Free tool · updated for AS 4970:2025
Tree Protection Zone (TPZ) Calculator
A tree protection zone is the area around a tree that must stay undisturbed during construction. Under AS 4970:2025 the theoretical area is the Notional Root Zone (NRZ) — trunk diameter at standard height multiplied by 12, capped at 15 metres — and "TPZ" now means the physical area actually fenced on site. Enter a trunk diameter below for the NRZ radius and area, the Structural Root Zone radius, and whether a proposed encroachment is minor, moderate or major.
Updated for AS 4970:2025, which superseded AS 4970-2009 on 19 December 2025. What changed, and what it means for your project →
Structural Root Zone (SRZ) Calculator
The SRZ is the area holding the roots that keep a tree standing. It is smaller than the NRZ and it is calculated differently — from trunk diameter measured at ground level, immediately above the root buttress, not at 1.4 metres. Its formula did not change in the 2025 revision.
The SRZ calculator is the second field above. The formula is radius = (D × 50)0.42 × 0.64, where D is that ground-level diameter in metres, with a minimum radius of 1.5 metres.
The distinction matters more than it used to. Losing part of the NRZ affects tree health and can often be compensated for. Under AS 4970:2025, any encroachment into the SRZ is automatically classed as major — there is no minor intrusion into the roots holding the tree up.
DBH calculator: how to measure DSH (formerly DBH)
DSH is Diameter at Standard Height — trunk diameter measured 1.4 metres above ground. The 2025 standard renamed it from DBH (diameter at breast height); the measurement point did not move, so any DBH figure you already have is a DSH figure.
Measure circumference with a tape and divide by π (3.1416) to get diameter, or use a diameter tape which does it for you.
- On a slope, measure 1.4 m from the highest point of ground contact.
- Where the trunk forks below 1.4 m, the standard gives a method for combining stem diameters. Measuring only the largest stem understates the NRZ.
- Where there is significant butt swell, measure above it and note the deviation in your report.
Encroachment: minor, moderate and major
This is the change most likely to affect a live project. AS 4970-2009 had a single boundary — above 10% of the protection zone was major. AS 4970:2025 replaced it with three bands:
| Band | Threshold | What it means |
|---|---|---|
| Minor | Up to 10% of the NRZ, outside the SRZ | Low risk where the tree is healthy and the lost area is compensated for in contiguous ground. Standard protection measures are normally enough, and detailed root investigation is not usually triggered. |
| Moderate | Between 10% and 20% of the NRZ, outside the SRZ | This band is new in the 2025 standard, and it is the one most projects now land in. It requires review by a project arborist — under the 2009 standard the same encroachment would simply have been called major. |
| Major | More than 20% of the NRZ, or any encroachment at all into the SRZ | The project arborist has to demonstrate the tree will remain viable, which in practice means root investigation before the design is locked in. Any intrusion into the Structural Root Zone is automatically major regardless of how small it is. |
The practical effect is a middle ground that did not exist before. A project taking 15% of the NRZ was major under the old standard and is moderate under the new one — reviewable by the project arborist rather than automatically requiring the full justification exercise.
Why twelve times the trunk diameter?
The multiplier is not arbitrary, but it is a generalisation. Root spread correlates loosely with trunk diameter across most species, and twelve times DSH captures a workable share of the root plate for the majority of trees in the majority of soils. It is a planning figure, not a measurement.
Which is why the standard allows it to be varied where there is evidence. A tree in shallow soil over rock has a wider, flatter root plate than the formula suggests. One in deep loam alongside a building may have far less root on the building side than a circle implies. Establishing that takes root investigation, and it is the difference between a design constrained by a circle on a drawing and one constrained by where the roots actually are.
What actually damages trees on a building site
Very little of it is the thing people expect. Trees are rarely killed by the structure itself — they are killed by what happens around it during construction, most of which is avoidable and none of which shows up for a season or two.
- Compaction. Vehicles, stockpiles and site huts inside the root zone crush soil pore space. Roots need air; compacted soil has none, and compaction is effectively permanent at depth.
- Fill over the root zone. Raising ground level buries the root collar and smothers surface roots. This is the single most common cause of urban tree decline, and it is usually done deliberately to level a site.
- Trenching. A service trench cut straight through the root plate severs structural roots on one side. The tree may stand for years and then fail in the first serious wind after.
- Altered drainage. A new driveway, retaining wall or slab changes where water goes. Trees adapted to one water regime frequently do not survive another.
- Washout and spoil. Concrete washout, render slurry and chemical spills inside the root zone change soil pH sharply and locally.
Every one of those is what a tree protection specification exists to prevent, and every one is cheaper to design around than to remediate.
This calculator does the arithmetic, not the assessment. The number is the straightforward part. What a council assesses is the report around it: the tree's condition, the justification for any encroachment, and the protection measures proposed. Use this for planning and feasibility — not as a substitute for a site inspection.
TPZ calculator questions
What changed in AS 4970:2025?
Three things that matter here. The theoretical root protection area is now called the Notional Root Zone (NRZ) rather than the Tree Protection Zone — the arithmetic is identical, twelve times trunk diameter, but the name moved. "TPZ" now means the physical area actually fenced off on site. Trunk diameter at 1.4 metres is now Diameter at Standard Height (DSH) rather than DBH; same measurement point, new name. And encroachment moved from a single 10% threshold to three bands: minor, moderate and major.
How is the Notional Root Zone calculated?
NRZ radius is the trunk diameter at standard height multiplied by 12. A tree with a 40 cm DSH has an NRZ radius of 4.8 metres. The standard caps it at 15 metres, so trunks above 125 cm do not increase it further. This is the same calculation the 2009 standard called the TPZ.
What counts as a major encroachment now?
Under AS 4970:2025 there are three bands rather than one threshold. Up to 10% of the NRZ is minor. Between 10% and 20% is moderate and requires project arborist review — that band is new. Above 20% is major. Separately, any encroachment at all into the Structural Root Zone is automatically major, however small, because those are the roots holding the tree up.
What is the difference between the NRZ and the SRZ?
The NRZ is the area needed to sustain the tree’s health, calculated from DSH at 1.4 metres. The SRZ is the smaller area holding the roots that keep it standing, calculated from trunk diameter measured at ground level above the root buttress. The SRZ sits inside the NRZ, its formula did not change in 2025, and encroaching on it is far more serious.
Where do I measure DSH from on a sloping site?
Measure 1.4 metres up the trunk from the highest point of ground contact. For multi-stemmed trees the standard gives a method for combining stem diameters rather than measuring the largest stem alone. Where there is significant butt swell the measurement point moves and the result changes materially — that is one of the situations where a calculator gets you close and an arborist gets you correct.
Will a council accept this calculation on its own?
For a straightforward single tree the arithmetic here is the same arithmetic an arborist would do, and it is a reasonable basis for early planning. What councils assess is the report around it — the site inspection, the tree condition, the encroachment justification and the protection measures. The number is the easy part.
Building near a protected tree?
Send through your plans and the trees involved. You will get a fixed price for the impact assessment and a straight answer on whether the encroachment you are planning is actually a problem.