Probability Calculator
Turn favorable and total outcomes into a probability percent, plus the odds for and against.
How it works
P = favorable ÷ total · odds in favor = favorable : unfavorable
Classical probability divides the outcomes you want by all equally likely outcomes, giving a number from 0 (impossible) to 1 (certain). Odds express the same fact as a ratio of favorable to unfavorable outcomes — a 3-in-8 probability means 3 favorable against 5 unfavorable, written 3 : 5.
Worked example
Drawing one of the 3 red marbles from a bag of 8: P = 3 ÷ 8 = 0.375 = 37.5%. The odds in favor are 3 : 5 — three winning marbles against five losing ones — and the odds against are 5 : 3.
Frequently asked questions
What’s the difference between probability and odds?
Probability compares wins to everything (3 out of 8); odds compare wins to losses (3 to 5). Convert with P = for ÷ (for + against) — 3 : 5 in favor means 3 ÷ 8 = 37.5%.
Why doesn’t a 50% chance mean alternating wins and losses?
Each independent try is fresh — probability describes the long-run average, not a pattern. Ten heads in a row leaves the next flip at exactly 50%; expecting otherwise is the gambler’s fallacy.
How do I combine probabilities of two events?
For independent events, multiply for "both" (P(A) × P(B)); for mutually exclusive events, add for "either". Two dice both showing six: 1/6 × 1/6 = 1/36.
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This calculator is for educational purposes. Double-check important results before acting on them.