Rex says
The fast lane for the math you almost remember from school. Type the numbers, get the answer, move on with your day.
Try a scenario
Click to load — tweak from there.Inputs
Result
A happens at least once
94.369%
Both A and B
10.000%
A or B
55.000%
Neither
45.000%
A but not B
15.000%
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter probability of a (%).
- 2Enter probability of b (%).
- 3Enter repeated trials of a.
- 4Read your a happens at least once on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
Computes the standard combinations of two independent event probabilities, plus the odds of an event happening at least once across repeated trials — the calculation that explains why unlikely things happen constantly at scale.
Worked example
Using the values the calculator loads with:
Inputs
- Probability of A: 25 %
- Probability of B: 40 %
- Repeated trials of A: 10
Results
- A happens at least once: 94.369%
- Both A and B: 10.000%
- A or B: 55.000%
- Neither: 45.000%
- A but not B: 15.000%
What each field means
Inputs
- Probability of A (%)
- The probability of a used in the calculation, measured in %. Starts at 25 % so you have a working example on load. Accepted range: 0–100 %.
- Probability of B (%)
- The probability of b used in the calculation, measured in %. Starts at 40 % so you have a working example on load. Accepted range: 0–100 %.
- Repeated trials of A
- The repeated trials of a used in the calculation. Starts at 10 so you have a working example on load.
Results
- A happens at least once
- Returned as a percentage and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Both A and B
- Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- A or B
- Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Neither
- Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- A but not B
- Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the probability calculator work?
Computes the standard combinations of two independent event probabilities, plus the odds of an event happening at least once across repeated trials — the calculation that explains why unlikely things happen constantly at scale. The underlying maths is: P(A and B) = P(A)P(B). P(A or B) = P(A) + P(B) − P(A)P(B). At least once = 1 − (1−P)ⁿ.
What do I need to enter?
3 values: probability of a, probability of b, and repeated trials of a. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the a happens at least once result mean?
It is returned as a percentage and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Results are rounded for display; the underlying calculation keeps full precision.
Who is this tool for?
It's built for students, developers, and anyone who needs the answer right now without setting up a spreadsheet.
Accuracy and limitations
- Results are rounded for display; the underlying calculation keeps full precision.
- Very large or very small inputs may hit floating-point limits in the browser.
- Inputs outside the accepted range are clamped rather than rejected.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Probability Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/probability
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/probability" target="_blank" rel="noopener">Probability Calculator — RevenueLab</a> (2026).</p>
Source: [Probability Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/probability) (2026).