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.
This includes working out the odds of a specific outcome repeating across multiple independent trials, such as calculating the probability of hitting the same low-probability event again over several repeated attempts.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 represents the likelihood of an event occurring, expressed as a number between 0 (impossible) and 1 (certain), or equivalently as a percentage between 0% and 100%. A common misunderstanding is treating probability as a prediction of what will definitely happen in a small number of trials, when it's actually a statement about the long-run frequency of an outcome across many repeated trials — a fair coin having a 50% chance of heads doesn't guarantee exactly 5 heads in 10 flips, only that the proportion converges toward 50% as the number of flips grows very large.
The Gambler's Fallacy — the mistaken belief that past independent events affect future probability, like assuming a coin is "due" for tails after several heads in a row — is one of the most common probability misconceptions, since each flip of a fair coin remains independently 50/50 regardless of previous outcomes.
Calculating the probability of multiple independent events all occurring requires multiplying their individual probabilities together, not adding them — the chance of flipping heads twice in a row is 0.5 × 0.5 = 0.25 (25%), not 0.5 + 0.5. Calculating the probability of at least one of several events occurring uses a different approach entirely, often calculated as 1 minus the probability that none of the events occur. Mixing up these two calculation methods is one of the most frequent sources of probability calculation errors.
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.
A 1/3578 probability equals approximately 0.028%. Enter this fraction above to see it converted to percentage and decimal form instantly.
Enter your total count and probability percentage above to calculate the expected number of occurrences, useful for estimating rare-event outcomes across a large sample.
Reciprocal odds convert a probability into a 1-in-X format by dividing 1 by the probability. Enter your probability value above to see the reciprocal odds instantly.
