Probability Calculator

How to use

1. 1Pick a mode tab: Single event, Two events, Bayes' theorem, Independent trials, or Odds and probability.
2. 2Type your numbers. Probabilities accept decimals (0.25), percentages (25%), and fractions (1/4). Whole-number fields accept spaces, commas, and underscores between digits.
3. 3Read the result panel for the answer as a decimal, a percentage, and (when integer counts are used) a reduced fraction. Inline formulas remind you of the rule that produced each value.
4. 4In Two events, leave P(A and B) blank to compare the independent and mutually exclusive answers side by side, or fill it in to get the actual joint, union, and conditional probabilities.
5. 5Click any quick example chip (coin flip, die roll, rare disease test, spam filter, 5 heads in 10 flips) to load a familiar textbook scenario in one click.
6. 6Use Copy result to grab a clean text summary of the formulas and answers for homework, a report, or a spreadsheet.

About this tool

Probability Calculator answers the five questions students, teachers, analysts, and quiz writers actually ask when they search for a probability tool, in one page with shared formatting. Mode one is single event probability: enter favorable and total equally likely outcomes and the tool returns P(A) as a reduced fraction, a decimal, and a percentage, along with the complement P(not A). Mode two is two events: type any combination of P(A), P(B), and the joint P(A and B) (probabilities, percentages, or fractions like 1/4 all parse), and the calculator returns P(A and B), P(A or B), P(A | B), and P(B | A), with a clear callout when the events look independent (P(A and B) = P(A) x P(B)) or mutually exclusive (P(A and B) = 0). If you leave the joint blank, the tool still shows the answers under the two textbook assumptions side by side so you can compare them. Mode three is Bayes' theorem: provide a prior P(A), a likelihood P(B | A), and a false positive rate P(B | not A), and the posterior P(A | B) is computed by P(B | A) P(A) / (P(B | A) P(A) + P(B | not A) P(not A)). Mode four is n independent trials with constant probability p: get P(none), P(at least one), P(all), the binomial P(exactly k) using C(n, k) p^k (1 - p)^(n - k), plus the mean n p and standard deviation sqrt(n p (1 - p)); the binomial coefficient is computed in BigInt so n up to 5000 stays exact. Mode five converts probability to odds in favor (a:b), odds against (b:a), and the decimal odds used by sports books, with two-way editing so you can type the value you have and read the others. Quick example chips load familiar textbook scenarios (coin flip, die roll, rare disease test, spam filter, ace from a deck, 5 heads in 10 flips). Inputs accept fractions, decimals, percentages, and comma decimal locales, and every error is reported in the row that caused it. Useful for high-school and college statistics homework, AP and IB exam prep, A/B test reasoning, medical and quality control screening intuition, gambling odds checks, and any time you need to remember whether independence or mutual exclusion changes the answer. The math runs locally in your browser; the numbers you type for class, work, or research stay on your device.

Free to use. Works in your browser. No signup, no login.