How a golf Handicap Index is calculated: best 8 of 20, and why there is no 0.96

The short version: average the best 8 of your last 20

A Handicap Index is built from your last 20 scores, and only the best 8 count. The Rules of Handicapping, published jointly by the USGA and The R&A, say to "average the lowest 8 of the most recent 20 Score Differentials" and round to the nearest tenth (Rule 5.2b, R&A Rules of Handicapping). The other 12 are in the record but do not enter the average, so an occasional blow-up round costs you nothing once it falls outside your best 8.

Two steps come before that average and two come after it. Before: each round has to be turned into a Score Differential, and a score is first adjusted so one disaster hole does not wreck the round. After: the result is checked against caps that stop an index rising too fast, and an exceptional score can pull it down.

Step 1: turn each round into a Score Differential

A Score Differential measures a round against the difficulty of the course you played. The Southern California Golf Association states it as 113 divided by the Slope Rating of the tees played, multiplied by the adjusted gross score minus the Course Rating minus the playing conditions adjustment (SCGA, Handicap Index calculation).

The number 113 is the Slope Rating of a course of standard difficulty, so the formula scales the gap between your score and the Course Rating up or down by how much harder or easier than standard the tees are. The playing conditions adjustment is normally 0 and moves only when the weather or the course set-up made scores that day unusually high or low.

The same 88 is worth 14.8 on a slope of 128, 13.6 on a harder slope of 140 and the full 16.8 on a slope of exactly 113. That is the whole purpose of the slope term: one score does not mean the same thing on every course.

Step 2: average the lowest 8 of the last 20

Sort your 20 most recent differentials, take the lowest 8, add them and divide by 8. The 20 differentials below are an example, not real data.

The worst 12 scores, up to 16.3, change nothing. Replace the 16.3 with a 30.0 and the index is still 10.9, which is why a Handicap Index describes what you can play to when you play well rather than what you shoot on average.

Why there is no 0.96 any more

The 0.96 belonged to the system before 2020. Maine Golf describes the old method as the best 10 of the most recent 20 differentials, averaged and then multiplied by .96, a step it calls the "bonus for excellence"; it says the move to the best 8 of 20 made that bonus unnecessary (Maine Golf, Basis of Handicap Index Calculation). Averaging fewer, better scores gives the same kind of weight to good rounds that the multiplier used to give by shaving the average.

Many calculators and articles still carry the old rule, so it is the most common reason a figure does not match your club's. Run the same 20 differentials both ways and the difference is visible.

Applying 0.96 to the lowest 8 would give 10.5, a figure no current rule produces. If a tool on the web gives you a number lower than your club's, check first whether it multiplies by 0.96.

Step 3: the soft cap and hard cap

The index is allowed to fall as fast as your scores do, but not to rise at the same speed. Compare the new average with your Low Handicap Index, the lowest index you have held in the past 12 months. If the average is more than 3.0 strokes above it, the soft cap applies: "the value above 3.0 strokes is restricted to 50% of the increase." After that, the hard cap holds the index to no more than 5.0 strokes above the Low Handicap Index (Rule 5.8, Rules of Handicapping).

At 14.0 the first 3.0 of the rise counts in full and the other 1.0 counts as 0.5, giving 13.5. At 20.0 the soft cap alone would allow 16.5 (3.0 plus half of 7.0), and the hard cap then pulls it back to 15.0. The caps are about protecting the index from a bad run, such as an injury or a long break, and they do not hold it down once your scores recover.

Step 4: exceptional scores

A round far better than your index pulls the whole record down at once. Where a Score Differential is 7.0 or more strokes lower than your Handicap Index, the Rules apply an exceptional score reduction to the 20 differentials in your scoring record: 1.0 stroke for a difference of 7.0 to 9.9 and 2.0 strokes for 10.0 or more, measured on the unrounded difference (Rule 5.9, Rules of Handicapping). A difference of 10.0 or more also generates a review notification.

What happens with fewer than 20 scores

A new golfer's Handicap Index is calculated after 54 holes, in any combination of 9-hole and 18-hole rounds (SCGA). Until the record reaches 20 scores, a table in Rule 5.2a sets how many of the lowest differentials are averaged and whether an adjustment applies, and an initial index above 54.0 is set to 54.0 (Rule 5.2a, Rules of Handicapping). The table is reproduced in the Rules of Handicapping and in your national association's version, so take the figures from there for the number of scores you hold.

What this page does not do

This page explains the calculation. It does not produce your Handicap Index, which only your national association or club can do from your submitted scores, and rules and figures can be revised, so the Rules of Handicapping and your association are the final word. Rounding, 9-hole scores, the maximum score allowed on a hole and a course's own rating are all set in those Rules.

To turn an index into the strokes you receive on a particular course, and to score the round itself, the stableford and net scores guide covers course handicap allocation and points, and the golf scorecard keeps the card. To find the average of any list of numbers, such as the lowest 8 of your own differentials, use the average calculator.