Probability Calculator
Single-event, AND/OR combined, and conditional probability, in one calculator.
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About the Probability Calculator
Calculates probability for a single event (favorable ÷ total outcomes), for two combined independent events (AND/OR), or for conditional probability P(A given B) — accepting input as either a decimal (0-1) or a percentage (0-100).
- Choose a scenario: single event, combined AND/OR, or conditional.
- For a single event, enter favorable outcomes and total possible outcomes.
- For AND/OR, enter P(A) and P(B) as independent event probabilities.
- For conditional, enter P(A and B) and P(B).
- Read the resulting probability as a percentage.
Rolling a single die and wanting a 4: 1 favorable outcome out of 6 total, so P = 1/6 ≈ 16.67%. Two independent coin flips both landing heads: P(A) = P(B) = 50%, so P(A and B) = 25%, and P(A or B) = 50% + 50% − 25% = 75%.
The AND/OR formulas assume A and B are independent — one event happening doesn't change the odds of the other. For dependent events, such as drawing cards from a deck without replacement, P(A)×P(B) no longer holds — use Conditional mode instead. P(A|B) is generally not the same value as P(B|A).
- • You can enter probabilities as either a decimal (0.5) or a percentage (50) — the calculator detects which one you mean.
- • Independence is the key assumption behind the AND/OR formulas — if one event's outcome changes the other's odds, use Conditional mode instead.
- • P(A or B) subtracts the overlap P(A and B) once — without that subtraction you'd double-count outcomes where both events happen.
- Khan Academy — Basic theoretical probability and compound events — accessed 2026-08-30