Tree Protection Zone Calculator

Understanding how close you can safely work around a tree is essential for construction, landscaping and development projects. This TPZ & SRZ Calculator gives you a quick estimate of the recommended protection distances based on the tree's trunk circumference. Using the standard AS 4970 method, the tool converts circumference to diameter, then calculates the Tree Protection Zone (TPZ), Notional Root Zone (NRZ) and Structural Root Zone (SRZ) to help guide safe, tree-friendly planning.

TPZ and SRZ Calculator

Enter trunk circumference at 1.4 m to estimate TPZ and SRZ in line with AS 4970-2025 concepts.

cm
Diameter at Standard Height (DSH) i
0.00 m
Notional Root Zone (NRZ) radius i
0.00 m
Tree Protection Zone (TPZ) radius i
0.00 m
Structural Root Zone (SRZ) radius i
0.00 m

The TPZ is the recommended area around the tree to protect roots during construction. The SRZ is the critical area needed to maintain tree stability. The TPZ and NRZ are often the same, but TPZ may be adjusted based on site specific arborist advice.

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What the TPZ, NRZ and SRZ Actually Mean

The Tree Protection Zone (TPZ) is the area around a tree that should remain undisturbed during construction. It’s designed to protect the absorbing roots responsible for water and nutrient uptake. The TPZ is usually equal to the Notional Root Zone (NRZ), which represents the theoretical spread of the root system.

The NRZ is calculated using the formula NRZ = DSH × 12, with a minimum radius of 2 metres and a maximum of 15 metres.

The Structural Root Zone (SRZ) is different. It represents the roots that support a tree’s stability. The SRZ ensures a tree stays upright during wind and weather events.

It’s calculated using SRZ = (DSH × 50)^0.42 × 0.64, with a minimum radius of 1.5 metres for smaller trees. Disturbing roots inside the SRZ significantly increases the risk of a tree failing.

How the Calculator Works

Everything begins with the trunk circumference measured at 1.4 metres above ground, a standard known as Diameter at Breast Height (DBH). The calculator converts circumference into diameter using:

DSH = circumference ÷ 0.142

Once the DSH (Diameter at Standard Height) is known:

  • NRZ = DSH × 12
  • Apply limits (minimum 2 m, maximum 15 m)
  • TPZ = NRZ (unless modified by an arborist)
  • SRZ = (DSH × 50)^0.42 × 0.64
  • Minimum SRZ = 1.5 m for small trees

The tool instantly evaluates these values so you can understand the recommended protection distances before planning excavation, construction or site changes. For certified measurements and project-specific advice, a qualified arborist can assess species, health, soil conditions and structural risks to refine the final TPZ and SRZ.

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