TPZ Calculator (NRZ + SRZ + Encroachment) — AS 4970-2025
A free Tree Protection Zone calculator built by NSW consulting arborists. Enter your tree’s DBH to get the Notional Root Zone, Structural Root Zone and encroachment classification under the latest AS 4970-2025 standard — the version councils across NSW now require.
Single or multi-stem?
Diameter at breast height in millimetres. AS 4970-2025 uses DBH as the input to the NRZ formula.
Trunk diameter at ground level (for SRZ)
If you only have DBH, leave this blank — the calculator will use DBH as a fallback. For accuracy on flared trunks, measure just above the root flare.
Encroachment zones (add one per impact)
Add a zone for each separate work or structure that intrudes into the NRZ — for example one zone for the proposed building, another for a driveway, another for a service trench. All zones are positioned in the same coordinate frame (offset zones face the trunk; corner zones occupy the same quadrant), so where they overlap they are counted once — not added twice.
Geometric zones (Straight edge + Corner) are combined via union — overlapping areas count once. Direct-area zones are added separately (they have no spatial position, so the calculator assumes you have not double-counted any overlap when entering an m² value).
Download your result as a PDF
Enter your details to download a branded PDF of this calculation. Use it as a project file note, share with your builder or architect, or attach to a quote request for a full Arboricultural Impact Assessment.
Built for the current standard. Most free TPZ calculators online were built against AS 4970-2009 and still return the old two-tier (Minor / Major) classification without the Moderate band. This calculator implements AS 4970-2025: NRZ vs TPZ terminology, three-tier encroachment, automatic Major classification on any SRZ intersection, and the AS 4970-2025 multi-stem combined-DBH rule. If a report you’ve received still cites the 2009 standard, your council assessor may ask for it to be revised.
Disclaimer & conditions of use
Purpose. This calculator is an educational tool that applies the published formulae and encroachment thresholds of AS 4970-2025 — Protection of trees on development sites to the values you enter. It is provided for information only.
Not professional advice. Output from this calculator is not an arboricultural opinion, not a report, and not a substitute for site-specific assessment by a qualified consulting arborist. Any recommendations shown (“Suggested actions”) are derived directly from the wording and intent of AS 4970-2025 and do not represent the professional opinion of Assurance Trees Pty Ltd, its directors or employees. Reliance on this calculator for a Development Application, planning matter, dispute, insurance claim, legal proceeding, council compliance, construction methodology, tree removal decision or any other purpose involving a duty of care is at the user’s sole risk.
Inputs. Results are only as accurate as the measurements you enter. Trunk diameter at breast height (1.4 m above ground on the high side of any slope) and trunk diameter at ground level (above the root buttress) must be measured to AS 4970-2025 methodology. Errors of a few millimetres in measurement can shift the encroachment classification. The calculator cannot detect input errors.
Factors not modelled. AS 4970-2025 requires the project arborist to consider species sensitivity to root disturbance, tree health and structural condition, soil type and depth, slope, drainage, microclimate, construction methodology, sequencing of works, root mapping evidence, retention value (e.g. IACA STAR), replacement-planting requirements, and the cumulative effect of multiple impacts. This calculator does not model any of those factors. The Tree Protection Zone (TPZ) that is actually implemented on a project is a professional decision made by the project arborist; it is not the NRZ this calculator returns.
Currency of standards. AS 4970-2025 is current at the date of this calculator’s release. Standards Australia may issue amendments, supplements or successor standards. Users should verify the current standard before relying on output.
No warranty, no liability. This calculator is provided “as is” without warranty of any kind, express or implied, including fitness for a particular purpose. To the maximum extent permitted by law, Assurance Trees Pty Ltd accepts no liability for any loss or damage (including indirect or consequential loss) arising from use of or reliance on this calculator, regardless of cause.
For any actual project, engage a qualified consulting arborist — an AQF Level 5 (or higher) practitioner experienced in the relevant work, such as those at Assurance Trees on 1300 859 510 or sales@assurancetrees.com.au.
Need this assessed properly for a DA?
An Arboricultural Impact Assessment translates these numbers into a council-ready report with TPP, TPS, encroachment mitigations and replacement plantings. AIA from $1,800 + GST — see what’s included or call 1300 859 510.
