Probability Calculator
Calculate probabilities for events, including combined and conditional cases.
The chance of an event
Probability measures how likely an event is, from zero (impossible) to one (certain), often expressed as a fraction or percentage. This calculates probabilities for single and combined events, helping you reason about chance.
Enter the event details to calculate the probability.
Where intuition fails
Probability is notorious for defying intuition, which is exactly why calculating it carefully matters. Combining events trips people up: the chance of two independent events both happening is the product of their probabilities, so requiring several things at once quickly becomes unlikely, while the chance of at least one of several happening is higher than any single one. Conditional probability — the chance of something given that something else has happened — is where even experts stumble, and it is the root of many famous paradoxes and real-world misjudgements, from medical test results to legal reasoning. Working probabilities out rather than guessing is genuinely valuable, because the honest answer is frequently the opposite of what feels right.
Frequently asked questions
How do I find the chance of two events both happening?
For independent events, multiply their probabilities — which is why requiring several things at once quickly becomes unlikely.
Why is probability so counterintuitive?
Combined and conditional probabilities often defy gut feeling, which is why famous paradoxes and real misjudgements arise. Calculating beats guessing.
Is anything uploaded?
No. The calculation happens in your browser.