What is the Probability Calculator?
Calculate probabilities online for free with this probability calculator. Find basic probability, complements, independent intersections, unions, conditional probability P(A|B), and binomial probabilities P(X = k), P(X ≥ k), and P(X ≤ k). Each result includes the formula, decimal, percent, and step-by-step work. Runs entirely in your browser with no signup.
How to use the Probability Calculator
- Choose a calculation type (basic, complement, union, conditional, binomial, etc.).
- Enter the required counts or probabilities.
- Click Calculate.
- Review the probability as a percent, decimal, and formula.
- Copy the result or the full summary with steps.
Common use cases
- Finding a 1/6 chance of rolling a specific die face
- Computing P(A ∪ B) when events overlap
- Calculating P(X ≥ 3) for 10 coin flips with p = 0.5
Frequently asked questions
- What probability formulas are supported?
- Basic P = favorable/total, complement P(A′) = 1 − P(A), independent P(A ∩ B) = P(A)×P(B), union P(A ∪ B) = P(A)+P(B)−P(A∩B), conditional P(A|B) = P(A∩B)/P(B), and binomial PMF/cumulative formulas.
- Can I enter percentages?
- Yes. Enter probabilities as decimals (0–1) or percentages (0–100); values above 1 are treated as percents.
- What is binomial probability?
- It models the chance of exactly k successes in n independent trials each with success probability p: P(X=k) = C(n,k) × p^k × (1−p)^(n−k).
- What are the limits?
- Binomial trials n are capped at 1,000. Basic total outcomes can be up to one billion.
