Probability calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Independent events: P(A and B) = P(A) × P(B) = 0.4 × 0.5 = 0.2.
- P(A or B) = P(A) + P(B) − P(A and B) = 0.7.
- These results assume A and B do not affect each other.
| Inputs | Answer |
|---|---|
| Probability of A: 0.4; Probability of B: 0.5 | If A and B are independent, P(A and B) = 0.2 and P(A or B) = 0.7. |
| Probability of A: 25%; Probability of B: 10% | If A and B are independent, P(A and B) = 0.025 and P(A or B) = 0.325. |
| Probability of A: 0.5; Probability of B: 0.5 | If A and B are independent, P(A and B) = 0.25 and P(A or B) = 0.75. |
How to use it
For independent events, the probability that both happen is P(A) × P(B), and the probability that at least one happens is P(A) + P(B) − P(A and B). The probability that an event does not happen is 1 minus its probability.
The calculator assumes the two events do not influence each other (a coin flip and a die roll, for example). If they do, you need the conditional probability, which this page does not calculate.
Frequently asked questions
How do I find the probability of A and B?
For independent events multiply: 0.4 × 0.5 = 0.2.
How do I find the probability of A or B?
Add the two and subtract the overlap: 0.4 + 0.5 − 0.2 = 0.7.
What are independent events?
Events where one happening does not change the chance of the other.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.