About this tool
Probability calculations follow specific rules depending on whether events overlap. Independent events (coin flip + dice roll) multiply. Mutually exclusive events (red AND blue from same item) cannot both happen. The 'or' rule subtracts the overlap to avoid double-counting.
How it works
Enter probabilities as decimals (e.g., 0.5 for 50%) or as percents and let the tool normalize. Pick whether the events are independent or mutually exclusive.
How to use the Probability Calculator
The Probability Calculator is a browser-based tool that runs entirely on your device. Inputs you enter never reach a server - all calculations happen client-side in JavaScript. This means:
- Privacy: nothing is logged, sent, or stored by 3Tej. Inputs disappear when you close the tab.
- Speed: results update as you type. No network round trip.
- Offline use: once the page is cached, it works without internet.
- No signup: no account, no email, no rate limits.
Step by step
- Enter your inputs in the form above. Each field is labeled with its unit (currency, percent, kg, etc.) and the expected range.
- Read the result as it updates. The number reflects the formula commonly accepted in Probability-related calculations.
- Adjust to see sensitivity: change one input at a time and watch how the output moves. This is the fastest way to understand which variable matters most.
- Copy or screenshot the result for later reference. The page state persists for the session if your browser allows it.
When you would use this
- Quick estimates: when you need a number now and don't want to open a spreadsheet.
- Sensitivity analysis: testing how a result changes as inputs vary, before committing to a real-world decision.
- Comparison: running the same calculation with different inputs to compare options side by side.
- Learning: building intuition for how the underlying math behaves.
- Documentation: capturing a snapshot of inputs and outputs at a point in time.
The formula explained
This calculator uses the following formula:
Independent: P(A∩B) = P(A)·P(B); Mutually exclusive: P(A∩B) = 0; P(A∪B) = P(A) + P(B) - P(A∩B)
The reason this formula works is rooted in the underlying physics, finance, or biology of the problem. Behind every calculator is a published, peer-reviewed equation or a widely accepted convention. We do not invent formulas; we apply standard ones from textbooks, government tables, professional bodies, and academic literature.
If you are curious about the math, the simplest way to verify is to plug in two known numbers and compare against a known result. The calculator should match published examples to within rounding precision.
Frequently asked questions
How do I know if events are independent?
If the outcome of one doesn't change the probability of the other. Coin flips are independent; drawing without replacement isn't.
What's mutually exclusive?
Both can't happen at once. Heads and tails on the same flip; red and blue from the same single ball.
Conditional probability?
P(A | B) = P(A∩B) / P(B). Use it for sequential or dependent events. Bayes' theorem extends this for diagnostic questions.
Why does 'or' need subtraction?
Without it you'd double-count overlap. P(A) + P(B) counts the AND case twice.
Is the Probability Calculator accurate?
The Probability Calculator applies the standard formula for probability. Accuracy is limited only by your input precision. For decisions with material consequences, use the result as a starting point and verify with a qualified professional or the relevant official source.
Is the Probability Calculator free?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads that appear around the tool but not inside the calculation flow.
Are my inputs saved?
No. Inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but does not see what you type into the form.
Can I use the Probability Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum guidance.
How do I report a bug or suggest improvement to the Probability Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours and update calculators when rules or formulas change.
How accurate is the Probability Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Probability Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Does the Probability Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
Can I share results from the Probability Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another probability tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.