Probability Calculator
Calculate your Probability result in seconds with the free Probability Calculator. No registration needed.
Probabilities
Result
What is a Probability Calculator?
A probability calculator computes the likelihood of single and combined events. It handles independent events and calculates the probability of both happening, either happening, or neither happening.
How to calculate probability manually (the formula)
P(A and B) = P(A) × P(B) for independent events. P(A or B) = P(A) + P(B) − P(A and B). P(neither) = (1 − P(A)) × (1 − P(B)). Probabilities are expressed as decimals between 0 and 1.
Example calculation
If P(A) = 0.5 and P(B) = 0.3: P(A and B) = 0.5 × 0.3 = 0.15 (15%). P(A or B) = 0.5 + 0.3 − 0.15 = 0.65 (65%). P(neither) = 0.5 × 0.7 = 0.35 (35%).
Common mistakes
- Adding probabilities without subtracting the overlap — P(A or B) is NOT P(A) + P(B) for non-mutually exclusive events.
- Forgetting probability must be between 0 and 1 — A probability less than 0 or greater than 1 is invalid.
- Confusing dependent and independent events — This calculator assumes independent events, where one event does not affect the other.
Frequently asked questions
What is probability?
A number between 0 and 1 measuring how likely an event is.
How do I calculate the probability of two independent events both happening?
Multiply P(A) × P(B).
How do I calculate the probability of either event happening?
P(A) + P(B) − P(A and B).
Can I enter probabilities as percentages?
Yes, values > 1 are auto-converted from percentages to decimals.
What is a complement in probability?
P(not A) = 1 − P(A).
Is this calculator free?
Yes, completely free.