To start building up a Handicap Index® you need to complete 54 holes, through either a combination of 18 or 9 hole rounds. At that point your H.I.™ will be based on the best Score Differential™.
9 Hole Rounds:
Under normal circumstances 9-hole rounds are usually scaled up to an 18 hole round so the correct calculation can be applied to your Handicap Index. However, as part of your initial 54 holes 9-hole rounds need to be paired.
Please see the following table to see how/what is included:
From that point your H.I. will become more reflective of your playing ability up to the point of completing 20 rounds, at which point it will be an average of your best 8 Score Differentials (plus taking your Low Handicap Index™ into consideration).
Following is a table that explains how your HI is calculated until you have completed 20 scores.
It is also worth noting that the score differential for any 9 hole round, that is played as part of the initial 54 holes, is not scaled up to 18. Therefore, the DIFF™ appears lower than what is expected. If there are 2 x 9 hole rounds they will be paired and the DIFF will be treated as if it was an 18 hole round.
Once a HI has been achieved all 9 hole rounds are automatically scaled up to an 18 round when calculating the DIFF.