About probability calculator
Probability measures how likely something is, from 0 (impossible) to 1 (certain). This calculator combines independent events.
Rules
- Both A and B happen (independent): P(A) × P(B). Two heads in a row is 0.5 × 0.5 = 0.25.
- Either A or B happens: P(A) + P(B) − P(A and B).
- Not A: 1 − P(A).
Independence matters
Events are independent when one does not change the other, like separate coin tosses. Drawing cards without putting them back is not independent.
A worked example
The chance that two dice both show a 6 is 1/6 × 1/6 = 1/36, or about 2.8 percent. The chance of at least one head in three coin flips is 1 − (1/2)³ = 7/8, or 87.5 percent.
Common mistakes
- The gambler's fallacy: past coin flips do not change the next one.
- Adding probabilities of events that can both happen, without subtracting the overlap.
- Treating dependent events as independent, such as drawing cards without replacement.
