Free tool · for homeowners and tree crews
Critical root zone calculator
By Umair Nawaz, founder of Lirevon Updated How we test tools
Planning a patio, trench, driveway or addition near a tree? Enter the trunk diameter. You get a rule-of-thumb root protection radius, the area to keep clear, the length of fence to put around it and a plain list of what to avoid. Results are estimates, not a site survey.
Related: critical root zone · DBH · tree library · tree care by state.
1. The tree
2. How careful to be
3. The planned work (optional)
Root zone estimate
Your tree
Protection radius from the trunk
–
DBH used
–
Zone across
–
Area to protect
–
Fence length
–
| Rule | Radius | Area |
|---|
Estimate only, from a rule of thumb (1 to 1.5 ft of radius per inch of DBH). Not a site survey or a tree protection plan. Check local tree rules before work.
How the math works
- DBH = trunk diameter in inches at 4.5 ft above the ground. From a tape: circumference ÷ 3.14.
- Several trunks: square each stem's diameter, add them up, then take the square root. Stems of 10, 8 and 6 inches give about 14.1 inches.
- Radius = DBH × feet per inch (1, 1.25 or 1.5). This is the rule of thumb.
- Area = 3.14 × radius × radius, in square feet.
- Fence length = 2 × 3.14 × radius. The length of fence to go all the way round.
- Your work: if the closest point is inside the 1 ft per inch ring, it is in the critical root zone. Between the rings is a caution area.
Before the digging starts
Construction cautions near trees
Root damage often does not show for one to several years. By the time the top thins out, it is too late. These steps are general good practice. Short explanation, not legal advice: some cities protect certain trees, so check local rules.
Fence it first
Put a sturdy fence at the edge of the zone before any machine arrives. Keep it up until the job ends. Tape alone gets moved.
No parking or storage
Trucks, skid steers, pallets and soil piles pack the soil. Packed soil holds less air and water, and roots suffocate.
Bore, do not trench
For pipes and cables, boring under the roots does far less harm than an open trench that cuts every root in its path.
Keep the grade
Adding a few inches of fill or scraping soil away inside the zone can starve or cut the shallow feeding roots.
Spread the weight
If machines must cross the zone, use ground protection mats or a thick layer of wood chips to spread the load.
Clean root cuts only
If a root must go, have a tree professional cut it cleanly. Torn roots from a bucket heal badly. Big roots near the trunk can hold the tree up.
Worried a tree is already unsafe? Read is my tree dangerous? and how to hire a tree service. Look up your species in our tree library.
Questions about the critical root zone
What is the critical root zone of a tree?
It is the area of soil around a tree that holds the roots the tree needs to stay healthy and standing. Damage inside it, from digging, soil compaction or a change in grade, can stress or kill the tree, sometimes years later. It is often drawn as a circle centered on the trunk.
How do I work out the critical root zone?
Measure the trunk diameter in inches at 4.5 feet above the ground. That is DBH. Multiply it by 1 to 1.5 feet. A 16-inch tree gives a radius of 16 to 24 feet. This is a rule of thumb, not a site survey. A certified arborist can set a protection zone that fits the species, age and site.
How do I measure DBH if I only have a tape measure?
Wrap the tape around the trunk at 4.5 feet above the ground on the uphill side. That is the circumference. Divide it by 3.14 to get the diameter. A 63-inch circumference is about a 20-inch DBH. This tool does that step for you.
Why would I use 1.5 feet per inch instead of 1 foot?
Older trees and species that do not cope well with root loss need a bigger buffer. Many arborist guidelines raise the radius for those trees. If you are not sure, use the larger number. It costs little to protect more ground, and a damaged tree is hard to save.
Is the critical root zone the same as the drip line?
No. The drip line is the edge of the canopy, where rain drips off the outer leaves. Roots often spread well past it, and on narrow or leaning trees the drip line says little about where the roots are. The trunk-diameter rule is usually a better starting point.
What should not happen inside the critical root zone?
Avoid trenching, grade changes, piling soil or mulch deep against the trunk, parking or driving machines, storing materials, washing out concrete or paint, and cutting large roots. Put up a sturdy fence at the edge of the zone before work starts and keep it up until work ends.
Can utilities or a driveway go inside the root zone?
Sometimes, with care. Boring or tunneling under roots does far less harm than open trenching. Ground protection mats or a thick layer of wood chips spread machine weight. Any root cutting should be planned and done cleanly by a tree professional. Check your city rules too, as some towns protect certain trees.
Does this calculator tell me if my tree will survive construction?
No. It gives a rule-of-thumb distance only. Survival depends on species, age, health, soil and how much root area is lost. For a valuable tree near a build, ask a certified arborist for a tree protection plan before the work starts.
Is anything I type sent to Lirevon?
No. The math runs in your browser. Your numbers are only kept in this browser on this device so the form is filled in next time. Clear your browser data to remove them.
I run a tree service. How can I use this with customers?
Use it on an estimate visit when a homeowner plans a patio, addition or new driveway near a tree. Print the summary, mark the fence line and explain what to avoid. It shows care, and that kind of advice is what gets you the call when the tree work comes up.
For tree service owners
Give good advice. Then catch every call.
Homeowners remember the crew that helped them protect a tree. Lirevon sets up missed-call text-back, an AI receptionist, follow-ups and review requests for tree companies, so that call gets answered when it comes.