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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.
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 thumb tack, and a calculator.
With the measuring tape, measure 4.5 feet up the trunk of the tree from the ground. Use a thumb tack to mark the height on the tree.
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.
Measure the length of string to get the circumference of the tree.
Convert the circumference measurement to diameter by dividing the circumference by pi (3.14).
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. If the tree forks well below 4.5 above the ground, measure the DBH of each fork individually, square each DBH, add the squares together, and take the square root of that sum. View this simplified guide to measuring DBH if measuring a particular tree is not straightforward.