Probability Calculator — Calculate the Likelihood of Any Event
Use this free probability calculator to calculate the probability of a single event, the combined probability of two independent events occurring together, or the probability of an event occurring at least once over multiple trials.
Probability Calculator
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How to Use the Probability Calculator
Choose your mode — Basic Probability for simple outcomes, Multiple Events for the probability of A and B both occurring, or At Least Once to find the probability of an event happening in multiple trials. Enter your values and click Calculate.
How Is Probability Calculated?
Basic probability is calculated by dividing the number of successful outcomes by the total number of possible outcomes:
P(event) = Successful Outcomes ÷ Total Outcomes
For example the probability of rolling a 3 on a six-sided die = 1 ÷ 6 = 0.1667 = 16.67%
Types of Probability
Basic probability — the likelihood of a single specific outcome occurring.
Combined probability — the probability of two independent events both occurring. Multiply the individual probabilities together.
At least once probability — the probability that an event occurs at least once in a number of trials. Calculated as 1 minus the probability of it never occurring.
FAQs
What is probability?
Probability measures how likely an event is to occur. It is expressed as a number between 0 and 1 or as a percentage. 0 means impossible, 1 means certain.
What is the complement of a probability?
The complement is the probability of the event NOT occurring. It equals 1 minus the probability. If there is a 30% chance of rain there is a 70% complement chance of no rain.
What is the difference between probability and odds?
Probability is successful outcomes divided by total outcomes. Odds is successful outcomes divided by unsuccessful outcomes. A 1 in 4 probability equals odds of 1:3.
What does independent events mean?
Two events are independent if the outcome of one doesn’t affect the other. Flipping a coin twice — each flip is independent. The Multiple Events mode assumes independent events.
Is this calculator free?
Yes completely free with no sign-up needed.