Calculators
About the Probability Calculator
The Probability Calculator is a free online calculator from Toolbench. Combined probabilities of two independent events and repeated trials. Enter your numbers and the answer — with the formula used — updates as you type.
- Category
- Calculators
- Price
- Free
- Account
- Not required
- Runs
- Entirely in your browser
How to use the Probability Calculator
- Enter the values (p(a) %, p(b) %, trials for "at least once").
- Read the result — it updates as you type.
- Check the breakdown rows to see exactly how it was calculated.
Worked example
| P(A) % | 50 |
|---|---|
| P(B) % | 30 |
| Trials for "at least once" | 5 |
| P(A and B) | 15% |
| P(A or B) | 65% |
| P(A xor B) | 50% |
| P(neither) | 35% |
| P(not A) | 50% |
| P(not B) | 70% |
| P(A at least once in 5) | 96.875% |
Formula: For independent events, P(A and B) = P(A) × P(B), P(A or B) = P(A) + P(B) − P(A) × P(B), and P(at least once in n trials) = 1 − (1 − P)ⁿ.
Inputs
| P(A) % | Default: 50 |
|---|---|
| P(B) % | Default: 30 |
| Trials for "at least once" | Default: 5 |
Privacy
Everything runs in your browser: what you type or paste is processed by JavaScript on your device, not on a server. If you are signed out, nothing is saved at all. If you sign in with Google, your inputs and settings are saved privately to your account so they sync across devices.
Frequently asked questions
Is the Probability Calculator free?
Yes. Probability Calculator is completely free with no sign-up, no usage limits and no watermarks.
Is my data private?
Yes. The Probability Calculator processes everything in your browser. Signed out, nothing is saved; signed in, your inputs sync privately to your own account.
How does the probability calculation work?
For independent events, P(A and B) = P(A) × P(B), P(A or B) = P(A) + P(B) − P(A) × P(B), and P(at least once in n trials) = 1 − (1 − P)ⁿ.
Does it work on my phone?
Yes. Toolbench runs in any modern browser on desktop, tablet or phone — there is nothing to install.
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