Free calculator
SRZ Calculator
Calculate a tree's Structural Root Zone (SRZ) radius and area to AS 4970:2025 from its Diameter Above Root Buttress (DARB). The SRZ is the area needed for a tree's stability — any intrusion into it is a major encroachment.
Enter trunk diameter
Structural Root Zone radius
2.67 m
SRZ area: 22.40 m²
Plan view
The SRZ is the area around the trunk needed for tree stability — any intrusion into it is a major encroachment, whatever the percentage of the NRZ.
How the SRZ is calculated
AS 4970:2025 defines the SRZ radius as (D × 50)0.42 × 0.64, where D is the Diameter Above Root Buttress (DARB) in metres. The SRZ has a minimum radius of 1.5 m, and no maximum.
Why the SRZ matters
Encroachment into the wider Notional Root Zone (NRZ) is classified as minor, moderate or major by percentage. The SRZ is different: it holds the woody roots that keep the tree standing, so any intrusion into it is a major encroachment in an Arboricultural Impact Assessment.