Simple Probability Calculator
Compute the probability of a single event from favorable and total equally-likely outcomes — as a fraction, decimal, percentage and odds.
How it works
P(event) = favorable outcomes ÷ total equally-likely outcomes · P(not) = 1 − P
For a set of equally likely outcomes, the probability of an event is the number of outcomes that satisfy it divided by the total number of possible outcomes. The result is always between 0 (impossible) and 1 (certain). The complement — the chance the event does not happen — is just 1 minus that probability, which is often the quicker quantity to reason about.
Worked example
Rolling a single die, the chance of rolling a 3 is 1 favorable outcome out of 6, so 1 ÷ 6 = 0.1667, or 16.67%. The odds in favor are 1 to 5, and the chance of not rolling a 3 is 83.33%.
Sources: Probability — Wikipedia
Frequently asked questions
What does "equally likely" mean?
It means every individual outcome has the same chance, like each face of a fair die or each ticket in a fair draw. The favorable-over-total formula only holds when outcomes are equally likely.
How do I turn a probability into odds?
Odds in favor compare favorable to unfavorable outcomes. A 1-in-6 probability is 1 favorable to 5 unfavorable, written 1 : 5.
What is the complement of an event?
It is the event not happening, and its probability is 1 minus the event’s probability. If something has a 16.67% chance, the complement has an 83.33% chance.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.