Probability Calculator
Calculate the probability of single events, combined events (AND/OR), and complements instantly.
How It Works
Enter favorable outcomes and total outcomes for P(A). Optionally add P(B) to calculate compound event probabilities.
Formula
P(A) = Favorable / Total
P(A and B) = P(A) × P(B) [independent]
P(A or B) = P(A) + P(B) − P(A and B)
P(not A) = 1 − P(A)
Example
Rolling a 3 on a six-sided die:
P(3) = 1/6 = 16.67%. P(not 3) = 83.33%.
When to Use This Calculator
Use for probability homework, game odds, risk percentages, or any likelihood calculation.
Common Mistakes to Avoid
- Confusing AND with OR — AND multiplies (lower); OR adds minus overlap (higher).
- Assuming independence — P(A and B) = P(A)×P(B) only when events are independent.
- Not checking P(A) + P(not A) = 1.
Frequently Asked Questions
Probability 0.5 means?
Equal chance of occurring or not — like flipping a fair coin.
What is a complementary event?
The complement of event A is everything except A. P(A) + P(not A) = 1 always.
Theoretical vs experimental probability?
Theoretical: calculated from known outcomes. Experimental: measured from actual trials.
Can probability exceed 1?
No. Always between 0 (impossible) and 1 (certain).
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Last Updated: July 4, 2026