MATH · PROBABILITY
Probability Calculator
Calculate simple event probability, the complement (probability of not A), or the joint probability of two independent events.
About This Calculator
Probability quantifies how likely an event is, expressed as a number between 0 (impossible) and 1 (certain). This calculator covers three fundamental probability operations: finding P(A), its complement P(A'), and the joint probability of two independent events P(A ∩ B).
How It Works
Select the mode, enter the values, and the calculator returns the probability as a decimal and a percentage. Simple mode divides favourable outcomes by total outcomes. Complement mode returns 1 minus that probability. Independent mode multiplies P(A) and P(B), which is valid only when the two events do not affect each other.
The Formula
P(A) = favourable / total P(A') = 1 − P(A) P(A∩B) = P(A) × P(B)
- P(A)
- probability of event A
- P(A')
- probability of the complement (not A)
Frequently Asked Questions
- What does it mean for events to be independent?
- Two events are independent when the outcome of one does not affect the probability of the other. Rolling a die and flipping a coin are independent. Drawing two cards from the same deck without replacement are NOT independent — the second draw depends on the first.
- Can P(A) and P(B) both be 1?
- Yes. P(A∩B) = 1 × 1 = 1. This means both events are certain, so their joint occurrence is also certain.
- What is the range of a probability?
- A probability is always between 0 and 1 inclusive. A probability of 0 means the event cannot happen; a probability of 1 means it is certain.