TPZ, NRZ and SRZ calculator (AS 4970:2025)
Enter the trunk measurements and get the Notional Root Zone, Structural Root Zone and encroachment percentage under AS 4970:2025. Free, no sign-up.
How the zones are calculated
Notional Root Zone (NRZ). The radius is 12 times the diameter at standard height, measured at 1.4 m above ground, with a minimum of 2 m and a maximum of 15 m (clause 3.2). Palms, cycads and tree ferns get no calculated NRZ, but theirs can't be less than 2 m.
NRZ radius (m) = DSH (m) × 12
Structural Root Zone (SRZ). Calculated from the diameter just above the root flare (DRF), not the DSH (clause 3.4). Trunks under 0.15 m get a 1.5 m SRZ. The SRZ is required when major encroachment is proposed, and doesn't apply to palms, cycads or tree ferns.
SRZ radius (m) = (D × 50)^0.42 × 0.64
Multi-stem trees. Stem diameters aren't simply added. The combined DSH is the square root of the sum of the squared stem diameters (Appendix A, informative).
Combined DSH = √(d1² + d2² + … + dn²)
Encroachment. The share of NRZ area taken by the proposed works sets the category (clause 3.3). In every category, an area equal to the encroachment is added to the TPZ elsewhere, unless the project arborist shows the tree stays viable without it.
| Category | Encroachment | What follows |
|---|---|---|
| Minor | 10% or less of NRZ area, outside the SRZ, no recent encroachment | Significant impact unlikely; protect the tree during works |
| Moderate | Over 10% up to 20%, outside the SRZ | Project arborist reviews the impact and shows how the tree stays viable, e.g. through design measures and a TPS and TPP |
| Major | Over 20%, or any encroachment into the SRZ | Explore alternative designs or show viability through detailed investigation such as root investigation; a TPS and TPP should support retention |
What the calculator doesn't do
The NRZ is a starting point, not a protection boundary. Under AS 4970:2025 the project arborist determines the TPZ, considering the tree's species, condition, root distribution and site conditions. Use this calculator to scope a site, not to replace an assessment.
Need the full report? ArborFlow runs these calculations for every tree on the site and produces the full report. Start free