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Math formula

How to calculate probability

Simple probability measures the chance of an event on a scale from 0 to 1.

See the formulaOpen calculator
Direct answer
Formula
P(event) = favorable outcomes / total outcomes

Use the table below to match each symbol with the right input. Keep units consistent before you start.

Probability formula
Start here when you only need the equation and variable names.
Formula
P(event) = favorable outcomes / total outcomes

Simple probability measures the chance of an event on a scale from 0 to 1.

Variable meanings
Check each symbol, meaning, and unit before you calculate.
SymbolNameDescriptionUnit
P(event)ProbabilityChance that the event happens.Depends on inputs
favorable outcomesFavorable outcomesOutcomes that meet the event condition.Depends on inputs
total outcomesTotal outcomesAll equally likely outcomes.Depends on inputs
When to use this formula
Check that your situation matches the formula before you trust the result.
  • Simple probability works when outcomes are equally likely, such as drawing one item or rolling a fair die.
Step-by-step method
Follow these steps when you are solving it by hand.
  1. 1Count the outcomes that match the event.
  2. 2Count all possible outcomes.
  3. 3Divide favorable outcomes by total outcomes.
  4. 4Multiply by 100 if you want a percent.
Examples
These sample numbers show the order of operations and units.
Colored balls
favorable outcomes
3 red balls
total outcomes
10 balls
  1. 1.P(red) = 3 / 10
  2. 2.P(red) = 0.3
Result
The probability is 0.3, or 30%.
Fair die
favorable outcomes
3 even faces
total outcomes
6 faces
  1. 1.P(even) = 3 / 6
  2. 2.P(even) = 0.5
Result
The probability is 0.5, or 50%.
Mistakes to avoid
Small input or unit errors can change the answer a lot.
  • Putting the wrong value in the numerator or denominator of the probability formula.
  • Skipping signs, order, or sorting rules that change the result.
  • Rounding before the last step.
  • Assuming a formula applies when the needed input is zero or missing.
Open the Probability Calculator
Open the calculator for the answer without rewriting the formula.

Enter your values in the related calculator, then compare the output with the hand method above.

Open Probability Calculator
FAQs
Short answers for common formula questions.
Can I calculate Probability by hand?

Yes. Write the units next to each value, convert rates or measurements first, and round only the final answer.

Why does my Probability result differ from another calculator?

Most differences come from rounding, unit conversions, rate timing, or a slightly different version of the formula.

When is the Probability Calculator better than hand math?

Open the Probability Calculator to check several scenarios or skip the hand arithmetic.