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How to Measure Tree Diameter (DBH)
A tape measure and a calculator will tell you whether your tree crosses a permit threshold. Here is where to wrap the tape, how to convert the reading, and when the number is close enough to call an arborist instead of guessing.
What DBH Is and Why Bylaws Use It
DBH stands for diameter at breast height: the width of a tree's trunk measured at a set height above the ground, rather than at the base where trunks flare out and give an inflated reading. Most GTA tree preservation bylaws set their permit threshold as a DBH figure, such as Toronto's 30 cm minimum or Markham's 20 cm minimum, so the measurement height matters as much as the tape reading itself. A few municipalities, including some in Durham Region, use zone-based permit rules instead of a diameter threshold, or have no per-tree bylaw at all.
Most municipalities in the region measure at 1.37 to 1.4 metres above the ground. Caledon's bylaw sets breast height at 1.37 m, and Markham does the same. Richmond Hill and Newmarket set it at 1.4 m, matching Toronto's own tree protection specifications. Vaughan measures differently: its bylaw defines the threshold by base diameter, taken at the trunk's base rather than at breast height, so a Vaughan property needs a different measurement point altogether.
The String Method
You do not need calipers to get a usable DBH reading. A flexible tape measure or a length of string and a ruler will do it.
- Find breast height on the trunk: 1.4 m above the ground for most cities, 1.37 m for Caledon and Markham. On sloped ground, measure from the higher side.
- Wrap the tape or string around the trunk at that height, level and snug against the bark, without compressing it.
- Mark or read where the tape overlaps. That is the circumference in centimetres.
- Divide the circumference by 3.1416 (pi) to convert it to diameter.
Worked example
A trunk measures 94 cm around at breast height. 94 ÷ 3.1416 = 29.9 cm, which rounds to 30 cm DBH. In Toronto, that is exactly at the 30 cm threshold, meaning the tree is protected and a permit is required before it can be removed.
Measuring Breast Height
Most GTA municipalities measure breast height at 1.4 m; Caledon and Markham use 1.37 m.
Multi-Stem Trees
A tree that splits into two or more trunks below breast height cannot be measured with a single wrap of the tape, and cities do not agree on how to combine the stems. Markham and Newmarket both confirm the same formula: square each stem's diameter, add the squared values together, then take the square root of the total. A tree with two 15 cm stems works out to roughly 21 cm DBH under that method, not 30 cm, because the formula weights the larger stem more heavily than a simple sum would.
Not every municipality has published a multi-stem formula. East Gwillimbury's bylaw does not confirm one, and other cities may differ from Markham and Newmarket's approach or stay silent on it entirely. If your tree has multiple stems, check your city's permit guide below or have an arborist confirm the number before you rely on it.
What Your Number Means
A DBH reading only tells you something once you compare it against your city's threshold, and those thresholds vary widely across the GTA, from 15 cm in Oakville to 30 cm in Toronto and Brampton. Some municipalities have no per-tree bylaw at all outside woodlots.
When to Have an Arborist Measure Instead
A string-method reading is good enough to tell you whether a tree is nowhere near the threshold or clearly well past it. It is not good enough when the reading lands within a few centimetres of the cutoff, when a lean, buttress roots, or a fork near breast height makes the wrap point ambiguous, or when a municipality or neighbour disputes your number.
A tree removal report carries weight your string measurement does not: it documents the exact measurement point, the method used, and the ISA Certified Arborist who took it, which is what a city's forestry department expects to see on a permit application.
Frequently Asked Questions
- At what height do you measure tree diameter?
- Most GTA municipalities measure diameter at breast height (DBH), 1.37 to 1.4 metres above the ground. Richmond Hill, Newmarket, and Toronto set that height at 1.4 metres. Caledon and Markham use 1.37 metres. Of these, Vaughan is the exception: its bylaw measures diameter at the base of the trunk instead of at breast height. Other GTA municipalities set their own heights too, so check your city's threshold before you measure.
- How do you measure DBH with a tape measure?
- Wrap a flexible tape around the trunk at the height your municipality specifies, keeping the tape level and snug without compressing the bark. Record the circumference, then divide by 3.1416 (pi) to get the diameter. A 94 cm circumference divides out to roughly 30 cm DBH.
- Where do you measure DBH on a leaning or multi-stem tree?
- On a leaning tree, measure along the trunk's length rather than straight up from the ground, so the tape stays perpendicular to the trunk at the correct height. Multi-stem trees are measured differently by city, and not every municipality has published a confirmed method. Where a formula is confirmed, it combines the stems rather than simply adding their diameters together.
Related Resources
Permit Checker
Look up your city's DBH threshold and permit process.
Check your city →Tree Permit Requirements by City
Thresholds, fees, and replanting rules compared across the GTA.
Compare cities →Tree Protection Zones in Toronto
How Toronto calculates a protection radius from DBH.
Read the guide →Tree Removal Reports
A certified DBH measurement and documentation for your permit application.
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