The formula
For 18 holes: Course Handicap = Handicap Index x (Slope Rating / 113) + (Course Rating - Par). The slope scales your index for how hard the tees play for a bogey golfer, and the rating minus par adjusts for how hard they play for a scratch golfer.
The unrounded Course Handicap is carried forward. Playing Handicap is the Course Handicap multiplied by the allowance for the format and rounded to a whole number as the very last step, with a half rounded up. Rounding early is the most common reason two apps disagree by a stroke.
Worked example: an index of 10.4 on tees with slope 130, rating 72.1 and par 72. 10.4 x 130 / 113 = 11.96, plus 0.1 is 12.06, so the Course Handicap is 12. At 95% the Playing Handicap is 12.06 x 0.95 = 11.46, which rounds to 11.
Nine holes
For nine holes, halve the Handicap Index (to one decimal) and use the nine-hole slope, rating and par: Course Handicap = (Index / 2) x (9-hole slope / 113) + (9-hole rating - 9-hole par). A published example: index 8.7, rating 35.3, slope 121 and par 36 gives a Course Handicap of 4.
Which holes get strokes
Strokes go to holes in order of stroke index, starting at 1. A Playing Handicap of 11 gets a stroke on holes with stroke index 1 to 11. Above 18, a second stroke starts again at index 1. A plus handicap works in reverse: you give strokes back starting at stroke index 18.
Which allowance should I choose?
Common allowances: 100% for individual match play, 95% for individual stroke play, and 85% for four-ball stroke play. In four-ball match play, the usual approach is 90% of the difference from the lowest player, which the off-the-low calculator handles. Your committee or group decides; these are the common starting points.
This tool only does arithmetic and stores nothing.