A golf handicap is calculated from a golfer’s ten best scores out of the last twenty rounds.
For each of those rounds, calculate a differential based on the following formula:
Differential = ( (Player Score – Course Rating) (113) ) / Slope
Then, use those differentials to average a handicap.
Handicap = (Average of Differentials) (.96)
In English: for each round you’re counting, subtract the course rating from your score. Then, multiply that result by 113. Finally, divide the previous result by the slope of the course that you played. That gives you the differential score for that round.
Handicaps often are misunderstood by players. Handicap is a measure of a player’s potential, and not his average. That’s why you take the ten best out of 20. It’s also why it would not be unusual for a player to perform far below expectations on any given day.
For a much more thorough discussion of handicaps, see this post on calculating handicaps.