If you’re working with trees—whether as part of a planning application, a biodiversity survey, or simply out of personal interest—you’ll often come across the term DBH, which stands for Diameter at Breast Height.
DBH is a standard way of measuring the width of a tree trunk, taken at 1.3 metres above ground level. This consistent reference point allows arboriculturists, ecologists, and planners to compare trees and assess their size, age, and potential constraints on development.
So, how do you measure it? There are several simple methods, depending on the tools you have available.
1. Using a Measuring Tape
The most straightforward tool is to use a standard measuring tape
STEP ONE: Wrap the tape around the tree trunk at 1.3m above ground to record the circumference.
STEP TWO: Measure out how many cm in the circumference.
STEP THREE: calculate the diameter using the formula. Make sure you calculate in cm.
Diameter = Circumference ÷ 3.142 (π)
For example, if the circumference is 54 cm, then:
54 ÷ 3.142 ≈ 17.2 cm DBH
If you're struggling, you can use our circumference tool.
2. Using a Ruler or Callipers
For smaller trees, you may be able to measure directly across the trunk.
Hold a ruler or callipers against the tree at breast height.
Record the straight-line measurement from one side of the trunk to the other.
This works best when the trunk is small enough to measure directly, but is impractical for larger trees. This is a much less reliable option than
3. Using a String and Ruler
If you don’t have a flexible tape measure:
Wrap a piece of string around the trunk at 1.3m.
Mark or cut the string to the exact circumference.
Lay the string flat and measure its length with a ruler.
Divide the measurement by 3.142 (π) to get the DBH.
Special Cases
Sloping ground: Always measure from the uphill side of the tree.
Forked trees: If the fork is below 1.3m, measure the diameter just below the fork. If it is above 1.3m, measure as normal at breast height.
Buttressed or irregular bases: Move the tape slightly higher, to where the trunk becomes more regular.