Work out the probability of an event, or of two independent events.
Calculated instantly in your browser.
How is probability calculated?
P(event) = favourable ÷ total. The chance it happens at least once in n trials = 1 − (1 − p)ⁿ. For independent events, P(A and B) = P(A)·P(B) and P(A or B) = P(A) + P(B) − P(A)·P(B). Probability runs from 0 (impossible) to 1 (certain). Example: a die has a 1/6 chance of any number; over 10 rolls the chance of at least one six is about 83.85%.
Understanding your result
Probability measures how likely something is, from 0 (impossible) to 1 (certain). The “at least once” result uses the complement rule — it is easier to find the chance of never happening and subtract from one. The AND/OR rules assume the two events do not influence each other.
Formula and method
P(event) = favourable ÷ total. At least once in n trials = 1 − (1 − p)ⁿ. For independent events: P(A and B) = P(A)·P(B); P(A or B) = P(A) + P(B) − P(A)·P(B).
Worked example
A die has a 1/6 ≈ 16.67% chance of any number; over 10 rolls the chance of at least one six is 1 − (5/6)¹⁰ ≈ 83.85%.
How to use this tool
- Choose a single event or two events.
- Enter the outcomes, or each probability.
- Read the probability and the breakdown.
Common mistakes to avoid
- Adding probabilities for “A or B” without subtracting the overlap.
- Treating dependent events as independent.
About the Probability Calculator
The Probability Calculator finds how likely an event is from the number of favourable and total outcomes, shows it as a percentage, fraction and odds, and works out the chance it happens at least once across repeated trials. It can also combine two independent events with the AND and OR rules.
Who should use this tool
Students, teachers, gamers and anyone weighing the odds of an outcome.
Benefits
- Probability as a percentage, fraction and odds.
- Chance of “at least once” over many trials.
- Combine two events with AND / OR rules.
- Shows the working, not just the answer.
Practical use cases
- Rolling a specific number on a die.
- The chance of at least one win in 10 tries.
- Combining the odds of two independent events.
Frequently asked questions
What does “at least once” mean?
It is the probability that an event happens one or more times over a number of independent trials, calculated as 1 minus the chance it never happens.
What are independent events?
Two events are independent when one occurring does not change the probability of the other — like two separate coin flips.