Online Tool Store
Probability Calculator
Calculate a single event from favorable and total outcomes, combine independent or mutually exclusive events, and find repeated-trial chances including exactly k successes. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Math and statistics
Probability Calculator
Single event
Enter favorable outcomes out of all equally likely outcomes.
Probability
Decimal
Odds
Two events
- Not A
- Not B
- Both A and B
- A or B (or both)
- Exactly one
- Neither
Repeated independent tries
Find the chance of at least one success, no success, every success, or exactly `k` successes.
At least once
Never
Every try
Exactly k
How it works
- Use Single event for favorable outcomes divided by total outcomes.
- Use Two events to choose independent or mutually exclusive before reading the combined results.
- Use Repeated independent tries for at least once, never, every try, or exactly k successes.
- Copy the results when you are ready. All calculations run in your browser.
The formulas
The calculator uses exact fractions for a single event and standard probability rules for the combined sections:
P(A and B) = P(A) x P(B)
P(A or B) = P(A) + P(B) - P(A and B)
P(at least once) = 1 - (1 - p)^n
P(exactly k) = C(n,k) x p^k x (1 - p)^(n-k)
FAQ
What can this probability calculator find?
It calculates a single event from favorable and total outcomes, combines two independent or mutually exclusive events, and evaluates repeated independent tries including at least once, never, every try, and exactly k successes.
Why is A or B not always the two percentages added together?
For independent events, adding the percentages counts the overlap twice. The calculator subtracts P(A) times P(B). Mutually exclusive events have no overlap, so their chances can be added when the total is no more than 100%.
What does exactly k successes mean?
It is the binomial probability of getting exactly the selected number of successes in n independent tries with the same chance on every try.
Can it calculate conditional probability or Bayes theorem?
Not directly. The two-event section assumes independence or mutual exclusivity. For dependent events, use a conditional-probability formula with the correct additional information.
How we compare
| Feature | Online Tool Store | Simple Number Tools | Zuma Tools |
|---|---|---|---|
| Single event with percentage, decimal, and odds | Yes | Yes | Not shown |
| Independent and mutually exclusive events | Yes | Independent only | Yes |
| Exactly k successes in repeated tries | Yes | Not shown | Not shown |
| At least once, never, and every try | Yes | Yes | Yes |
| Copy results and no account | Yes | Save available | Not shown |
For everyday probability questions, this keeps single events, combined events, and repeated trials together with explicit assumptions and readable outputs.