Implied probability turns a price into the chance the price suggests. For decimal odds, divide 1 by the odds. For fractional odds, divide the second number by the sum of both. When the implied probabilities of every outcome in a market are added up, the total is above 100%. The extra amount is the bookmaker’s margin, also called the overround. Comparing your own estimate of a chance with the implied probability is how bettors judge whether a price is fair.
Example
Decimal odds of 4.00 imply 25%. If a two outcome market is 1.91 on each side, each implies 52.4% and the total is 104.7%.