Probability for equally likely outcomes
The basic formula P(event) = favourable outcomes ÷ total outcomes applies when each outcome is equally likely. The result can be shown as a decimal, fraction, percentage, or odds.
Example
A fair six-sided die has six equally likely outcomes. Two outcomes, 5 and 6, satisfy “greater than 4,” so the probability is 2/6 = 1/3, or approximately 33.33%.
Know the model's limits
Real-world outcomes are not always equally likely or independent. Historical frequency does not guarantee a future event, and this simple calculator does not handle conditional probability, dependent events, distributions, or uncertainty in the input assumptions.
Probability as a Fraction, Decimal and Percentage
The same probability can be written in different forms. A fraction of 1/4 equals the decimal 0.25 and the percentage 25%. Convert a decimal to a percentage by multiplying by 100.
Probability Compared with Odds
Probability compares favorable outcomes with all outcomes. Odds in favor compare favorable outcomes with unfavorable outcomes. With one winning outcome among four equally likely outcomes, probability is 1/4 while odds in favor are 1:3.
When the Basic Formula Does Not Apply
Real-world outcomes are not always equally likely or independent. Historical frequency does not guarantee a future event. This calculator does not model conditional probability, dependent events, probability distributions, sampling bias, or uncertainty in the input assumptions.
