Most golfers know their handicap index—but fewer understand how it's calculated. It's not a simple average of your scores. The World Handicap System (WHS) uses a specific formula based on score differentials, and it deliberately favors your best rounds rather than treating all of them equally. Once you understand why, the number makes a lot more sense.
What Is a Score Differential?
A score differential measures how well you played relative to the difficulty of the course on a given day. The formula is:
Differential = (Adjusted Gross Score − Course Rating) × 113 ÷ Slope Rating
The Course Rating represents what a scratch golfer would score on that course under normal conditions. The Slope Rating (ranging from 55 to 155, with 113 as the baseline) measures how much harder the course plays for a bogey golfer versus a scratch golfer. Dividing by 113 and multiplying by the actual slope normalizes every round to a common scale, so a round at a difficult course and an easy course can be compared fairly.
For example: if you shoot 89 on a course with a rating of 71.2 and slope of 128, your differential is (89 − 71.2) × 113 ÷ 128 = 15.7.
Why the Best 8 of 20?
Under WHS, your handicap index is calculated from the lowest 8 differentials out of your most recent 20 rounds, averaged and multiplied by 0.96. That 4% reduction is called the "bonus for excellence"—it encourages golfers to play their best rather than sandbag.
Using the best 8 of 20 means your handicap reflects your demonstrated potential, not your median performance. A few bad rounds don't drag your index up; a few great rounds pull it down. This makes the index a better predictor of what you're capable of on a good day, which is the goal of net stroke play—giving everyone a realistic chance when they're playing well.
What This Means in Practice
If you've been playing for a while, you'll notice your index tends to be lower (better) than what you shoot on most days. That's intentional. On any given round there's roughly a 25% chance you'll play to your index or better, and the WHS is designed around that expectation.
Xordinari Golf calculates differentials automatically after every round you log, keeps your rolling 20-round history, and updates your index without any manual entry. It also applies Playing Conditions Calculation (PCC) adjustments on rounds where weather or course conditions significantly affected everyone's scores—another WHS feature that most apps ignore but that meaningfully improves index accuracy over time.