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Free tool · AS 4970
Tree protection zone calculator
A tree's protection zone is the area of roots and soil that has to be protected during building work. Under the Australian Standard AS 4970 its radius is 12 times the trunk diameter measured 1.4 m above the ground, between 2 m and 15 m. This free calculator works it out from your tape measure, draws it on an aerial photo of your property and shows how much of it your pool, extension or driveway would take.
- Step 1 Find your property.
- Step 2 Draw what you're building.
- Step 3 Place your trees and enter their trunk size.
Step 1 Find your property
No address, or not in NSW?
Your property will appear here once you've found it in Step 1.
Planned works Root zone (NRZ) Structural root zone
Choose what it is, then tap each corner of it on the map. Press Finish shape when you're done. Add as many as you need.
Your result
Thanks, we've got it.
Josh or the office will call you back. Urgent? Ring us on 02 9154 5133.
- Standard
- AS 4970-2025, Protection of trees on development sites
- Root zone radius
- 12 × trunk diameter at 1.4 m (2–15 m)
- Reports by
- Josh Baber, AQF Level 5 consulting arborist
- Structural root zone
- (D × 50)0.42 × 0.64, at least 1.5 m
What a tree protection zone is
Most of a tree's roots sit in the top 60 cm of soil and spread well past the edge of the canopy. Dig a pool, pour a slab or run a trench through them and the tree can decline for years before it dies, or fall over much sooner. The Australian Standard for protecting trees on development sites, AS 4970, sets out how big that root area is, how much of it building work can take, and how the rest is protected while the work is done.
The 2025 edition, which replaced the 2009 standard in May 2025, splits the old "tree protection zone" in two. The notional root zone (NRZ) is the root and soil area, worked out from the trunk. The tree protection zone (TPZ) now means the physical area fenced off and protected on site. Most councils, architects and builders still say "TPZ" for both, and the calculation is the same.
How the zones are worked out
| Zone | Formula | Limits |
|---|---|---|
| Notional root zone (old TPZ) | radius = 12 × trunk diameter at 1.4 m | at least 2 m, at most 15 m |
| Several trunks | diameter = √(d₁² + d₂² + …) of the stems at 1.4 m | then × 12 as above |
| Structural root zone | radius = (D × 50)0.42 × 0.64, D = diameter above the root flare in metres | at least 1.5 m |
| Encroachment | area of works inside the root zone ÷ root zone area | shown in m² and % |
Worked example. A spotted gum measures 141 cm around the trunk at 1.4 m. That is a 45 cm diameter (141 ÷ π), so its root zone radius is 12 × 0.45 = 5.4 m and its area is π × 5.4² = 91.6 m². Just above the root flare it is 54 cm across, giving a structural root zone of (0.54 × 50)0.42 × 0.64 = 2.6 m. A new driveway that covers 18 m² of that circle is about a 20% encroachment.
How much encroachment councils accept
Under the 2009 standard, up to 10% of the root zone was a "minor" encroachment that needed little justification. Published summaries of AS 4970-2025 describe a wider range: moderate encroachments of up to 20%, and major encroachments of 20 to 30% where root mapping and a tree-sensitive design (piers instead of strip footings, bridging slabs, permeable paving, building above existing levels) show the tree will stay healthy. The calculator flags these bands:
- Minor (up to 10%): usually manageable with protection fencing and ground protection.
- Moderate (10–20%): an arborist must show the tree stays viable, normally in a report with your application.
- Major (20–30%): expect root investigation and a tree-sensitive design.
- Over 30%, or any work in the structural root zone: expect council to ask for a redesign, or for an application to remove the tree.
Councils decide, not the calculator. A protected tree can still need a permit for pruning or removal whatever the encroachment, and the rules differ from council to council. Check your council's tree rules.
Measuring your tree properly
- Measure on the uphill side of a sloping block, 1.4 m above the ground.
- Wrap the tape level around the trunk and read the distance around it. Choose "around the trunk" and the calculator converts it.
- If the trunk splits below 1.4 m, measure each stem and add it with "several trunks".
- A bulge, branch or lump at 1.4 m? Measure just below it.
- Don't forget trees on the neighbour's side and street trees: their roots count too. Estimate them, then let an arborist measure.
If the result shows an encroachment
Talk to an arborist before the design is locked in. Moving a pool a metre, switching to pier footings or bridging a driveway over the roots is cheap on paper and expensive once council has refused the application. Our consulting arborist, Josh Baber (AQF Level 5), visits the site, measures every tree properly and writes the report your council or certifier asks for, with tree protection measures for the build. He is usually on site within about a week. A few councils, including Woollahra and Lane Cove, only accept reports from an arborist with no link to tree work; if yours is one of them, we'll tell you up front and can still quote the approved work. See arborist reports and permits and what goes in an arborist report.
Questions
Tree protection zones: common questions.
What is a tree protection zone?
The area around a tree where roots, soil and the canopy must be protected during building work, so the tree survives it. AS 4970-2025 calls the root and soil part the notional root zone (NRZ) and keeps "tree protection zone" for the fenced area protected on site. Its radius is 12 × the trunk diameter at 1.4 m, at least 2 m and at most 15 m.
How do I measure my tree?
Measure 1.4 m up from the ground (about chest height). Use a tape measure around the trunk and type in the distance around it; the calculator divides by π for you. For a tree that splits into several trunks below 1.4 m, measure each one and add them as extra stems. For the structural root zone, measure across the trunk just above where it flares out at the base.
What is the structural root zone (SRZ)?
The inner circle of woody roots that hold the tree up. It is worked out from the trunk diameter just above the root flare: (D × 50)^0.42 × 0.64, at least 1.5 m. Digging, trenching or footings inside it can make a tree unstable, so councils rarely accept work there without a redesign.
How much encroachment is allowed?
The 2009 standard treated up to 10% of the root zone as a minor encroachment. Published summaries of AS 4970-2025 describe moderate encroachments of up to 20%, and major encroachments of 20 to 30% where root mapping and a tree-sensitive design show the tree stays viable. Either way an arborist has to justify it, and the structural root zone should be left alone.
Do I need an arborist report for my development application?
Usually, if a protected tree, including a neighbour's or a street tree, has a root zone that overlaps your works. Councils ask for an arborist report to AS 4970 setting out the zones, the encroachment and the protection measures. Your council's page says when its rules apply, and our arborist report guide explains what goes in one.
Can your arborist write the report?
Yes. Josh Baber, our AQF Level 5 consulting arborist, writes development-site reports and tree protection plans. A few councils, including Woollahra and Lane Cove, only accept reports from arborists with no link to tree work. Where that applies we tell you up front. See arborist reports and permits.
Is this calculator an arborist report?
No. It is an indicative check based on the published AS 4970 formulas, your own measurements and an aerial photo, which is not survey-accurate. Use it to see whether a tree is likely to affect your plans, then have a consulting arborist confirm it on site.
Is anything I enter saved?
Not unless you choose. Your measurements and drawing stay in your browser, and the PDF is made on your own device. The address you search is only looked up to show the map. If you email the result to yourself, we send it from our office mailbox and keep your email address and result with our enquiries; if you send the quote form, we receive what you put in it.
Building near a tree?
Ask before you design.
Josh measures the trees, maps the root zones and writes the report your council asks for. Run the calculator above and press Ask for a quote, and your map and results come with it.