Probability measures the likelihood of an event occurring, expressed as a number between 0 (impossible) and 1 (certain), or as a percentage (0% to 100%).

What does P(A or B) mean?

P(A or B) is the probability that either event A occurs, or event B occurs, or both. For mutually exclusive events: P(A or B) = P(A) + P(B).

What does P(A and B) mean?

P(A and B) is the probability that both events A and B occur together. For independent events: P(A and B) = P(A) × P(B).

What is the complement of an event?

The complement P(not A) is the probability that event A does NOT occur. P(not A) = 1 - P(A). If rain probability is 30%, no-rain probability is 70%.

What are mutually exclusive events?

Events that cannot occur at the same time. For example, getting heads AND tails on a single coin flip is impossible - they are mutually exclusive.

Probability Calculator

Calculate single event probability, combined events (A or B, A and B), and complements.

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Probability of A (%)

Enter as percentage (0-100)

Visualization

P(A)

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Probability of event A occurring

P(A)

50%

P(not A)

P(A or B)

P(A and B)

Other Formats

Decimal

Odds (for)

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1 in X

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Documentation

Probability Rules

Probability quantifies uncertainty. Values range from 0 (impossible) to 1 (certain). Combined probabilities depend on whether events are independent or mutually exclusive.

Complement

Union

Independent

Frequently Asked Questions

How to Use

About the Probability Calculator

What it calculates

This probability calculator solves mathematical and statistical problems using standard formulas. Calculate simple, independent, and conditional probabilities.

When to use it

Use for homework, data analysis, exam prep, or quick verification of manual calculations.

Example

Adjust the inputs above to model your specific scenario with the Probability Calculator.

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Probability Calculator

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Visualization

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Other Formats

Probability Rules

About the Probability Calculator

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