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Finding the Diameter of a Tree

Written by Jeff Justice, Project Manager

Finding the diameter of a tree is used for all sorts of things like determining value, finding correct dosage for insecticides and injectables, or just seeing how much the tree has grown over time. And depending on how it is done, the results can vary a lot! If you find yourself needing to measure a tree, you can use these standard guidelines to measure just like the pros.

First, make sure you are using diameter tape. Different from a tape measure, this tool is designed for finding the diameter of a cylinder. It is very useful if your diameter tape has standard tape measure increments on the back side of the tape.

Next you will want to look at the tree in question. Is it straight and even? Is it leaning or have swelling? If it is straight and has no obvious bulges, you will measure normally to find the DBH (diameter at breast height), which is the diameter of the trunk at 4.5 ft from the ground. Wrap your diameter tape around the tree, making sure it stays at 4.5 ft all around the trunk, and record your measurement as soon as you can so you don’t forget!

Trees don’t always have nice, straight, single trunks for us to measure. Refer to the below guidelines for all sorts of different tree measuring situations.

  • If the tree is on a slope, always measure 4.5 ft from the ground on the uphill side of the tree.
  • If the tree has obvious swelling or knobs at 4.5 ft, or if it just looks larger there than it does below, it becomes necessary to now measure at the narrowest point below 4.5 ft that you can find.
  • If the tree has a lean, make sure you measure 4.5 ft from the base of the trunk and not just 4.5 ft up from the ground. Make sure you are measuring perpendicular to the trunk like shown in diagram.
  • If the tree splits into multiple trunks below four and one-half feet, take the diameter of the largest trunk plus half (1/2) the diameter of each additional truck.