Understanding NRZ, TPZ and SRZ under AS 4970-2025
AS 4970-2025 (Protection of Trees on Development Sites) introduced a critical terminology change. The acronym TPZ has shifted meaning, and a new zone — the NRZ — now sits alongside it. Older online calculators built on the 2009 standard get this wrong; this calculator implements the 2025 definitions.
NRZ — Notional Root Zone
The theoretical, formula-derived protective area calculated from DBH. This is what the 2009 standard called the “TPZ”. If you’ve used a TPZ calculator before, it was almost certainly returning what AS 4970-2025 now calls the NRZ.
NRZ radius (m) = DBH (mm) × 0.012, with a minimum of 2 m and a maximum of 15 m.
TPZ — Tree Protection Zone
The physical, implemented protected area on site — the actual fenced zone. The project arborist decides this based on the NRZ, root mapping, species, construction methodology and site constraints. It may be smaller or larger than the NRZ.
This calculator returns the NRZ as the default starting point. The TPZ for your specific project is an arborist’s professional decision.
SRZ — Structural Root Zone
The smaller zone around the trunk where the structural roots holding the tree up are concentrated. Any encroachment into the SRZ is automatically classified as Major under AS 4970-2025 and almost always means the tree cannot be retained without structural compromise.
SRZ radius (m) = ((D×50)0.42) × 0.64, where D is trunk diameter at ground level in metres. Minimum 1.5 m.
Encroachment classification — three tiers under AS 4970-2025
AS 4970-2009 had a two-tier system (Minor / Major). AS 4970-2025 formalises a Moderate band and treats any SRZ intrusion as automatically Major. These are the thresholds your council assessor and the project arborist now apply:
| Encroachment | Class | Typical mitigation |
|---|---|---|
| < 0.05% of NRZ area | None | No mitigation required |
| ≤ 10% of NRZ area, outside SRZ | Minor | Standard tree protection — TPF fencing, ground protection, supervision |
| > 10% and ≤ 20% of NRZ area, outside SRZ | Moderate | Project arborist assessment + specific mitigation: root mapping, hand-digging, compensatory TPZ offset, hydration regime |
| > 20% of NRZ area — OR any SRZ encroachment | Major | Detailed assessment, may not be approvable. Tree retention often needs to be reconsidered. |
Watch the SRZ. A small encroachment that’s only 8% of the NRZ area might look “minor” on paper, but if it clips the SRZ it’s automatically Major. The calculator above flags this for you in offset mode — for area mode, tick the SRZ checkbox if your overlap polygon enters the structural root zone.
Worked example
A Spotted Gum (Corymbia maculata) with DBH 600 mm and trunk diameter at ground level 720 mm:
- NRZ radius = 600 × 0.012 = 7.20 m (NRZ area = 162.9 m²)
- SRZ radius = ((0.72 × 50)0.42) × 0.64 = 2.88 m (SRZ area = 26.1 m²)
- A proposed building face at 5.5 m offset from the trunk creates a circular segment of about 10.8 m² inside the NRZ = 6.6% — Minor
- If the same building face were at 2.0 m offset, the segment would be ~53.0 m² = 32.6% — clearly Major, and at 2.0 m it also enters the 2.88 m SRZ, so Major regardless
Try the worked example yourself: enter 600 in the DBH field and 720 in the trunk-at-ground-level field, then switch encroachment mode to Building offset and try the two distances.
Moderate or Major encroachment? You’ll need a root investigation — we do them in-house.
AS 4970-2025 indicates non-destructive root investigation (typically Air Spade) for any encroachment above the Minor band and for any works near the SRZ. Assurance Trees owns the Air Spade and the 250 CFM trailer-mounted compressor — most consultants don’t, which means they have to specify an outside vac-truck contractor on a separate engagement, on someone else’s schedule, at a much higher cost. We mobilise within the same week, the AQF 5 reads the roots on site as the soil is moved, and you get the report you need to support retention — or to redesign before the DA is rejected. See how our Root Investigations work or call 1300 859 510.
Frequently asked questions
What’s the difference between TPZ and NRZ under AS 4970-2025?
The NRZ (Notional Root Zone) is the formula-derived theoretical protection area — this is what the 2009 standard used to call the “TPZ”. The TPZ (Tree Protection Zone) under the 2025 standard is the physical zone you actually fence and protect on site — a project arborist’s professional decision that takes the NRZ as a starting point and may adjust based on root mapping, species sensitivity, construction methodology and site context. So “TPZ” now means something different from what most older calculators returned.
