How this calculator works
A tape measures girth around the trunk, whereas diameter crosses the centre. Division by pi assumes a circular section. Subtract twice the radial bark thickness because bark appears on both sides. For several stems, the equivalent diameter is the square root of their squared diameters, preserving total area. Bark is removed from each stem before combining its wood area. This equal-area diameter is not a universal arboricultural reporting rule.
Formula
d = C/pi; d_wood = d - 2t; d_equivalent = sqrt(sum(d_i²)).
Worked example
A circumference of 100 cm gives a diameter of 31.831 cm. With 1 cm bark thickness, the wood diameter is 29.831 cm. Stems 30 cm and 40 cm across combine to an equal-area diameter of 50 cm.
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Frequently asked questions
Why subtract bark twice?
The supplied thickness is radial. A line through the trunk crosses bark on both sides.
Can I use the widest trunk measurement?
A single caliper width may misrepresent a non-circular trunk. Girth produces a circular approximation; use the inventory protocol for irregular trunks.
Does an equivalent diameter replace a field DBH?
Only when your protocol calls for an equal-area combination. Keep the original stem measurements for reporting.