Tree Root Zone Calculator

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, and often not until 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 sit in the top 60 cm (24 inches) of soil and reach out 2 to 3 times the drip line, which is the edge of the canopy. The feeder roots that take up water and nutrients are shallower still, often in the top 15 cm (6 inches).

Damage to as little as 25 to 40% of the root zone can start a decline. Construction traffic does it without ever cutting a root: driving over the zone compacts the soil, oxygen and water stop moving through it, and the roots suffocate.

The International Society of Arboriculture (ISA) Standard

The most widely used formula in professional arboriculture:

CRZ Radius (feet) = Trunk Diameter (inches) × 1.5

Trunk diameter here means Diameter at Breast Height (DBH), measured 4.5 feet (1.37 m) above the ground. Measure lower and you catch the root flare, which is wider and would inflate the answer.

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.

The same rule in metric

The ISA formula is written in imperial, so converting it takes two steps. One centimetre of trunk is 0.394 inches, and 1.5 feet is 0.457 metres, so:

CRZ radius (metres) = Trunk diameter (cm) × 0.18

Check it against the example above. A 10 inch trunk is 25.4 cm, and 25.4 × 0.18 = 4.57 m, which is the same 15 feet. Watch out for the version of this rule that circulates as “cm × 0.6”: that factor gives the answer in feet, not metres, and using it as metres puts the protection fence more than three times too far out.

For the conservative 1:1 standard the metric factor is 0.12, and for the generous 2:1 it is 0.24. The calculator handles all three.