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Online Calculator for TPZs and SRZs as per AS4970-2009

Calculate the TPZs (tree protection zones) and SRZs (structural root zones) as per the Australian Standard AS4970 quickly and easily using this calculator.
Ensure all values entered are in metres (m), for example 0.25m is entered not 250mm.
TPZs enter the DBH (diameter at breast height) which is the diameter of the tree at 1.4m above grade.
SRZs enter the diameter just above the trunk flare or buttresses.


Diameter at DBH or 1.4m high for TPZ calculation in m
Diameter at base for SRZ calculation in m

To obtain the DBH or diameter a simple method is to get the circumference with a tape measure and then divide it by 3.1416 (Pi)

TPZ as radius from centre of trunk =

SRZ as radius from centre of trunk =

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Multiple Stem or Trunk DBH Calculator

Often there may be more than one stem at 1.4m high. In the calculator below you can simply enter up to four stem diameters and the answer will be the combined DBH you use for TPZ and SRZ above. Measure stems at right angles. If there is more than 4 stems then simply calculate the value for 4 stems, use the answer of the combined value of the 4 stems as 1 new stem and continue adding in the rest.


Diameter 1 in m
Diameter 2 in m
Diameter 3 in m
Diameter 4 in m


Total diameter in m at 1.4m high =




Calculating Segment Areas

Often there may be an intrusion into the TPZ, it could be a trench, a path or driveway or even a building. The calculator below works out what the area of the segment is. Simply input the known radius of the TPZ and the distance (segment height) the intrusion is from the edge of the circle.


Radius in m
Segment Height in m

Segment Area =

Total Circle Area =




Disclaimer:
Use of the calculator within this website is free. Whilst every effort has been made to ensure the accuracy of the calculator published within this website, you choose to use them and rely on any results at your own risk. We will not under any circumstances accept responsibility or liability for any losses that may arise from a decision that you may make as a result of using the calculator. @ Eric Frei consulting arborist 2010


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