Most of the emails we get asking whether a tree needs a permit could have been answered with a tape measure. The answer almost always turns on one number, the trunk’s diameter at breast height (DBH). Get that number and compare it to your city’s threshold before you call anyone.
Each City Sets a Diameter Threshold
Most GTA tree bylaws set their permit threshold as a trunk diameter. Toronto’s Chapter 813 of the Municipal Code sets the cutoff at 30 cm. Markham’s By-law 2023-164 sets it at 20 cm. Mississauga’s Private Tree By-law sets it at 15 cm. A tree at or above the local threshold needs a permit before it can be removed or injured. A tree below it is still regulated when it is city-owned, on a boundary, inside a ravine or woodland, protected by a development condition, or caught by another municipal rule.
Not every municipality works this way. Whitby, Ajax and Clarington use zone or location rules instead of a diameter threshold. Oshawa has no bylaw for an individual private tree.
Diameter isn’t measured at the base, where trunks flare out and inflate the reading. It’s measured higher up the trunk, at a height each municipality sets in its bylaw. Toronto measures at 1.4 metres. Markham measures at 1.37 metres. Vaughan’s Tree Protection By-law 177-2025 also uses 1.37 metres, measured outside the bark, for its 20 cm non-public-tree threshold. Base Diameter is a separate definition for a tree already cut below 1.37 metres. For multiple trunks, use Vaughan’s three-largest-trunks calculation instead of comparing each trunk separately. Check your own city’s height and threshold on our permit checker or the full bylaw comparison by city before you pick up a tape.
The Method, Briefly
Wrap a flexible tape around the trunk at the right height, level and snug against the bark, and divide the circumference by 3.1416 to get the diameter. A trunk that splits into two or more stems below breast height needs a different formula, not a simple sum of the stems, and a lean or fork near breast height throws the reading off if you measure straight up from the ground instead of along the trunk. The full method, worked example, and diagram are on our how-to-measure-DBH page.
When the Reading Is Close Enough On Its Own
If your tree measures well below your municipality’s threshold, the tape settles the diameter question. It does not settle ownership, ravine, woodland, development or site-specific protections. Rule those out before you decide you need no permit or report.
When a Tape Measure Stops Being Enough
If the number lands within a few centimetres of your city’s cutoff, rounding and tape placement can change the answer. If the trunk has a fork, buttress roots, or a pronounced lean right at breast height, the wrap point itself is ambiguous, and two people measuring the same tree can land on different numbers. And if a neighbour or the city disputes your reading, your own tape measurement carries no documentation behind it.
A city forestry department reviewing a permit application wants a measurement that records the point on the trunk, the method used and who took it. A homeowner’s tape reading does not. A tree removal report from an ISA Certified Arborist records all three, so the number holds up if anyone questions it.