Free tool · updated for AS 4970:2025
Tree Protection Zone (TPZ) Calculator
This free tree protection zone (TPZ) calculator works out a tree's protection zones under AS 4970:2025 from its trunk diameter. Enter the diameter at 1.4 m for the Notional Root Zone (NRZ) radius and area, add the diameter above the root buttress for the Structural Root Zone (SRZ), and add a proposed encroachment to see whether it is minor, moderate or major — with the working shown under every figure.
Updated for AS 4970:2025, which replaced AS 4970-2009 when it was published on 30 May 2025. What changed, and what it means for your project →
Structural Root Zone (SRZ) Calculator
The SRZ is the area around the base of a tree holding the woody roots that keep it standing. It is far smaller than the NRZ and comes from a different measurement — trunk diameter just above the root buttress, not at 1.4 metres. Enter that figure in the second field above for the SRZ radius. Any encroachment inside the SRZ is major under AS 4970:2025, whatever the percentage — losing part of the NRZ affects health and can often be compensated for, but there is no minor intrusion into the roots holding the tree up.
How to calculate SRZ (Structural Root Zone)
SRZ radius = (D × 50)0.42 × 0.64, where D is the trunk diameter in metres, measured just above the root buttress flare. The formula is unchanged from the 2009 standard.
- Measure the trunk just above the root buttress. With a tape, take the circumference and divide by π (3.1416).
- Convert to metres and multiply by 50.
- Raise that to the power of 0.42.
- Multiply by 0.64. The answer is the SRZ radius in metres, from the centre of the trunk.
Worked through for a 55 cm trunk: 0.55 × 50 = 27.5, and 27.50.42 = 4.02, and 4.02 × 0.64 = 2.57 m.
Trunks under 0.15 m have a fixed SRZ of 1.5 m. For a multi-stemmed tree the project arborist decides whether to measure around all the stems or work from their combined cross-sectional area, and records why. The SRZ is not calculated for palms, cycads or tree ferns.
The standard requires the SRZ to be calculated when a major encroachment — more than 20% of the NRZ — is proposed. It is a notional figure: tree height, crown size, soil type and moisture, and rocks or footings all change where the structural roots really are, and root investigation is how that gets established.
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 up from the ground level at the centre of the trunk — midway between the uphill and downhill sides.
- 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.
- On a buttressed trunk, measure just above the buttress; where the trunk is deformed at 1.4 m, just above the deformity. Note the deviation in your report.
TPZ Encroachment table
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, each measured as a share of the NRZ's area:
| Band | Threshold | What it means |
|---|---|---|
| Minor | Up to 10% of the NRZ, outside the SRZ, with no recent encroachment | Unlikely to significantly affect the tree. Tree protection goes in for the works, and an area equal to the encroachment is added to the TPZ elsewhere, unless the project arborist shows the tree will stay viable without it. |
| Moderate | More than 10% and up to 20% of the NRZ, outside the SRZ | New in 2025. A project arborist reviews the impact and investigates as needed to show how the tree will stay viable, for example through design measures and construction controls in the TPS and TPP. The same compensating area applies. Under the 2009 standard this was major. |
| Major | More than 20% of the NRZ, or any encroachment inside the SRZ | The project arborist explores alternative designs with the design team and/or demonstrates the tree will stay viable, backed by a more detailed investigation: root investigation, soil analysis, the history of the tree and site. The SRZ has to be calculated, and the same compensating area applies. |
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. The project arborist still has to show the tree will stay viable, but through design measures and construction controls rather than the alternative designs and detailed investigation a major encroachment calls for.
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 zone actually protected on site, which the project arborist sets from the NRZ. 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, and sets a 2 metre minimum, which covers any trunk under about 17 cm. Palms, cycads and tree ferns are not calculated at all — they get the 2 metre minimum. This is the same calculation the 2009 standard called the TPZ.
Why is NRZ & TPZ 12x the trunk diameter?
The multiplier is a generalisation rather than a measurement. Root spread correlates loosely with trunk diameter across most species, and twelve times DSH captures a workable share of the root plate for most trees in most soils. Strictly it is the NRZ that is twelve times DSH. The TPZ starts from that figure and is then set by the project arborist from what is actually there — where the roots are, the soil, the species, existing structures — so it can end up a different shape. A tree in shallow soil over rock has a wider, flatter root plate than the formula suggests, and one beside a building may have far less root on that side than a circle implies. Root investigation is what establishes the difference, 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 is SRZ (structural root zone)?
The structural root zone is the area around the base of a tree holding the large woody roots that keep it standing. AS 4970:2025 treats it as a notional radius from the centre of the trunk: (D × 50)^0.42 × 0.64, where D is the trunk diameter in metres measured just above the root buttress, with a fixed 1.5 metres for trunks under 0.15 m. It covers stability only — the root area a tree needs to stay healthy is much larger, which is what the NRZ is for. The standard requires it to be calculated when a major encroachment is proposed, and any encroachment inside it is major whatever the percentage.
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 from the ground level at the centre of the trunk — midway between the uphill and downhill sides, which is how Figure A.1 of the standard draws it. 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.