Tree Diameter Calculator

Tree diameter is one of the fundamental measurements used in forestry, arboriculture, ecology, and urban tree inventories. Because a standing trunk is easier to wrap with a tape than to measure through its center, field workers frequently record circumference, also called girth. For a nearly circular cross-section, diameter equals circumference divided by pi. This calculator applies that relationship and converts the answer among millimeters, centimeters, meters, and inches. If a diameter is entered directly, it preserves that dimension while converting units and also reports the corresponding circumference. Foresters normally report diameter at breast height, abbreviated DBH. The exact standard varies by jurisdiction, but a height of about 1.3 meters or 4.5 feet above ground is common. A consistent reference point makes repeat measurements and comparisons meaningful. Measure from the uphill side on sloping terrain. If the reference height falls on a branch, swelling, wound, or other irregularity, follow the measurement rules used by the relevant inventory program rather than moving the tape arbitrarily. Keep the tape level and perpendicular to the trunk axis. Remove loose vines or debris without damaging bark, and do not pull a flexible tape so tightly that it compresses thick bark. A forestry diameter tape is graduated to display diameter directly from circumference; an ordinary tape works just as well when paired with this calculator. Trees are not perfect cylinders, so caliper measurements taken along one direction can differ from girth-derived diameter. Two perpendicular caliper readings and their average may better represent an oval stem. DBH supports calculations of basal area, biomass, growth, stocking, and timber volume, but diameter alone does not determine tree safety, health, age, or monetary value. Buttressed trees and multi-stem specimens require special protocols, and comparisons over time should use the same method, height, units, and instrument. Record the measurement date and any unusual trunk condition along with the result. For formal surveys, carbon reporting, appraisals, or hazard assessments, follow the governing standard and consult a qualified forester or arborist.