Probability Calculator — Single & Combined Events
Find the probability of a single event from favorable and total outcomes, or combine two independent events with AND / OR logic — all shown as decimals, percentages, and odds.
📝 Single Event Probability
🔄 Two Independent Events
📊 Single Event Result
🔄 Combined Event Result
Disclaimer: This tool performs standard mathematical calculations for educational and reference purposes. Always double-check results independently for graded coursework, exams, or decisions involving real risk or money.
Understanding Probability
Probability measures how likely an event is to happen, on a scale from 0 (impossible) to 1 (certain). For a single event with equally likely outcomes, the probability is simply P(A) = favorable outcomes / total outcomes. That same value can be expressed three interchangeable ways: as a decimal (0.25), a percentage (25%), or odds (1 in 4) — pick whichever format fits how you're communicating the result.
Combining Two Independent Events
When two events don't influence each other, you can combine their individual probabilities using two different rules depending on what you're asking. P(A and B) — the chance both happen — is found by multiplying: P(A) × P(B). P(A or B) — the chance at least one happens — is found by adding the two probabilities and subtracting the overlap so it isn't double-counted: P(A) + P(B) − P(A and B).
Worked Example
A weather report gives a 30% chance of rain (event A) and, independently, a 20% chance a flight is delayed for unrelated mechanical reasons (event B).
- Step 1: P(A and B) = 0.30 × 0.20 = 0.06, or 6% — the chance both happen
- Step 2: P(A or B) = 0.30 + 0.20 − 0.06 = 0.44, or 44% — the chance at least one happens