Tree Root Zone Calculator
Calculate the critical root zone (CRZ) of a tree based on trunk diameter.
Determine safe distances for construction, excavation, and landscaping near trees.
The Critical Root Zone (CRZ), also called the Tree Protection Zone (TPZ), is the area around a tree that contains the majority of its structural and feeder roots. Disturbing this zone through construction, trenching, soil compaction, or grading causes root damage that can kill even a healthy, established tree — sometimes years after the disturbance.
Why Root Zones Matter
Tree roots do not grow like a carrot straight down. The majority of a tree’s roots are in the top 60 cm (24 inches) of soil and extend 2–3 times the drip line (canopy edge) in all directions. The feeder roots that absorb water and nutrients are at the very surface — often in the top 15 cm (6 inches) of soil.
Damage to even 25–40% of the root zone can cause decline. Construction equipment driving over the root zone compacts soil, reducing oxygen and water penetration — the roots essentially suffocate.
The International Society of Arboriculture (ISA) Standard
The most widely used formula in professional arboriculture:
CRZ Radius (feet) = Trunk Diameter (inches) × 1.5
Where trunk diameter is measured at Diameter at Breast Height (DBH) — 4.5 feet (1.37 m) above the ground.
Example: A tree with a 10-inch DBH has a CRZ of 10 × 1.5 = 15 feet radius.
Alternative Formulas
Some jurisdictions use:
- 1 foot of radius per 1 inch of trunk diameter (more conservative)
- The “dripline radius + 50%” rule for trees with large canopies
Construction Near Trees
When construction must occur near trees, protective fencing should be installed at the CRZ boundary before any work begins. Trenching within the CRZ should be done by hand or air spade, not mechanical equipment.
Root Zone in Metric
DBH in centimeters × 0.6 = CRZ radius in meters (Equivalent formula: cm × 38.1 mm per cm = mm, then × 1.5 / 25.4)
This calculator provides CRZ radius in feet, meters, and square area.