9. Prevent Compaction within the PRZ Calculating the Critical Root Radius, CRR of the Protected Root Zone, PRZ 1. First measure the tree’s circumference (distance around) at a 4.5 foot height, in inches. 2. Divide the number by 2. 3. Express the results in feet. CRR Example: Circumference = 30 inches 30 2 = 15 CRR = 15 feet dripline Protected Root Zone (PRZ)