Probability Calculator
This calculator computes basic probabilities using fundamental rules. Enter probabilities for events A and B, and the calculator shows P(A), P(not A), P(A or B), and P(A and B) for independent events. Probability is the foundation of statistics and risk analysis, essential for decision-making under uncertainty.
Probability rules
P(not A) = 1 - P(A)
P(A and B) = P(A) * P(B) [for independent events]
P(A or B) = P(A) + P(B) - P(A and B)
P(A and not B) = P(A) * (1 - P(B))
Key concepts
- Complement: P(not A) is the opposite of event A.
- Intersection (and): Both events occur together.
- Union (or): At least one event occurs.
- Independence: One event does not affect the other.
Probability: frequently asked questions
What is probability?
Probability is the likelihood of an event occurring, expressed as a number between 0 and 1. P = 0 means impossible, P = 1 means certain. P = 0.5 means equally likely.
What is P(A and B) for independent events?
For independent events: P(A and B) = P(A) * P(B). The events do not influence each other.
What is P(A or B)?
P(A or B) = P(A) + P(B) - P(A and B). We subtract P(A and B) to avoid double-counting when both occur.
How do I find P(not A)?
P(not A) = 1 - P(A). The complement of an event is everything except that event.
Official sources
- NIST/SEMATECH e-Handbook: NIST Handbook.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.