How the handicap index is calculated
Under the World Handicap System, the handicap index is based on the best eight score differentials from your most recent twenty rounds. Each differential is calculated as (113 / Slope Rating) multiplied by the difference between your adjusted gross score and the course rating. The average of the best eight differentials is then multiplied by 0.96 to produce the handicap index.
If you have fewer than twenty rounds, the system uses a smaller number of best differentials according to a lookup table. With only three rounds, for example, only the lowest differential is used minus a further adjustment.
Differential = (113 / Slope) x (Score - Course Rating)
113 is the standard slope rating for an average-difficulty course.