Probability Calculator Formula

Understand the math behind the probability calculator. Each variable explained with a worked example.

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Formulas Used

Probability

probability = favorable / total

Probability (%)

percentage = (favorable / total) * 100

Odds in Favor

odds_for = favorable / (total - favorable)

Variables

VariableDescriptionDefault
favorableFavorable Outcomes3
totalTotal Outcomes10

How It Works

How to Calculate Probability

Formula

P(A) = Favorable outcomes / Total outcomes

Probability measures how likely an event is to occur, ranging from 0 (impossible) to 1 (certain). Multiply by 100 to express as a percentage. The odds in favor are the ratio of favorable to unfavorable outcomes.

Worked Example

A bag has 3 red balls out of 10 total. What is the probability of drawing a red ball?

favorable = 3total = 10
  1. 01Favorable outcomes = 3 (red balls)
  2. 02Total outcomes = 10 (all balls)
  3. 03P(red) = 3 / 10 = 0.3
  4. 04As a percentage: 0.3 * 100 = 30%
  5. 05Odds in favor: 3 / (10 - 3) = 3/7 ≈ 0.4286

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