TPZ / NRZ calculator
Enter a trunk diameter measured at 1.4 metres and get the notional root zone radius and area, worked from AS 4970:2025 — the current edition.
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Calculate
Notional root zone
The radius is the trunk diameter in metres multiplied by twelve, floored at 2 metres and capped at 15. The area is the circle that radius describes. Add the buttress diameter and the proposed works and the rest follows.
1The tree
Where exactly do I measure?
- Level groundMeasure the diameter 1.4 m above ground level.
- Sloping groundTake ground level on the upslope side, then measure 1.4 m above that.
- Leaning trunkMeasure 1.4 m along the lean, not 1.4 m vertically off the ground.
- Fork above 1.4 mOne trunk at that height, so measure it as a standard tree.
- Fork below 1.4 mEach stem is measured, then combined. Use the multi-stemmed tree type.
- Buttressed treeMeasure just above the buttress, where the trunk stops widening into the roots.
Enter a trunk diameter and the notional root zone appears here.
2Structural root zoneOptional
Enter the buttress diameter and the structural root zone appears here.
3The proposed works
A straight edge — a slab, wall or footing
The encroachment needs a trunk diameter in step 1 first.
Your figures
Enter a trunk diameter and the zones appear here. Nothing on this page is a Tree Protection Zone — that is determined by a project arborist.
- NRZ radius
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- NRZ area
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- From DSH
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- SRZ radius
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- Encroached area
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- Encroachment
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- It does not give you a Tree Protection Zone. The notional root zone is the calculated starting point; the final TPZ is determined by a project arborist after considering the tree, the site and the proposed works.
- It does not tell you whether a development complies, whether a council will approve anything, whether a tree can be removed, or whether works are safe to proceed.
- It does not replace a site inspection. The measurements have to be taken on the tree, and site-specific professional assessment may still be required.
- It does not handle palms, cycads or tree ferns, because the supplied Standard does not support applying the standard method to them.
- It does not decide how a multi-stemmed tree should be measured. AS 4970:2025 gives the project arborist two methods and asks them to justify the one they pick; this uses the cross-sectional area method and says so.
Common questions
About this calculator
No. It is the Notional Root Zone — the calculated starting point used when determining the Tree Protection Zone. The final TPZ may change after a project arborist considers the tree, the site and the proposed works. Writing the calculated figure onto a plan and fencing to it is the most common error on a development site.
Trunk diameter at 1.4 metres above ground level — diameter at standard height. Measure the diameter, or measure the circumference and divide by pi. On sloping ground, ground level is taken on the upslope side.
Those are the limits in the supplied rules. A calculated radius below 2 metres is raised to 2, and one above 15 metres is capped at 15. The calculator says so when a limit has been applied.
Each stem is measured at 1.4 metres and combined into a single diameter by cross-sectional area, which is the method set out in Appendix A of AS 4970:2025. The Standard gives the project arborist a choice between that and measuring around all stems, and asks them to give their reasons for the method they use — so the figure here is a starting point calculated one particular way, not a decision about how your tree should be measured.
Use it to understand the scale of what you are dealing with. An application needs a report prepared by a suitably qualified arborist from measurements taken on site — a figure from a web calculator is not that, and no calculator output establishes compliance or approval.
Contact
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For many protected vegetation questions, the property address is enough to start checking. Call, or send the details below.