Probability Calculator
Calculate event probabilities, at-least-once chances and combined events.
All calculations happen locally in your browser — nothing is uploaded, stored or tracked.
Pick a mode
Simple, at least once, or combined events.
Enter the values
Outcomes, probabilities or trials as needed.
Read the result
Probability as a fraction, percentage and odds.
What Is a Probability Calculator?
A probability calculator works out how likely an event is. The basic definition is the count of favourable outcomes divided by the total number of possible outcomes — rolling a 4 on a fair die is 1/6 ≈ 16.7%.
Beyond simple events, three practical situations are covered: the probability of an event happening at least once across several trials (like "at least one six in three dice rolls"), and combining two independent events with "and" and "or" rules.
The Rules
Your data never leaves your device.
Every calculation on this site runs in your browser using vanilla JavaScript. Nothing is uploaded, stored or tracked.
- 100% local, client-side calculations
- No data stored on any server
- No tracking, no analytics, no ads scripts
- Works offline once loaded
Frequently Asked Questions
How do I calculate the probability of an event?
Divide the number of favourable outcomes by the total number of possible outcomes.
What is the probability of rolling a 6?
1/6 ≈ 16.7%, since a die has six equally likely faces.
How do I calculate "at least once" probability?
Use 1 − (1 − p)ⁿ, where p is the per-trial probability and n the number of trials. For three rolls of a die: 1 − (5/6)³ ≈ 42.1%.
What does "independent events" mean?
Events whose outcomes do not affect each other, like separate coin flips. Their joint probability is the product of the individual probabilities.
What are odds?
Odds compare favourable to unfavourable outcomes: probability 1/4 is odds of 1:3.