Probability Calculator
Calculate probabilities of events.
How this calculator works
Formula
P(A and B) = P(A) · P(B) ; P(A or B) = P(A) + P(B) − P(A and B)
Method
Assumes independent events. Also reports P(neither) and P(not A).
Example
If P(A)=0.5 and P(B)=0.3, then P(A and B)=0.15, P(A or B)=0.65.
The complete guide to the Probability Calculator
Probability Calculator is a free tool from Morecalcs designed to make a common calculation fast, transparent, and accurate. Calculate probabilities of events. Below is a complete guide to what the tool computes, the formula it uses, when to reach for it, and the pitfalls to avoid.
What Probability Calculator does
Calculate probabilities of events. You enter your values, the tool computes the result instantly, and shows a breakdown of the numbers that went into it. There's no signup, no data leaves your browser, and the calculator is optimized for both mobile and desktop.
How it works under the hood
Assumes independent events. Also reports P(neither) and P(not A). The math runs entirely client-side, so results update the moment you type. This makes it fast to explore 'what if' scenarios without waiting for a server round-trip.
The formula
The core equation is P(A and B) = P(A) · P(B) ; P(A or B) = P(A) + P(B) − P(A and B). If you're doing this by hand, plug your numbers in directly — the calculator saves you the arithmetic and, more importantly, the errors that come with it.
A worked example
If P(A)=0.5 and P(B)=0.3, then P(A and B)=0.15, P(A or B)=0.65. Enter these values above and confirm the result matches. Then change one input at a time to build intuition for how each variable affects the outcome.
When to use this calculator
Reach for Probability Calculator whenever you need a quick, reliable number for planning, comparison, or verification. It's especially useful when the calculation would be tedious or error-prone by hand, or when you want to compare several scenarios side by side. Because it's free and requires no account, you can bookmark it and use it repeatedly.
Common mistakes to avoid
The most common error is entering the wrong unit — for example, using annual instead of monthly figures, or millimeters when the calculator expects centimeters. Read the label above each field carefully. The second most common issue is trusting a single result: whenever the outcome will guide a real decision, run the calculation two or three times with slightly different assumptions to see how sensitive the answer is.
Related tools on Morecalcs
Morecalcs organizes calculators by category so related tools are one click away. From the Probability Calculator page you'll find companion tools in the sidebar — they use the same design and share input conventions, so switching between them is seamless.
Why we built it this way
Morecalcs is designed to be the calculator you keep bookmarked because it never gets in your way. No modal ads, no signup wall, no dark patterns. Every tool has a consistent layout: inputs on the left, results on the right, with an explainer below for when you want to understand the math instead of just get an answer.
Sharing your result
Every calculator page has a stable URL and a share-friendly preview, so you can send the tool to a friend and they'll see the same layout. Bookmark this page if you'll use it often — it loads in under a second on any modern connection.
Frequently asked questions
Is Probability Calculator free to use?
Yes. Every Morecalcs tool is free with no signup required.
Does the tool store my data?
No. Calculations happen in your browser. Nothing you enter is sent to a server or stored.
How accurate is the result?
The calculator uses the standard textbook formula for this problem. Results are as accurate as your inputs.
Can I use this on mobile?
Yes. Every Morecalcs page is fully responsive and works on any device.