Free calculator
NRZ & TPZ Calculator
Calculate a tree's Notional Root Zone (NRZ) to AS 4970:2025 from its Diameter at Standard Height (DSH), then set the Tree Protection Zone (TPZ) fence and draw proposed works on a scaled plan to see how much of each zone they take.
Enter trunk measurements
Notional Root Zone radius
6.00 m
NRZ area: 113.10 m²
TPZ area: 113.10 m²
The TPZ fence encloses the full NRZ.
Plan view
Tap the plan to add points, then tap the first point (or press Enter) to close the shape. Drag points to adjust. Grid squares are 1 m, with heavier lines every 5 m.
Dashed green is the NRZ, the solid dark line is the TPZ fence. Optionally draw the footprint of proposed works to measure how much of each zone it takes.
How the NRZ is calculated
AS 4970:2025 defines the Notional Root Zone radius as the tree's DSH multiplied by 12, with a minimum of 2 m and a maximum of 15 m. For example, a tree with a 500 mm (0.5 m) DSH has an NRZ radius of 6 m. DSH is measured 1.4 m above ground — the same point as the DBH used under AS 4970-2009.
NRZ vs TPZ
The NRZ is the ideal, theoretical root space a tree needs for long-term health. The TPZ is the area actually fenced off and protected on site during construction. The project arborist sets the TPZ starting from the NRZ, and it is the TPZ that is shown in a Tree Protection Management Plan. Under AS 4970-2009 the term "TPZ" covered both ideas.
Drawing proposed works
The plan is drawn to scale in metres around the trunk. Draw one or more shapes for the footprint of proposed works — overlapping shapes are counted once, and only the part inside the NRZ counts towards encroachment. The result is classified as minor (up to 10% of the NRZ), moderate (more than 10% and up to 20%) or major (more than 20%, or any intrusion into the Structural Root Zone).