Free · browser-only · no sign-up
Calcul De La Probabilité
Type in two probabilities — or a handful of outcomes — and read the union, intersection, conditional and Bayes results instantly as decimal, percent, simplified fraction and odds. Nothing leaves your browser.
Probability calculator
Edit any field — the answer updates as you type.
How it works
Single event counts, two events with a relationship, or a conditional / Bayes setup.
Decimals (0.25) or percents (25%) both work; counts must be whole numbers.
Decimal, percent, simplified fraction and odds — refreshed on every keystroke.
Formulas used
Everything below is computed in your browser with these standard identities.
Independent: P(A) + P(B) − P(A)·P(B). Mutually exclusive: P(A) + P(B). General: P(A) + P(B) − P(A ∩ B).
Independent: P(A)·P(B). Mutually exclusive: 0. General: read straight from your input.
Bayes' rule in its joint-probability form; undefined when P(B) = 0.
The complement — also the numerator of the odds against.
Reported as a whole-number ratio, e.g. P = 0.25 → 1 : 3 in favour.
Decimals are converted with continued fractions and reduced, so 0.166666… reads as 1/6.
FAQ
Independent or mutually exclusive — which do I pick?
Independent means one event happening does not change the other's probability (two coin flips). Mutually exclusive means they cannot happen at the same time (one dice roll landing on 2 or 5). If you already know the joint probability P(A ∩ B), choose general.
Can I type a percentage instead of a decimal?
Yes — enter 25% or 0.25. Anything outside 0–1 (0%–100%) is rejected with an inline message instead of producing an impossible result.
What happens when P(B) = 0 in conditional mode?
P(A | B) = P(A ∩ B) / P(B) has no answer, so the tool prints "undefined (P(B) = 0)" instead of Infinity or NaN.
Why does the result update while I type?
The whole calculator is plain JavaScript running in your browser — there is no submit button, no rounding round-trip and no data sent anywhere.
How accurate are the fractions?
Counts are reduced exactly (3 of 6 → 1/2). Decimal inputs use a continued-fraction approximation with a denominator cap, which is exact for any short decimal like 0.125 → 1/8.