Probability Calculator

A probability calculator computes the likelihood of events from the basic rules: a single event's probability is favorable ÷ total outcomes (a die showing 6: 1/6 ≈ 16.7%); two independent events both happening multiply — P(A) × P(B) (two sixes: 1/36 ≈ 2.8%); either happening adds minus the overlap — P(A) + P(B) − P(A and B).

Intuition fails at exactly these combinations: "either" feels like plain addition (it double-counts the overlap), long runs feel due for a change (independent events have no memory), and small percentages repeated many times accumulate faster than expected — the calculator replaces the feeling with the number.

Enter your event probabilities and get single, AND, OR and complement results instantly — as fractions of certainty, percentages and implied odds.

The everyday translations: games (drawing an ace: 4/52 = 7.7%; at least one six in four rolls: 1 − (5/6)⁴ = 51.8% — the historical Chevalier de Méré bet); risk stacking (a 1% failure chance across 100 independent trials is 1 − 0.99¹⁰⁰ = 63.4%, not 100% and not 1%); and the "at least once" pattern behind both — complement the never-case: P(at least once) = 1 − P(never), the single most useful trick in applied probability.

The independence caveat the math rests on: multiplying requires events that don't influence each other. Card draws without replacement, correlated failures and streaky processes violate it — and conditional probability (a different calculator's job) takes over.