A golf handicap differential is calculated subtracting a course's rating from a player's adjusted gross score, then multiplying the sum by 113 divided by the course's Slope rating. A handicap index represents a golfer's average handicap differential multiplied by 0.96.
The handicap differential formula can be written as: (adjusted gross score - course rating) x (113 / Slope rating) = handicap differential.
In the United States the Slope and course ratings are assigned by the USGA. 113 represents the average Slope rating. A player's adjusted gross score is the player's average over a minimum of five 18-hole rounds. The course rating is equivalent to par for the course. The Slope rating is a difficulty rating, which most golf courses make available to golfers at the clubhouse and sometimes online.