The formula
differential = (score − course rating) × 113 ÷ slope · index = average of the best
Where it comes from
A handicap does not come from the raw score but from the differential, which corrects the result for how hard the course is. The course rating says how many strokes a scratch player takes there, and the slope says how much harder it gets for someone who is not; 113 is the slope of a course of average difficulty, which is why it appears in the formula as the reference unit. Then you do not average every round: only the best eight of the last twenty, because a handicap measures what you are capable of, not what you average.
How to work it out by hand
- Turn each round into its differential using the rating and slope of that course
- Sort them and keep the best ones — eight if you have twenty rounds
- Average those and apply the adjustment if you have few rounds
- For the course you are playing, multiply by its slope over 113 and add rating minus par
What is worth knowing
With fewer than twenty rounds the system is more conservative: with three it counts only the best and takes two strokes off as well, and that penalty fades as more rounds come in. Index and course handicap are not the same thing, and mixing them up is the commonest confusion: the index is yours and does not change from course to course, while the course handicap is the one on the card and depends on the course and the tees. The same 13.7 index can be 15 on a hard course and 12 on an easy one.