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How to Measure Tree Diameter (DBH)
A tape measure and a calculator tell you whether your tree crosses a permit threshold. If the reading lands within a few centimetres of it, have an arborist measure.
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. Ajax, Whitby, Clarington and Pickering use zone-based rules instead of a diameter threshold. Milton, King Township and Oshawa have no per-tree bylaw.
Most municipalities in the region measure at 1.37 to 1.4 metres above the ground. Markham, Burlington, Oakville, Brampton, Caledon, East Gwillimbury and Clarington set breast height at 1.37 m. Toronto, Mississauga, Richmond Hill and Newmarket set it at 1.4 m. Whitchurch-Stouffville uses 1.3 m. Vaughan's Tree Protection By-law 177-2025 also defines Diameter at 1.37 metres above existing grade, measured outside the bark. Its separate Base Diameter definition applies only when a tree has already been cut below 1.37 metres.
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 in Toronto, Mississauga, Richmond Hill and Newmarket, 1.37 m in Markham, Vaughan, Burlington, Oakville, Brampton, Caledon, East Gwillimbury and Clarington, 1.3 m in Whitchurch-Stouffville. 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 gives about 29.92 cm diameter, below 30 cm. Do not round that up and call it exactly at Toronto's 30 cm threshold. At this reading, have an arborist measure it before you rely on it.
Measuring Breast Height
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, Burlington, Richmond Hill and Newmarket use 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.
Vaughan uses the three largest trunks: square each diameter, add those squares, then take the square root. Three 12 cm trunks give about 20.78 cm, above the 20 cm non-public-tree threshold, even though each trunk is smaller.
Mississauga, Oakville, East Gwillimbury and Whitchurch-Stouffville add the stem diameters together. Oakville and East Gwillimbury also accept another method their Designated Official approves. Brampton and Caledon treat each stem as its own tree, and a stem must reach the threshold on its own. Toronto measures each stem and regulates the whole tree if one stem reaches 30 cm. Pickering's bylaw sets no multi-stem method.
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. Milton, King Township and Oshawa have no per-tree bylaw 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 records the exact measurement point, the method and the ISA Certified Arborist who took the measurement. A city's forestry department expects that record 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. Toronto, Mississauga, Richmond Hill and Newmarket set that height at 1.4 metres. Markham, Vaughan, Burlington, Oakville, Brampton, Caledon, East Gwillimbury and Clarington use 1.37 metres. Whitchurch-Stouffville uses 1.3 metres. Vaughan measures outside the bark. Its separate Base Diameter definition applies when a tree has already been cut below 1.37 metres, not to the ordinary standing-tree threshold.
- 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 gives about 29.92 cm diameter, below 30 cm. Do not round a reading across a permit threshold.
- 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. Markham, Burlington, Richmond Hill, Newmarket and Vaughan square each stem's diameter, add the squares and take the square root. Vaughan counts only the three largest stems. Mississauga, Oakville, East Gwillimbury and Whitchurch-Stouffville add the stem diameters together. Brampton and Caledon treat each stem as its own tree.
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.
Learn more →Get a quote
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