other. That assumption is essential for multiplying the two probabilities to find their overlap.
Worked example
Use the example inputs below to reproduce this result. These are the form's starting values, not recommended targets. Changing your inputs updates your result above; this worked example stays fixed for comparison. Results are rounded for display.
Example inputs
- Probability A
- 0.5
- Probability B
- 0.25
Example result
- Both A and B
- 0.125
- A or B (inclusive)
- 0.625
- Neither
- 0.375
- Exactly one
- 0.5
Method and assumptions
Assumes independent events. Probabilities range from 0 to 1.
Define the event before calculating
A probability or count depends on what qualifies as an outcome. Decide whether order matters, whether items can repeat and whether events are independent before choosing inputs. Two questions using the same numbers can have different answers when these assumptions differ. Express a probability consistently as a fraction, decimal or percentage when comparing results.
Common question
Does A or B include the case where both happen?
Yes. The inclusive A-or-B result counts A, B and their overlap once. The exactly-one result excludes the overlap.
Guide updated .