Tree Diameter Calculator — DBH from Circumference
Measure the girth of a tree trunk and instantly convert it to diameter at breast height (DBH), the universal forestry measurement. Also calculates the basal cross-sectional area used in timber and ecology surveys.
cm
Unit
Diameter at breast height — circumference ÷ π
- 1
Radius from girth
125 ÷ (2 × π) = 19.89Rearranging C = 2πr gives r = C ÷ (2π). - 2
Diameter (DBH)
2 × 19.89 = 39.79
How does this calculator work?
Divide trunk circumference (girth) by π to get the Diameter at Breast Height (DBH). Basal area = π × (DBH/2)². Enter the girth measured at 1.3 m (4.5 ft) above ground — the universal forestry standard — and choose your unit.
Formula
How this is calculated
Diameter at breast height (DBH) is the standard measure of tree trunk size in forestry, ecology, and arboriculture. It is always measured at exactly 1.3 m (4.5 ft) from the ground on the uphill side — a height chosen to be consistent and above any root flare. Because a tree trunk is roughly circular, measuring the circumference (girth) with a tape is far easier than measuring diameter directly, and DBH is derived by dividing the girth by π (≈ 3.14159).
Basal area is the cross-sectional area of the trunk at breast height (π × r²). In forest inventories, basal area per hectare is a key stand density metric that correlates with timber volume, habitat, and competition for resources.
The calculator assumes a circular trunk cross-section, which is a standard simplification. Irregular, fluted, or multi-stemmed trunks may require measuring diameter in two perpendicular directions and averaging. Trees on slopes should be measured on the uphill side at 1.3 m up the slope.
Frequently asked questions
DBH (Diameter at Breast Height) is the standard trunk diameter for forestry and ecology. The 1.3 m height (4.5 ft in the US) is a globally accepted standard set above typical root flare but still reachable without a ladder, ensuring consistent, comparable measurements across surveys.
Wrap a flexible tape measure around the trunk at exactly 1.3 m from the ground on the uphill side. Keep the tape horizontal and snug against the bark. On slopes, measure from the uphill base. Use the reading directly as girth — dividing by π gives the diameter.
Basal area (the cross-sectional trunk area) is summed across all trees in a forest stand to get stand basal area per hectare — a key indicator of timber volume, crown competition, and habitat quality. Individual tree basal area also appears in commercial timber valuation formulas.
Also known as
TG we-Calculate Editorial Team. (2026). Tree Diameter Calculator — DBH from Circumference [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/tree-diameter-calculator
TG we-Calculate Editorial Team. "Tree Diameter Calculator — DBH from Circumference." TG we-Calculate. 2026. https://we-calculate.com/calculator/tree-diameter-calculator.
TG we-Calculate Editorial Team, "Tree Diameter Calculator — DBH from Circumference," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/tree-diameter-calculator
@misc{wecalculate_tree_diameter_calculator, title = {Tree Diameter Calculator — DBH from Circumference}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/tree-diameter-calculator}}, year = {2026}, note = {TG we-Calculate} }
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