To calculate Moneyline probability when betting on a favorite, add 100 to positive odds. Then, divide 100 by the result. If you bet on an underdog, subtract 100 from negative odds and divide negative odds by the result. To get an implied probability for the fractional format, do this: the top number/ (the top number + the bottom one) x 100. The formula for the decimal: (1/ odds) x 100.