How to measure tree diameter?

Trees do not grow like cylinders, but rather taper upward; a tree’s diameter gets smaller as one gets closer to the top of the tree. Trees also have butt swell, a thickening of the wood and bark at the base of the tree to support the tree’s mass (Figure 3.2). Butt swell can create a very large diameter on trees exposed to heavy wind, on steep slopes and in sparsely populated stands.  Thus, to obtain truly useful diameter data, the question becomes, “Where on the tree should I measure?”

To make tree diameter measurements meaningful and easy to perform, a standard location and protocol have been developed. Diameters are measured outside the bark at 4.5 feet above the ground on the uphill side of the tree (Figure 3.3). This location, called diameter at breast height or DBH, is above most butt swell and brush. It is also at a comfortable arm position for most people.

graphic showing 4.5 feet above the ground on the uphill side of a tree
Figure 3.3. The standard location for measuring tree diameter is at DBH, 4.5 feet above the ground on the uphill side of the tree. Source: [FS] 1990.
 

For most trees in the forest, measuring DBH is quite straightforward. However, there are plenty of irregular trees that require adaptations, as described in Figures 3.4 – 3.10.  (All illustrations from or adapted from [FS] 1990.)

graphic showing forking above and below dbh
Figure 3.4. On forked trees, measure as one tree if fork occurs at or above 4.5 feet (left). Measure as two trees if fork occurs below 4.5 feet (right).

 

graphic showing branches at dbh; measurement point is directly above them. Graphic on right shows an enlarged stump with a measuring point 1.5 feet above the swelling
Figure 3.5. Measure directly above a bulge or branch whorl (left). On trees with extensive butt swell, measure at least 1.5 feet above the butt swell (right).

 

graphics depicting condition described in caption
Figure 3.6. For a large burl or canker, measure above the deformity and adjust the diameter down slightly (left), or take two measures equidistant from DBH above and below the deformity, and use the average (right).

 

graphics depicting condition described in caption
Figure 3.7. On leaning trees, the tape is held perpendicular to the tree bole, and is measured on the uphill side of the tree if on a slope (left); on the short side of the lean if on flat ground (right).

Further useful knowledge pages

How to measure tree height?

How to measure tree diameter?