Calculate probability, odds and likelihood of events instantly.
Basic Probability
Multiple Events (P(A) and P(B))
Probability is the mathematical measure of how likely an event is to occur, expressed as a number between 0 and 1 (or 0% to 100%). A probability of 0 means the event is impossible; a probability of 1 means it is certain. Our probability calculator online helps you quickly calculate probabilities for single events, multiple events, conditional probability, and more.
P(Event) = Number of favorable outcomes / Total number of possible outcomes. For example, the probability of rolling a 4 on a fair dice = 1/6 ≈ 0.167 or 16.7%. The probability of drawing a red card from a standard deck = 26/52 = 0.5 or 50%.
Addition Rule (OR): P(A or B) = P(A) + P(B) − P(A and B). For mutually exclusive events: P(A or B) = P(A) + P(B). Multiplication Rule (AND): For independent events: P(A and B) = P(A) × P(B). For dependent events: P(A and B) = P(A) × P(B|A).
Probability is used everywhere. Weather forecasting — "70% chance of rain" means that in 100 similar weather conditions, rain occurred 70 times. Insurance companies use probability to assess risk and set premiums. Medical testing uses probability for diagnosis accuracy and treatment effectiveness. Finance uses probability for risk assessment and portfolio management. Understanding probability helps you make better decisions in uncertain situations.
Probability is the measure of how likely an event is to occur, expressed as a number between 0 (impossible) and 1 (certain), or as a percentage.
Theoretical probability is calculated mathematically. Experimental probability is based on actual results from repeated trials.
The probability that an event does NOT occur equals 1 minus the probability that it does occur. P(not A) = 1 − P(A).
Two events are independent if the occurrence of one does not affect the probability of the other. Example: flipping a coin twice — each flip is independent.
