How to Measure a Tree

Diameter at breast height, or DBH, is the standard for measuring trees. DBH refers to the tree diameter measured at 4.5 feet above the ground.
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How to Measure Tree Diameter Video 

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Measuring Single Trunk Trees on Level Ground

Diameter at Breast Height (DBH) can be measured quickly with a specially calibrated diameter tape, often referred to as a d-tape, that displays the diameter measurement when wrapped around the circumference of a tree. If you don't have access to a d-tape, you can find the diameter of the tree using a string, a measuring tape, a thumbtack, and a calculator.

  1. With the measuring tape, measure 4.5 feet up the trunk of the tree from the ground. Use a thumbtack to mark the height on the tree.
  2. Wrap your string around the tree trunk at 4.5 feet. Make sure the string is straight and tight around the trunk, and mark or cut the circumference on the string.
  3. Measure the length of string to get the circumference of the tree.
  4. Convert the circumference measurement to diameter by dividing the circumference by pi (3.14).
Measuring Tree DBH Image
Figure 80-4 Measuring (DBH) at 4.5'

Measuring Trees at an Angle or On a Slope

When the trunk is at an angle or is on a slope, the trunk is measured at right angles to the trunk 4.5 feet along the center of the trunk axis, so the height is the average of the shortest and the longest sides of the trunk; see Figure 80-5.

A drawing of two trees showing a trunk that grows at an angle and a tree growing on a slope

Measuring Trees with a Split Trunk

When the trunk branches or splits less than 4.5 feet from the ground, measure the smallest circumference below the lowest branch. See Figure 80-6. If the tree has a branch or a bump at 4.5 feet, it is better to measure the diameter slightly below or above the branch/bump.

A drawing of a tree showing a trunk that splits above 4.5 feet above the ground

Measuring Multi-stemmed Trees

For multi-stemmed trees, the size is determined by measuring all the trunks, and then adding the total diameter of the largest trunk to one-half the diameter of each additional trunk (see Figure 80-7). A multi-stemmed tree has trunks that are connected above the ground and does not include individual trees growing close together or from a common rootstock that do not have trunks connected above the ground.

A drawing of a tree showing multiple trunks

Finding the Diameter

Diameter formula image
Image of how to calculate diameter with circumference measurement