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Probability Calculator – Compute Single, Compound & Conditional Probabilities

Calculate probabilities for single events, compound events (AND/OR), conditional probability, and complementary probability. Free online probability calculator with fractions, percentages, and odds.

Updated for 2026

By the GlobalCalqulate team, founded by Pavan Kusunuri · About our editorial standards

Calculation Type

Type

Single Event Inputs

Favorable Outcomes

Number of outcomes considered success.

Total Possible Outcomes

All equally likely outcomes.

Compound Probability Inputs

P(A) Probability of Event A (0 to 1)

Probability of first event, e.g., 0.5 for 50%.

P(B) Probability of Event B (0 to 1)

Probability of second event, e.g., 0.5 for 50%.

Operation

—

✓ Reviewed and updated for accuracy using current official rates and widely accepted formulas.

How to Use This Calculator

Select Calculation Type

Choose single event, compound (AND/OR), conditional, or complement probability.

Enter Your Values

Input favorable/total outcomes for single events, or decimal probabilities (0 to 1) for compound/conditional.

Review Results

Results appear instantly showing probability as decimal, percentage, and odds. Interpretations help you understand likelihood.

Understanding Your Results

Probability Basics
Probability ranges from 0 (impossible) to 1 (certain). Formula: P(A) = Favorable / Total. A fair coin flip has P(heads) = 1/2 = 0.5 = 50%. The probability of rolling a 6 on a die is 1/6 ≈ 0.1667 = 16.67%.
AND Rule
For independent events, the probability of both occurring is the product: P(A and B) = P(A) × P(B). Example: flipping heads (0.5) AND rolling a 6 (0.1667) = 0.5 × 0.1667 = 0.0833 = 8.33%.
OR Rule
For mutually exclusive events (cannot happen together), add probabilities: P(A or B) = P(A) + P(B). The probability of rolling 1 OR 6 = 1/6 + 1/6 = 2/6 = 0.3333 = 33.33%.

Key Tips

✓
Probability is always between 0 and 1. A result > 1 or negative indicates incorrect inputs.
✓
Use decimal form (0.5 not 50) for compound probability inputs.
✓
The complement rule: P(not A) = 1 − P(A) — useful for calculating the probability something won't happen.

Help & FAQs

Frequently Asked Questions

Clear answers to common questions to help you use this calculator confidently.

How do I use this calculator?

Tap to view the answer

Enter your values in the input fields and results appear instantly. Adjust any parameter to recalculate automatically.

Is this calculator free to use?

Tap to view the answer

Yes, it's completely free. All calculations run directly in your browser with no limits.

Why are my results not what I expected?

Tap to view the answer

Double-check your inputs. Make sure you're using the correct units or format. If uncertain, consult the educational content or FAQs below.

Where are the formulas used?

Tap to view the answer

All formulas are mathematically verified against standard mathematical references before release.

Need more help? Contact support or email support@globalcalqulate.com

We typically reply within 24–48 hours.

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How to Use This Calculator

Select Calculation Type

Choose single event, compound (AND/OR), conditional, or complement probability.

Enter Your Values

Input favorable/total outcomes for single events, or decimal probabilities (0 to 1) for compound/conditional.

Review Results

Results appear instantly showing probability as decimal, percentage, and odds. Interpretations help you understand likelihood.

Understanding Your Results

Probability Basics
Probability ranges from 0 (impossible) to 1 (certain). Formula: P(A) = Favorable / Total. A fair coin flip has P(heads) = 1/2 = 0.5 = 50%. The probability of rolling a 6 on a die is 1/6 ≈ 0.1667 = 16.67%.
AND Rule
For independent events, the probability of both occurring is the product: P(A and B) = P(A) × P(B). Example: flipping heads (0.5) AND rolling a 6 (0.1667) = 0.5 × 0.1667 = 0.0833 = 8.33%.
OR Rule
For mutually exclusive events (cannot happen together), add probabilities: P(A or B) = P(A) + P(B). The probability of rolling 1 OR 6 = 1/6 + 1/6 = 2/6 = 0.3333 = 33.33%.

Understanding Probability Theory

What Is Probability?

Probability measures the likelihood of an event occurring. It ranges from 0 (impossible) to 1 (certain) and can be expressed as a fraction, decimal, or percentage. Probability was formalized in the 17th century by Blaise Pascal and Pierre de Fermat while analyzing gambling problems. Today, probability underlies fields from weather forecasting to medical diagnosis, finance, and artificial intelligence.

Single Event Probability

The simplest case: P(Event) = Favorable Outcomes / Total Possible Outcomes. For a fair six-sided die, the probability of rolling any specific number is 1/6. For a standard deck of cards, the probability of drawing an Ace is 4/52 = 1/13. The key assumption is that all outcomes are equally likely — this is the classical definition of probability.

Compound and Conditional Probability

Compound probability covers multiple events. For independent events (one doesn't affect the other), use multiplication: P(A and B) = P(A) × P(B). For mutually exclusive events, use addition: P(A or B) = P(A) + P(B). Conditional probability P(A|B) asks: given that B occurred, what's the probability of A? Bayes' Theorem formalizes conditional probability and is fundamental to machine learning, spam filtering, and diagnostic testing.

Probability Calculator – Compute Single, Compound & Conditional Probabilities

Calculation Type

Single Event Inputs

Compound Probability Inputs

How to Use This Calculator

Understanding Your Results

Key Tips

Frequently Asked Questions

How do I use this calculator?

Is this calculator free to use?

Why are my results not what I expected?

Where are the formulas used?

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Calculation Type

Single Event Inputs

Compound Probability Inputs

How to Use This Calculator

Understanding Your Results

Key Tips

Understanding Probability Theory

What Is Probability?

Single Event Probability

Compound and Conditional Probability

Example: Rolling a Six on a Die

Understanding Probability Theory

What Is Probability?

Single Event Probability

Compound and Conditional Probability

Example: Rolling a Six on a Die

Calculation Type

Single Event Inputs

Compound Probability Inputs

How to Use This Calculator

Understanding Your Results

Key Tips

How do I use this calculator?

Is this calculator free to use?

Why are my results not what I expected?

Where are the formulas used?

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Calculation Type

Single Event Inputs

Compound Probability Inputs

How to Use This Calculator

Understanding Your Results

Key Tips

Understanding Probability Theory

What Is Probability?

Single Event Probability

Compound and Conditional Probability

Example: Rolling a Six on a Die

Understanding Probability Theory

What Is Probability?

Single Event Probability

Compound and Conditional Probability

Example: Rolling a Six on a Die