Tree Diameter Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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When a diameter tape is not available, circumference is often the easiest trunk measurement to collect. The Tree Diameter Calculator converts that circumference into diameter using the geometry of a circle, then shows supporting values that are useful in forestry and tree measurement.

What this calculator does

Enter the circumference of a single stem and the calculator returns the corresponding trunk diameter. The result card also reports diameter in inches, radius, cross-sectional area, and the normalized circumference used by the calculation. This makes the tool useful when another forestry formula requires diameter at breast height but your field measurement was taken with an ordinary tape.

How to use it

Wrap the tape around the stem at the required measurement height and keep it perpendicular to the trunk axis. Record the full circumference, not half-circumference or width viewed from one side. Enter the value in centimeters or inches using the unit selector. For buttressed, leaning, forked, or irregular trees, follow the measurement convention required by your forestry protocol before using the conversion.

How the calculation works

For a circular stem, circumference C and diameter d are related by C = πd, so d = C/π. The calculator then divides the diameter by two for radius and uses πr² for cross-sectional area. Unit conversions happen before the derived values are displayed, so the same physical stem should produce the same diameter whether circumference is entered in centimeters or inches.

Example

With the default circumference of 100 cm, the calculator returns a diameter of about 31.83 cm. That is approximately 12.53 in, with a radius of about 15.92 cm and a circular cross-sectional area of roughly 795.77 cm². These values all describe the same idealized circular cross-section.

How to interpret the result

The main result is a geometric diameter, not a direct measure of wood volume, basal area per hectare, biomass, or age. It is often the input needed for those later calculations. If repeated measurements are being used to track growth, measure at the same marked height and with the same protocol each time so apparent diameter change is not caused by measurement position.

Limitations and notes

Real stems are rarely perfect circles. Deep bark furrows, elliptical trunks, buttresses, vines, wounds, swelling, and multiple stems can make circumference-derived diameter differ from caliper measurements. The formula itself is exact for a circle; uncertainty comes from how well the measured stem cross-section matches that assumption and from field technique.