How is the TPZ calculated under AS 4970-2025?
The TPZ itself isn’t calculated by a single formula — it’s decided by the project arborist. What the formula produces is the NRZ: NRZ radius (in metres) = DBH (in millimetres) × 0.012, clamped to a minimum of 2 m and a maximum of 15 m. For a tree with a measured DBH of 500 mm, that’s NRZ = 500 × 0.012 = 6.0 m radius. The arborist then takes that NRZ as the default TPZ unless site evidence supports adjusting it.
How is the SRZ calculated?
SRZ radius (in metres) = ((D × 50)0.42) × 0.64, where D is the trunk diameter at ground level (just above the root buttress) measured in metres. AS 4970-2025 sets a minimum SRZ of 1.5 m. For a tree with trunk diameter at ground level of 0.5 m (500 mm), the SRZ radius is ((0.5 × 50)0.42) × 0.64 = about 2.47 m. If you only have DBH measured at 1.4 m, you can use that as an approximation, but trunk diameter at ground level is usually 10–20% larger because of the root flare.
What’s a Minor, Moderate or Major encroachment?
Under AS 4970-2025: Minor is up to 10% of the NRZ area, outside the SRZ — usually acceptable with standard tree protection. Moderate is more than 10% and up to 20% — requires the project arborist to specify mitigations like root mapping, hand-digging or compensatory protection. Major is more than 20% or any SRZ encroachment — significant intervention required and the tree may not be retainable. The 2009 standard only had Minor and Major, so this Moderate band is new.
Can I encroach into the SRZ at all?
Practically, no — any SRZ encroachment is automatically classified as Major under AS 4970-2025. The SRZ contains the structural roots that hold the tree up. Cutting them risks instability, and the tree’s wind-firmness can’t be guaranteed afterwards. If a proposed building or service trench enters the SRZ, the project arborist will usually recommend redesign or removal rather than protection.
What if my tree has multiple stems?
AS 4970-2025 uses a combined DBH for multi-stem trees: combined DBH = √(D₁² + D₂² + D₃² + …). Switch to Multi-stem mode in the calculator above and enter each stem diameter at 1.4 m. The NRZ is then calculated from the combined DBH. The SRZ should be calculated from the trunk diameter measured below the union (where the stems converge), not from the sum of stems.
Does this calculator replace an Arboricultural Impact Assessment?
No — it’s an indicative tool. A formal AIA assesses each tree’s retention value (IACA STAR), species sensitivity to root disturbance, soil conditions, the construction methodology, replacement plantings required by council, and the project arborist’s professional decision on the implemented TPZ. For a Development Application or any planning, dispute or insurance matter, you need a written report from a qualified consulting arborist. Assurance Trees provides council-ready AIAs from $1,800 + GST — see what’s included.
Do NSW councils accept AS 4970-2025?
Yes — AS 4970-2025 superseded AS 4970-2009 and is the standard NSW councils now apply to Arboricultural Impact Assessments, Tree Protection Plans and Tree Protection Specifications. If you receive an older report citing only the 2009 standard, the council may issue a Request for Further Information asking for the assessment to be revised under the current standard.
How accurate is this calculator?
The formula outputs are exact to AS 4970-2025. The accuracy of your result depends entirely on the accuracy of your inputs: a DBH measured with a callipers at the correct height (1.4 m above ground on the high side of any slope) is typically within ±5 mm, which translates to about ±0.06 m on the NRZ radius. For multi-stem trees, slight under-counting of small stems can meaningfully reduce the combined DBH. Trunk diameter at ground level is the trickier measurement and is best taken with the assistance of a consulting arborist.
Definitions — key terms used in this calculator
All definitions below are drawn from or aligned with AS 4970-2025 — Protection of trees on development sites and the surrounding Australian arboricultural literature. Where AS 4970-2025 wording is paraphrased it is marked as such.
- AS 4970-2025
- The Australian Standard Protection of trees on development sites, 2025 edition. It superseded AS 4970-2009 and is the standard NSW councils and consent authorities now apply to Arboricultural Impact Assessments (AIAs), Tree Protection Plans (TPPs) and Tree Protection Specifications (TPSs).
- DBH — Diameter at Breast Height
- The trunk diameter measured at 1.4 m above natural ground level, taken on the high side of the trunk where ground level slopes. For multi-stem trees see “combined DBH” below.
