The girth also called circumference or the diameter of the trunk is the most often used way to measure trees. Diameter at breast height, or DBH, is the standard for measuring trees. DBH refers to the tree diameter measured at 1.3m above the ground.
DBH can be measured quickly with a specially calibrated diameter tape,that displays the diameter measurement when wrapped around the circumference of a tree. If you don’t have access to a diameter 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 1.3m 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 1.3m. 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).
The aim is to measure girth at 1.3m above the ground from where the tree germinated or sprouted, or was planted as a small tree. This is assumed to be at the centre of the trunk. A 1.3 m stick is useful to stand upright next to a tree as a height marker and handy for gaining access through bracken or nettles!
The pictures below show different circumstances in measuring trees.