Diameter tape
A diameter tape, or D-tape, is a measuring tape used to estimate the diameter of a cylindrical object, especially tree trunks. The tape is marked so that when you wrap it around the object, the reading shows the diameter (in either metric or imperial) because the scale is reduced by the value of π. It assumes the object is a perfect circle and gives an approximate diameter, useful in dendrometry. The tape usually has the diameter scale on one side and standard measurements on the other.
Diameter tapes are typically made of cloth or metal. They can be used to measure tree stems, logs, and other parts of trees, but accuracy decreases if the trunk isn’t close to round.
Diameter measurements are usually taken at a standard height above ground called diameter at breast height (DBH) or standard diameter height (SDH). In many places, DBH is 4.5 feet (1.4 meters) above ground; in Canada, Europe, Thailand, and Vietnam it’s 4.27 feet (1.30 meters). Measuring at ground level is avoided because of butt swell, where the base is thicker than the trunk above it.
Why measure diameter: Combined with height, diameter helps estimate a tree’s volume and basal area. For single-stem trees, DBH is straightforward; for multi-stem trees, DBH can be harder, and a different height may be more practical.
How to measure: Wrap the tape around the trunk at the correct height, perpendicular to the trunk’s axis. Align the zero with the rest of the tape, and read the diameter on the diameter side. Readings are most reliable for fairly round trunks.
Cautions: If the trunk is not circular, the reading is only an approximation. For more accuracy on irregular shapes, precision diameter tapes or other methods may be used.
Precision diameter tapes: In metalworking, there are high-precision diameter tapes made from 1095 clock spring steel with a vernier scale. These are checked for accuracy to about 0.001 inch (0.025 mm) for tapes up to 144 inches, and are used in industries like aerospace and plastics.
This page was last edited on 3 February 2026, at 19:48 (CET).