Taking the diameter of a tree, also known as “Diameter at Breast Height” or “DBH,” is one of the most essential skills in forestry. It is at the core of properly measuring timber and wood volume (known as forest mensuration), and landowners often make important decisions about the management of their trees and forests based on these numbers. Though it may seem like a simple, straightforward task, there is a standardized method to measure tree diameter that must be used to assure proper, consistent results. When you read academic or industry materials that reference “tree diameter,” this is the specific measurement they refer to.
Below, we will take you through the proper methodology so you can measure trees like a true forester and scientist! After, we will show you a few great ways you can use tree diameter for both utility and enjoyment of your trees. After reading this article, if you want to learn more about forest measurements and managing your woodlands, get a copy of our free guide with the form below:
Get Our Free 76-Page Forestry Guide
Subscribe to our newsletter and get our free guide to become a better forester.
- D-Tape or flexible measuring tape
Step 1: Find “Breast Height” at 4.5′ High
As mentioned previously, tree diameter is known in the industry and scholarship of forestry as “Diameter at Breast Height” or “DBH” as shorthand. Breast height is specifically defined as being 4.5′ above the ground, and this is where one must measure the diameter of a tree. It can be overly cumbersome and inefficient to measure this height on every tree, so to make life easier, you must familiarize yourself with where 4.5′ lands on your body and use that mark to determine where to measure diameter on a tree.
Being 6′ tall myself, 4.5′ lands perfectly at my breast, as you can see in the photo below, so DBH holds particularly true–as well as easy to remember. Everyone’s “breast height” will be different, however, so it is important to measure for yourself.
Step 2: Carefully Wrap the Tape Around the Tree
For the next step you will need either a “D-tape,” or a flexible tape measure (such as one used in tailoring). I do, however, highly recommend buying yourself a proper D-tape, particularly if you are a landowner and plan on measuring trees frequently. Not only will a D-tape come with a specialized hook for grabbing into bark, making it much easier to measure large diameter trees, but it will automatically convert circumference into diameter, saving you a step later on.
Whichever tool you choose to use, wrap the tape around the tree at 4.5′, keeping it as straight as possible, avoiding any sag in the tape in the back. Avoid measuring over any defects or protrusions. If you do find there is a protrusion in the tree at 4.5′ that will affect the measurement, simply measure directly above the defect. If the tree is on a slope, try to measure on the uphill side of the tree. Remember, trees taper slightly as you go up the trunk, so whenever there is any doubt about where exactly to measure, it is best to move the tape upward slightly to avoid overestimation.
If the tree shape is irregular or oblong in some fashion, don’t worry about it. Because we are deriving diameter from a circumferential measurement, we are automatically creating a sort of average of the tree’s shape.
Step 3: Take the Reading
After taking the tape completely around the tree, bring it back and match up the end up the tape with zero. My D-tape reads in centimeters, so this tree is 60.3 centimeters in diameter, or about 23.7 inches. There you have it. That’s the diameter. There are D-tapes that are double sided, so be sure you are reading the correct side before recording your information.
If you are using a regular tape measure, this number is only the circumference of the tree, and further calculation is necessary to measure the true diameter.
Step 4: Convert the Measurement as Necessary
A forester’s D-tape is designed to automatically measure diameter from circumference, but if you use a standard measuring tape, you will have to do this yourself. Luckily, it isn’t too difficult.
To convert your circumference into diameter, simply divide by 3.14 or pi. That’s it.
To use the above tree as an example, if you were to measure the diameter with a standard tape measure, it would read 74.4 inches, which would be the circumference. Divide 74.4 by 3.14, and you get the real diameter: 23.7 inches.
Alternative Method: Using a Biltmore Stick
In place of using a D-tape, a Biltmore stick can be used to take an expedient estimate. Biltmore sticks work by using the geometry of your perception to estimate diameter based on the stick’s distance from the edge of the stem. The larger the tree, the larger the distance between the stick and the stem. Thus, you will perceive each additional inch in diameter as a smaller gain. For this reason, the increments between each inch on a Biltmore stick decrease gradually to give an accurate measurement.
To use a Biltmore stick to measure DBH, simply hold the stick 25 inches from your eyes level and line up the zero mark with the left end of the stem. The reading on the right side will be your DBH. In the picture below, you can see the reading would give a diameter of 10.5 inches.
While Biltmore sticks are quick and easy, they can be more prone to error than other methods, so be judicious with their use.
What You Can Do With Tree Diameter Measurements
The ability to accurately measure tree diameters comes with great benefits. For large landowners interested in timber cruising, properly measuring diameter comes with the ability to accurately assess standing inventories and make management decisions based on those numbers. However, you don’t need to have a complex cruising program to benefit from measuring trees. Having the ability to measure tree diameter opens up a host of opportunities to better monitor and manage your forest, as well as simply enjoy nature.
With diameter measurements, you can easily estimate individual tree volume using volume tables, such as this cords per tree table below:
Particularly for landowners who harvest timber on a small scale, such as for firewood, estimating the volume of individual trees prior to harvest allows for more targeted and accurate management, so you can be sure you don’t over-harvest based on your forest’s growth rate.
If you want more information on using the table above and estimating the volume of a tree, we have an article for you here.
Even if you have no interest in forest management and only want to measure a few trees in your yard, you can still derive enjoyment and learn about trees by measuring diameter to monitor growth. Simply find a few trees and measure and record their diameter. Keep the records in a safe place and repeat the process every year or every few years. You will be amazed at how quickly your trees grow!
Particularly if you have young kids, this can be a great and engaging way for them to see progressions that would otherwise be imperceptible to them. When they are older, they will appreciate looking back on your measurements and pondering how much smaller the trees were so many years ago. It is a great way to gain perspective.
So What are you waiting for? Get out there and start measuring!
Become a Forester.
Build an Asset.
Subscribe to our newsletter and get a free 76-page guide to understanding and managing your forest.