Probability Calculator
Calculate basic probability from favorable and total outcomes.
Enter values to see the result.
P(A) = favorable / total
P(A and B) = P(A) x P(B) (independent)
P(A or B) = P(A) + P(B) - P(A and B)
Guide: Probability
Probability is a measure of certainty of an event occurring, expressed as a number between 0 and 1. P = 0 means impossible event, P = 1 means certain event. For example, the probability of getting heads in a coin flip is 0.5 or 50%.
Basic probability is calculated as the ratio of favorable outcomes to the total number of possible outcomes. In a 6-sided die, if there are 3 favorable outcomes, P = 3/6 = 0.5 or 50%. This classical probability assumes each outcome is equally likely.
P(A and B) - multiplication rule: For two independent events, the probability of both occurring is P(A) × P(B). For example, the probability of two heads in a row is 0.5 × 0.5 = 0.25. Events are independent if the occurrence of one does not affect the other.
P(A or B) - addition rule: For two events, the probability of at least one occurring is P(A) + P(B) - P(A and B). We subtract the intersection to avoid double counting. For mutually exclusive events (cannot occur together), simply add them.
Applications: Probability is used in insurance for risk assessment, in medicine for drug efficacy, in finance for asset pricing, in gambling for strategy, and in business decision-making.