Probability quantifies likelihood on a scale from 0 (impossible) to 1 (certain). It underpins statistics, gambling analysis, risk assessment, and machine learning.
Basic Formula
P(event) = favorable outcomes / total outcomes (for equally likely outcomes). Use our Probability Calculator for quick computations.
Addition and Multiplication Rules
For mutually exclusive events: P(A or B) = P(A) + P(B). For independent events: P(A and B) = P(A) * P(B).
Complement Rule
P(not A) = 1 - P(A). If there's a 30% chance of rain, there's a 70% chance it won't rain.
Going Deeper
Learn fundamental probability rules, formulas, and real-world applications. This guide connects theory to practice — use the related calculators linked at the bottom to verify each example with your own numbers.
Practical Tips
- Write down given values and unknowns before opening the calculator.
- Check units and rounding rules appropriate to your context (class, lab, or business).
- Compare manual working with the calculator result to build confidence.
Common Mistakes to Avoid
- Rushing inputs without reading field labels carefully.
- Confusing similar formulas that use different variables or units.
- Reporting results with more precision than your inputs justify.
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