Free calculator
Encroachment Zone Calculator
Measure how much proposed works encroach into a tree's Notional Root Zone (NRZ), using either a straight edge or shapes drawn to scale, and classify the result under the AS 4970:2025 minor, moderate and major encroachment bands.
Enter tree and encroachment details
Encroachment area inside the NRZ
4.50 m²
4.0% of the NRZ · NRZ radius 6.00 m, area 113.10 m²
Minor encroachment (4.0% of the NRZ, outside the SRZ). Unlikely to significantly affect the tree, provided there has been no recent encroachment.
Compensating area to add to the TPZ elsewhere: 4.50 m², unless the project arborist demonstrates the tree remains viable without it.
Enter the DARB to check for intrusion into the Structural Root Zone — any SRZ intrusion is a major encroachment regardless of percentage.
Minor ≤ 10% · Moderate > 10–20% · Major > 20% or any SRZ intrusion
Plan view
Red shading shows the proposed encroachment into the Notional Root Zone (dashed green).
How encroachment area is calculated
In straight-line mode, the works are treated as a straight boundary cutting into the NRZ circle at a given depth, and the resulting circular-segment area is calculated. In drawing mode, you draw one or more shapes on a plan scaled in metres around the trunk; overlapping shapes are counted once and only the part inside the NRZ counts.
Minor, moderate and major encroachment
AS 4970:2025 measures encroachment as a percentage of the NRZ area. Up to 10% is minor, more than 10% and up to 20% is moderate, and more than 20% — or any intrusion into the Structural Root Zone (SRZ) — is major. In each case an area equal to the encroachment is generally added to the Tree Protection Zone elsewhere around the tree, unless the project arborist demonstrates the tree will remain viable without it. Moderate and major encroachments need the project arborist to show how the tree will remain viable.
Next step
If your proposed encroachment is moderate or major, a project arborist should review the specific root architecture before the Arboricultural Impact Assessment is finalised.