Golf Handicap Differential
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A handicap differential is a single round's performance translated into a standardized number that accounts for the course's difficulty, used as one of several rounds averaged together to calculate a player's overall handicap index. It lets rounds played on different courses be compared fairly.

The formula is (score − course rating) × 113 / slope rating. Course rating reflects the expected score of a scratch golfer, and slope rating reflects how much harder the course plays for a bogey golfer relative to a scratch golfer, with 113 being the average slope value used as a scaling constant.

Course rating alone reflects difficulty for an expert player, but doesn't capture how much harder a course plays for an average or higher-handicap golfer. Slope rating adjusts for that gap, ensuring the differential reflects difficulty appropriately across all skill levels, not just elite ones.

113 is defined as the slope rating of a course of 'standard' difficulty, the baseline value in the slope rating system which typically ranges from 55 to 155. Using it as the denominator's reference point means a differential calculated on an average-difficulty course needs no further adjustment.

Modern handicap systems (like the World Handicap System) typically average the best 8 of a player's most recent 20 rounds' differentials, applying further adjustments for things like exceptional rounds. Only using the best scores, rather than all of them, is intentional, meant to reflect a player's demonstrated potential rather than their average performance.