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Tree Protection Zone (TPZ), Structural Root Zone (SRZ), Diameter At Breast Height (DBH) calculatorWork out how much a proposed building, driveway, trench, or excavation will encroach into a tree's protected root area, calculated to AS 4970:2025 Protection of Trees on Development Sites, the current Australian Standard for assessing development impacts on trees.
This tool gives you a fast, standards-based starting figure. It does not replace a site assessment by a qualified Project Arborist, and the sections below explain why that distinction matters more than the percentage on the page.
Enter a tree's trunk diameter and the distance from the trunk to the nearest edge of the proposed works, and the calculator returns:
You can set up an assessment two ways, separately or together:
Both methods feed the same calculation, so you can mix them: type in a quick distance for an initial check, then switch to drawing if the footprint turns out to be more complex than a single number can capture.
The calculator gives you two independent, separately labelled work areas, S1 and S2, that can be entered or drawn at the same time. Beyond simply assessing two unrelated proposals at once, this is particularly useful for comparing a new proposal against an existing structure:
That overlap figure matters because it changes how a given percentage should be read. A large total encroachment can look alarming in isolation, but if most of that area sits under a structure that's already there, the tree's roots have typically already adapted to that loss, so the genuinely new impact of the proposed works is usually smaller than the headline percentage suggests. Where the new and existing footprints barely overlap, the full encroachment area represents real, new root loss and should be weighed accordingly. Either way, a Project Arborist should make the final call on how much weight to give the overlap. The calculator surfaces the figures; it doesn't make the judgement.
The 2025 revision renamed and re-scoped several core terms. If you learned this standard under the 2009 version, the definitions below have shifted, particularly TPZ, which now means something different.
| Term | Meaning under AS 4970:2025 |
|---|---|
| DSH (Diameter at Standard Height) | The trunk diameter measured at 1.4 m above ground. This replaces the old term DBH; the measurement itself is unchanged. |
| NRZ (Notional Root Zone) | The theoretical circular area required to sustain the tree's long-term health, calculated from DSH. This is what used to be called the "TPZ" under the 2009 standard. |
| TPZ (Tree Protection Zone) | The actual, physical area protected on site during construction: fencing, ground protection, and exclusion zones. The TPZ is informed by the NRZ but is the implemented, on-the-ground outcome, and can be reshaped or offset across the site. |
| SRZ (Structural Root Zone) | The smaller, innermost zone containing the structural roots that physically anchor the tree. Encroachment here is a stability issue, not just a health one. |
In short: the NRZ is the calculation, the TPZ is what actually gets fenced off on site, and the SRZ is the zone you essentially shouldn't touch without expert investigation.
NRZ radius (m) = DSH (m) × 12Minimum 2 m radius; capped at 15 m unless a Project Arborist provides specific justification for a larger figure, which may be relevant for very large, old, or multi-stemmed trees.
SRZ radius (m) = (D × 50)^0.42 × 0.64Where D is the trunk diameter, in metres, measured immediately above the root buttress, not at 1.4 m. Minimum SRZ radius is 1.5 m.
Worked example: a tree with a DSH of 0.5 m has an NRZ radius of 6 m, an NRZ area of approximately 113 m². With a diameter above the root buttress of 0.55 m, its SRZ radius is approximately 2.5 m.
AS 4970:2025 replaced the old "minor vs major" split with three defined tiers. Where your calculated encroachment falls determines what happens next, and none of these outcomes is automatic; each is a trigger for a decision, not a verdict.
| Tier | Threshold | What it means |
|---|---|---|
| Minor | Up to 10% of the NRZ area, entirely outside the SRZ, with no other recent NRZ encroachments on the same tree | Generally low risk. Standard tree protection measures are usually sufficient. |
| Moderate | More than 10% and up to 20% of the NRZ area, outside the SRZ | The Project Arborist must review the proposal and demonstrate the tree's viability can be maintained, often via site-specific controls such as non-destructive digging, root mapping, or revised footings. |
| Major | More than 20% of the NRZ area, or any encroachment into the SRZ at all | Requires serious investigation: alternative design options, non-destructive root investigation, and a clear, evidenced case for whether the tree remains viable. |
The NRZ/SRZ formulas treat every tree as a uniform cone of roots radiating from a trunk of a given width. Real trees aren't that consistent, and a calculator can't see what a qualified arborist can.
A young, vigorous tree often has real capacity to regenerate fine roots after disturbance and can tolerate proportionally more encroachment than the raw number suggests. A mature or veteran tree is the opposite case: its root system has taken decades to establish, its capacity to compartmentalise damage and regenerate roots is lower, and the consequences of getting it wrong aren't recoverable on any timeframe relevant to a development.
Some species tolerate root disturbance reasonably well; others decline rapidly or become structurally unstable after even modest root loss, particularly species prone to root or butt decay, shallow or plate-like root systems, or known sensitivity to changes in soil moisture and compaction.
A tree already under stress from drought, prior root damage, disease, poor previous pruning, or compromised structure has far less reserve capacity to absorb additional root loss than a tree in good condition.
Urban trees, particularly larger and older ones, provide cooling, shade, stormwater interception, biodiversity support, amenity, and wellbeing benefits. Many of these benefits scale with canopy size and age, meaning they take decades to replace.
None of this is a reason to ignore the numbers. It's a reason not to stop at them.
Engage a suitably qualified consulting arborist, minimum AQF Level 5 / Diploma in Arboriculture, before finalising a site design, not after a permit application stalls. You should be talking to a Project Arborist if:
A Project Arborist's documentation under AS 4970:2025 should result in a Tree Protection Plan (TPP), the site drawing showing fencing and protection measures, and Tree Protection Specifications (TPS), the written detail of how, when, and by whom those measures are implemented.
This calculator answers the question for one tree, at your desk. The ProofSafe app does the same standards-based job for an entire site, in the field, on your phone or tablet:
If you're already using this calculator for a one-off check, the ProofSafe app is the natural next step for teams assessing trees, or running any other kind of field-based data collection, across multiple sites and projects.