Probability Calculator
Calculate the chance of two independent events, of something happening at least once in several tries, or of exactly k successes in n trials.
Result
Choose a calculation and enter probabilities.
How to use this calculator
- Choose a calculation.
- Enter probabilities as decimals (0.25), percentages (25%) or fractions (1/4).
Formula
- A and B: P(A) × P(B); A or B: P(A) + P(B) − P(A)P(B)
- At least once in n tries: 1 − (1 − p)ⁿ
- Binomial: P(X = k) = C(n, k) pᵏ (1 − p)ⁿ⁻ᵏ
Worked example
Rolling a six
The chance of at least one six in 4 rolls is 1 − (5/6)⁴ ≈ 51.8%. Exactly 3 heads in 10 fair flips: 120 ÷ 1024 ≈ 11.7%.
Assumptions and limitations
- Everything runs in your browser; nothing you enter is uploaded or stored.
- Events and trials are assumed independent.
Frequently asked questions
What does independent mean?
One event does not change the chance of the other, like separate dice rolls. Drawing cards without replacing them is not independent.
Why is at least once not just p × n?
Because the tries can overlap — that formula could exceed 100%. 1 − (1 − p)ⁿ counts every way except never.