Root zone calculator for the 2025 standard
Work out the Notional Root Zone and Structural Root Zone from your trunk measurements, and see how an encroachment classifies. Free, no sign up, and it runs entirely in your browser so it works on site with no reception.
Your measurements
Enter what you measured at the tree. Results update as you type.
Measure each stem at 1.4 m and enter them separately. The standard combines them as the square root of the sum of the squares, which is not the same as adding them up.
Add a proposed encroachment area above to see how it classifies under AS 4970:2025.
How these are worked out
Read what changed in AS 4970:2025 · How to measure DSH properly
D is the trunk diameter above the root buttress, in metres. Encroachment is measured as a percentage of the NRZ area, not its radius, which is why a small looking incursion at the edge can still come out under ten percent.
What changed in 2025
Same maths, different words.
AS 4970:2025 superseded the 2009 version on 30 May 2025. If you learned this standard as TPZ and DBH, here is what moved.
TPZ did two jobs
One term covered both the figure that came out of the formula and the area you fenced off on site. In practice people used it to mean whichever they needed at the time, which caused no end of argument in planning responses.
NRZ and TPZ are separate
The NRZ is the calculated figure, the starting point. The TPZ is the physical area actually protected on site, which the project arborist determines after looking at the tree and the ground. The formula gives you the first. Your judgement gives you the second.
DBH, and loose encroachment
Diameter at Breast Height, which was never a defined height. Encroachment was handled largely as minor or not, with ten percent as the usual line and a fair bit of interpretation either side of it.
DSH, and three defined tiers
DSH is Diameter at Standard Height, measured at the same 1.4 m as before. Encroachment now sits in three tiers by percentage of NRZ area: minor at ten percent or less, moderate above ten and up to twenty, major above twenty or any SRZ intrusion.
What this calculator is, and what it is not
It gives you a reference figure, not an issuable one. TIMBR Field puts it more precisely than most: a calculated zone is reference only until a practising arborist attests to it, and no attestation is recorded here. This page does arithmetic. It does not assess trees. The NRZ is a starting figure, and whether it is the right protection distance for a particular tree on a particular site is a judgement for the project arborist, taking in species, age, health, soil, aspect and site history. Plenty of trees warrant more than the formula gives, and the standard expects you to say so.
The SRZ result assumes a single stemmed tree measured above the root buttress. Multi stemmed trees, leaning trees and trees on slopes need a different approach and a set of eyes on site.
These figures are drawn from published summaries of AS 4970:2025 rather than from the standard document itself, which Standards Australia sells. Before you rely on this for a report, check it against your own copy. If you find a discrepancy, tell us on hello@timbrfield.com.au and we will fix it the same day.
Where this came from
The app does this while you are still standing at the tree.
This calculator is one small piece of TIMBR Field, which is a tree assessment app for consulting arborists. In the app the same figures are worked out as you enter the measurements, and they stay attached to the tree record with your photos, your notes and your recommendation, then sync into your own Microsoft 365 tenant ready for the report.
Built by an arborist who got tired of doing this on the back of a form in the wind.