IGA Handicap / Index Calculations
How is a Handicap Index produced?
It is essential to the true spirit and etiquette of golf to possess a proper golf handicap / index. According to the USGA such a proper handicap index is created through a series of precise steps. The overall formula used is as follows:
Differential = (113 * ( Adjusted Score  Rating)) / Course Slope
A handicap differential is the difference between the adjusted gross score and the actual Course Rating, multiplying the difference by 113, dividing the results figure by the Slope Rating and rounding off to the nearest tenth. The following is an example for a course with a 70.5 Course Rating and a 124 Slope Rating.
Adjusted Gross Score 
92 
Course Rating 
70.5 
Difference 
21.5 
Slope Rating 
124 
Handicap Differential 
113 x 21.5 / 124 = 19.6 

Handicap = .96 * (Avg. of best 10 differentials from 20 last rounds)
Equitable Stroke Control
A calculation is made on the 20 most recent golf scores. These scores must be adjusted as follows:
Equitable Stroke Control (ESC) is a procedure whereby abnormally high individual hole scores are adjusted downward prior to the score being posted. ESC sets a maximum number that a golfer can post on any hole, depending on the golfer's Course Handicap (not his Handicap Index).
Course Handicap Index 
Maximum Number on any Hole 
9 or less 
Double Bogey 
10 through 19 
7 
20 through 29 
8 
30 through 39 
9 
40 or more 
10 

For each of these 20 most recent rounds you will need the following from each related score:
Date
Your Current Handicap Index
Course SSS Rating (Standard Scratch Score)
Course Slope Rating (if no slope indicated then 113)
If you have fewer than 20 differentials available then use the following table to select the right number of cards to use in the calculation.
N° Registered 
Result 
14 
No INDEX issued 
56 
Lowest 1 
78 
Lowest 2 
910 
Lowest 3 
1112 
Lowest 4 
1314 
Lowest 5 
1516 
Lowest 6 
17 
Lowest 7 
910 
Lowest 8 
19 
Lowest 9 

Once you have recorded the 20 differentials, pull out the 10 best. Add those 10 best together, and then divide the sum by 10. This will give you the average of your top 10 differentials. Then, multiply that average by .96 and the result will be your Handicap Index.