Example of open Area per 20' Length

2 rows .20" slots on 120 degree centers. 1/4" spacing.
= 31.92 sq. inches open area per 20' length

2 rows 1/2" holes on 120 degrees centers. 5" spacing
= 18.4522 sq. inches open area per 20' length

This drawing shows the advantage that slotted pipe has over perforated pipe in zone coverage. A row of slots cover a greater range of the pipe's circumference than a row of holes.