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Probability Calculator

Calculate probabilities of events using addition rule, conditional probability, and Bayes' theorem with interactive Venn diagrams and step-by-step solutions.

Probability Calculator
∪
Union
P(A or B)
|
Conditional
P(A given B)
B
Bayes
Reverse P(A|B)
'
Complement
P(not A)
Quick Examples: Union Example Conditional Example Bayes Example Complement Example
P(A)
Probability of event A (0 to 1)
P(B)
Probability of event B (0 to 1)
P(A ∩ B)
Probability of both A and B
P(B|A)
Probability of B given A occurred

Embed Probability Calculator Widget

About Probability Calculator

Welcome to the Probability Calculator, a comprehensive tool for calculating probabilities of events using fundamental probability rules. Whether you need to find the probability of a union, calculate conditional probability, apply Bayes' theorem, or compute complement probabilities, this calculator provides step-by-step solutions with interactive visual diagrams.

What is Probability?

Probability is a measure of the likelihood that an event will occur, expressed as a number between 0 and 1, where 0 indicates impossibility and 1 indicates certainty. Understanding probability is essential in statistics, data science, risk assessment, decision making, and everyday reasoning.

$$P(A) = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}}$$

Probability Calculation Types

Basic Probability (Union)

The addition rule calculates the probability that at least one of two events occurs. The general formula accounts for overlap between events:

$$P(A \cup B) = P(A) + P(B) - P(A \cap B)$$

For mutually exclusive events (events that cannot occur simultaneously), the formula simplifies to:

$$P(A \cup B) = P(A) + P(B)$$

Conditional Probability

Conditional probability measures the probability of event A occurring given that event B has already occurred. It allows us to update probabilities based on new information:

$$P(A|B) = \frac{P(A \cap B)}{P(B)}$$
Real-world example: What is the probability that a student passed the exam given that they attended all lectures? Here, passing is event A and attendance is event B.

Bayes' Theorem

Bayes' theorem allows us to reverse conditional probabilities. Given P(B|A), we can calculate P(A|B):

$$P(A|B) = \frac{P(B|A) \times P(A)}{P(B)}$$

Bayes' theorem is widely used in medical diagnosis, spam filtering, machine learning, and Bayesian statistics.

Complement Rule

The complement of an event A, denoted A' or Ac, represents all outcomes where A does not occur:

$$P(A') = 1 - P(A)$$

How to Use This Probability Calculator

  1. Select Problem Type: Choose from Basic Probability (union), Conditional Probability, Bayes' Theorem, or Complement Rule based on what you need to calculate.
  2. Enter Probability Values: Enter the required probability values (between 0 and 1) for your selected problem type. The calculator displays which fields are needed for each type.
  3. Calculate Probability: Click Calculate Probability to compute the result with step-by-step solutions.
  4. Review Visual Diagram: Examine the interactive Venn diagram or probability tree to visualize how the probabilities relate.
  5. Study Additional Results: Review computed values like complement probabilities and intersection values.

Understanding Venn Diagrams

Venn diagrams visually represent relationships between events. In probability:

Probability Rules Summary

Addition Rule

For any two events A and B:

Multiplication Rule

For finding joint probabilities:

Complement Rule

Common Applications

Medical Testing

Bayes' theorem is crucial in medical diagnosis. Given a positive test result, what is the actual probability of having the disease? This depends on the test's sensitivity, specificity, and the disease prevalence.

Risk Assessment

Probability calculations help assess risks in finance, insurance, and project management. What is the probability that at least one of several potential issues will occur?

Quality Control

In manufacturing, probability helps determine defect rates and the likelihood of multiple defects occurring in a batch.

Games and Gambling

Understanding probability is essential for calculating odds in games, lotteries, and betting scenarios.

Frequently Asked Questions

What is probability?

Probability is a measure of the likelihood that an event will occur. It ranges from 0 (impossible) to 1 (certain). For example, P(A) = 0.5 means event A has a 50% chance of occurring.

How do I calculate P(A or B)?

Use the addition rule: P(A or B) = P(A) + P(B) - P(A and B). If events are mutually exclusive (cannot occur together), then P(A or B) = P(A) + P(B). Enter P(A), P(B), and optionally P(A and B) in the Basic Probability mode.

What is conditional probability?

Conditional probability P(A|B) is the probability of event A occurring given that event B has already occurred. The formula is P(A|B) = P(A and B) / P(B). It helps us update probabilities based on new information.

What is Bayes theorem used for?

Bayes theorem is used to reverse conditional probabilities. Given P(B|A), P(A), and P(B), you can calculate P(A|B) using the formula: P(A|B) = P(B|A) times P(A) divided by P(B). It is widely used in medical diagnosis, spam filtering, and machine learning.

What is the complement rule?

The complement rule states that P(not A) = 1 - P(A). If an event A has probability 0.7, then its complement (A not occurring) has probability 0.3. The event and its complement always sum to 1.

Additional Resources

For further learning about probability theory:

Reference this content, page, or tool as:

"Probability Calculator" at https://MiniWebtool.com/probability-calculator/ from MiniWebtool, https://MiniWebtool.com/

by miniwebtool team. Updated: Jan 12, 2026

Developer API available: Run this tool from your app, automation, or agent with one JSON HTTP request. View API docs

You can also try our AI Math Solver GPT to solve your math problems through natural language question and answer.

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