- DAB / DAGL — Diameter Above Buttress (Diameter at Ground Level)
- The trunk diameter measured immediately above the root buttress — the flared zone where the trunk meets the roots. Used as the input to the SRZ formula. On strongly buttressed species it can be considerably larger than DBH.
- Circumference
- The girth measurement around the trunk — the value obtained by a measuring tape passed around the trunk at the relevant height. Diameter = Circumference ÷ π. Surveyors and field operators often measure circumference because a tape is more practical than callipers; this calculator converts circumference to diameter when you select the Circumference input mode.
- NRZ — Notional Root Zone
- The theoretical, formula-derived protective area around a tree, expressed as a circular disc with the trunk at its centre. Under AS 4970-2025 the NRZ radius equals DBH (in millimetres) × 0.012, with a minimum of 2 m and a maximum of 15 m. Under AS 4970-2009 this same value was called the “TPZ”; that usage is now superseded.
- TPZ — Tree Protection Zone
- The physical, implemented protection area on a site — the area that is fenced off and managed during construction. Under AS 4970-2025 the TPZ is determined by the project arborist using the NRZ as a starting point, then adjusting for root mapping, species sensitivity, construction methodology and site constraints. The TPZ is therefore a professional decision, not a formula output.
- SRZ — Structural Root Zone
- The smaller zone around the trunk containing the structural roots that hold the tree upright. Under AS 4970-2025 the SRZ radius (in metres) equals ((D × 50)0.42) × 0.64, where D is trunk diameter at ground level in metres, with a minimum of 1.5 m. Any encroachment into the SRZ is automatically classified as Major under AS 4970-2025 and generally means the tree cannot be retained without structural compromise.
- Combined DBH (multi-stem)
- For trees that fork below 1.4 m, AS 4970-2025 specifies a combined DBH equal to the square root of the sum of the squares of each stem’s DBH: combined DBH = √(D₁² + D₂² + D₃² + …). The NRZ is then calculated from this combined value.
- Encroachment
- Any proposed work (building footprint, paving, excavation, services, vehicle hardstand, fill, cut, change in soil level, etc.) that intrudes into the NRZ. AS 4970-2025 classifies encroachment into three tiers: Minor (≤10% of NRZ area), Moderate (>10% to ≤20%), and Major (>20% — or any SRZ intersection regardless of NRZ percentage).
- Tree Protection Fencing (TPF)
- Physical fencing installed around the TPZ for the duration of works to exclude construction activity, vehicles, materials storage, and trades from the protected zone. Specifications are set by the project arborist and detailed in the Tree Protection Plan / Tree Protection Specification.
- Project Arborist
- The arborist nominated under the consent or contract to specify and supervise tree protection for the duration of works. Typically an AQF Level 5 (Diploma of Arboriculture) or higher practitioner. Responsible for the implemented TPZ, mitigations, on-site decisions when conditions change, and the final compliance certificate.
- Root investigation / Air Spade investigation
- Non-destructive excavation of the soil within the NRZ to expose the actual root system, using compressed air (Air Spade) or, less commonly, hydro-vac. AS 4970-2025 indicates root investigation as a typical mitigation for Moderate and Major encroachments because it lets the project arborist see the actual root distribution rather than rely on the geometric NRZ assumption.
- Straight-edge encroachment (Type A)
- An encroachment where a single straight building edge or works boundary cuts a chord across the NRZ disc. The encroachment area is the circular segment of the disc lying on the works side of that edge.
- Corner encroachment (Type B)
- An encroachment where a building corner sits inside the NRZ disc, with two perpendicular edges cutting into the disc. The encroachment area is the disc area lying inside the building’s “corner quadrant” — the region where both perpendicular edges have been crossed.
- AQF Level 5 Arborist
- Australian Qualifications Framework Level 5 — the Diploma of Arboriculture qualification typically required for consulting arboricultural work (writing AIAs, TPPs, TPSs and acting as a project arborist) for NSW councils and consent authorities.
Need a council-ready AIA, TPP or Project Arborist?
Assurance Trees is a Hunter-based NSW consulting arboricultural firm: Licensed NSW Builder + AQF Level 5 Consulting Arborist. We write AIAs and TPPs to AS 4970-2025 for residential builders, civil contractors, architects, town planners, government and councils across NSW.
1300 859 510or email sales@assurancetrees.com.au — 24-hour quote turnaround.
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