Probability calculator
Calculate the probability of an event from the favorable cases and the total possible cases.
Probability = favorable ÷ total cases. Ranges from 0 (impossible) to 1 (certain).
How the calculation works
Probability measures the chance of an event, from 0 (impossible) to 1 (certain) — or 0% to 100%. In the classic form:
P = favorable cases ÷ total possible cases
Step-by-step example
Rolling a 5 on a 6-sided die: 1 favorable case in 6 → 1/6 ≈ 16.67%.
Combined events
- "AND" (independent): multiply. Two 6s in a row: 1/36.
- "OR" (exclusive): add. A 5 or 6: 1/3.
- Complement: P(not happening) = 1 − P(happening).
Examples
- Heads on a coin: 1/2 = 50%.
- Drawing an ace from 52 cards: 4/52 ≈ 7.7%.
Frequently asked questions
Can probability exceed 100%?
No. The maximum is 100% (a certain event).
Chance of two events together?
For independent events, multiply the probabilities. E.g. two heads in a row = 1/4.
What is complementary probability?
The chance of something NOT happening: 1 minus the event's probability. Often the easiest route.
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See all →Updated on June 18, 2026 · by Rafael Rossi · Methodology & sources