Probability Calculator
Calculate single, combined, and conditional probabilities with odds and complements.
Pick a probability question
Enter each probability as a decimal between 0 and 1 (for example, 0.25 means a 25 percent chance).
P(A)
0.5
50%
- Complement
- 0.5
- Odds for
- 1:1
- Odds against
- 1:1
Probability of event A.
Frequently Asked Questions about the Probability Calculator
What is probability?
A number between 0 and 1 that expresses how likely an event is. 0 means impossible, 1 means certain, and 0.5 means a 50 percent chance.
How do I calculate P(A and B) for independent events?
Multiply the two probabilities: P(A and B) = P(A) times P(B). Use this only when one event does not affect the other.
How do I calculate P(A or B)?
Add the probabilities and subtract the overlap: P(A or B) = P(A) + P(B) - P(A and B). This avoids double counting.
What is conditional probability?
P(A|B) is the probability of A given that B has happened. Compute it as P(A and B) divided by P(B), provided P(B) is greater than 0.
How do odds relate to probability?
Odds compare favorable to unfavorable outcomes. For a probability p, odds for are p:(1-p). A probability of 0.25 equals odds of 1:3 in favor.