Probability Calculator

Formula and method

Single-event probability divides favorable outcomes by total equally likely outcomes. Complement uses 1 − P(A). The AND mode multiplies P(A) and P(B) only after the user explicitly selects independent events. Union subtracts the supplied intersection. Conditional probability divides the joint probability by a positive P(B).

Worked example

With 3 favorable outcomes among 10 equally likely outcomes, P(A) = 0.3, or 30%. If P(A) = 0.5, P(B) = 0.4, and the intersection is 0.2, then P(A or B) = 0.7.

Assumptions and boundaries

• Outcome counts in single-event mode are nonnegative integers and favorable outcomes cannot exceed the total.
• Decimal and percent formats are selected explicitly; all internal probabilities are from 0 through 1.
• AND mode applies only to independent events. Independence is never inferred silently.
• This tool does not calculate distribution probabilities, gambling odds, or investment expected value.

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