Free tool · AS 4970-2025
NRZ & SRZ Calculator
Enter a trunk diameter and get the Notional Root Zone and Structural Root Zone, drawn to scale, with the arithmetic shown. Nothing is uploaded and nothing is stored — it all runs in the browser.
Everything on this page is in metres. The digits fill from the right, like a till: type 55 for 0.55 m, 100 for 1.00 m.
Measured 1.4 m above ground. This drives the NRZ.
Measured immediately above the root flare. This drives the SRZ.
NRZ radius
— m
Diameter — m
Area — m²
SRZ radius
— m
Diameter — m
Area — m²
The working
NRZ = DSH × 12
SRZ = (DAB × 50)0.42 × 0.64
More than one tree
Type the diameters and the zones follow. Metres, filling from the right, the same as above.
| Tree | DSH (m) | DAB (m) | NRZ (m) | SRZ (m) | NRZ area (m²) |
|---|
Past a dozen trees this stops being a calculator problem and starts being a data problem. ArbGen reads the schedule you already have — spreadsheet, shapefile or the surveyor's PDF — derives every zone, and draws them on the plan.
What the two zones are
The Notional Root Zone (NRZ) is a circle of radius twelve times the trunk diameter at 1.4 m. It is an estimate of the root area a tree of that size needs, floored at 2 m so a sapling still gets a zone, and capped at 15 m so a very large tree does not get an unusable one. AS 4970-2025 lets a project arborist argue past the cap where the site justifies it.
The Structural Root Zone (SRZ) is the smaller inner circle: the soil volume holding the tree up. It is keyed off the diameter above the buttress, not the diameter at 1.4 m, and it has a 1.5 m floor. Under AS 4970-2025 any encroachment into it at all is a major encroachment.
The Tree Protection Zone (TPZ) is not the same thing as the NRZ, and 2025 is the revision that separated them. The NRZ is the calculation. The TPZ is what the project arborist actually sets on site, starting from the NRZ and adjusting for what is in front of them. A calculator can give you the first. Only you can give the second.
The formulas
NRZ
radius = DSH × 12
minimum 2 m · maximum 15 m · DSH in metres
SRZ
radius = (DAB × 50)0.42 × 0.64
minimum 1.5 m · DAB in metres, above the buttress
Area
area = π × radius²
AS 4970-2025 Protection of trees on development sites is an advisory standard, not a regulatory instrument, and this page is not a site assessment. Confirm any figure with a qualified project arborist before it goes near a development application.