DBH calculator: circumference to diameter, and where to measure.

Circumference to DBH

Wrap a flexible tape around the trunk at 4.5 feet, keeping it level and snug, and read the circumference. Divide by 3.1416 for the diameter. A diameter tape (D-tape) has the division printed on it and reads DBH directly; a logger's tape or a cloth tape needs the table.

Where 4.5 feet is

On level ground, 4.5 feet up the trunk from the soil surface, not from the top of the mulch. On a slope, measure from the ground on the uphill side. On a leaning tree, measure 4.5 feet along the trunk on the underside of the lean, and hold the tape perpendicular to the trunk, not level with the ground. A trunk that forks above 4.5 feet is one tree with one DBH; a fork below it is two stems.

Mark the height once with a stick cut to 4.5 feet or a mark on your tape handle. Eyeballing it drifts, and on a tree with taper a few inches of height is an inch of diameter.

Bulges, wounds, and branches at breast height

If 4.5 feet lands on a swelling, a burl, a wound, a branch collar, or a graft union, the measurement will overstate the tree. Measure at the narrowest point below the abnormality and record where you measured. For dosing, a smaller honest number beats a larger inflated one; the label is written for the wood that will carry the product, not the burl.

Multi-stem trees and clumps

Measure each stem that reaches 4.5 feet, at 4.5 feet, and record them individually. What you do next depends on the label and the purpose. For injection dosing, most labels have you dose each stem as its own tree and inject each one; a stem too small to inject gets no product. For a single "equivalent DBH" for a risk form or a price, the forestry convention is the square root of the sum of the squared diameters: three 8 inch stems are the square root of 192, about 13.9 inches equivalent, not 24. For pricing, plenty of companies sum the stems because the time and ports sum too. Pick one, write it on the record.

How DBH drives the dose

Every systemic label doses by DBH, as a rate per inch or a table by band. The band matters: on the Propizol table a 20 inch oak is 190 to 210 mL low, 380 to 420 mL high (label band 19 to 21 in), while a 22 inch oak is 220 to 240 mL low, 440 to 480 mL high (label band 22 to 24 in). On TREE-äge G4 a 12 inch birch is 30 to 35 mL low, 35 to 90 mL medium, 90 to 120 mL high (label band 10 to 12 in) and a 16 inch birch is 42 to 55 mL low, 55 to 135 mL medium, 135 to 180 mL high (label band 16 to 18 in). Two inches of measurement error moves a tree across a band. The rate calculator takes the DBH and returns the dose, the port count, and the per-port volume; the number it starts from is yours.

Recording it

DBH goes on the plant record with the date it was measured. Trees grow: a 20 inch oak adds a quarter inch to a half inch a year, so a two-year retreat on the same number is close enough, and a ten-year-old record is not. In PHC Rx the DBH on the plant record feeds the calculator, the prescription price, and the risk assessment form, so it is measured once per visit and typed once.

Metric

Diameter at breast height in centimeters is measured at 1.3 meters in most of the world and 1.37 meters (4.5 feet) in the United States. Circumference in centimeters divided by 3.1416 gives DBH in centimeters; divide by 2.54 for inches. A 100 cm circumference is 31.8 cm DBH, or 12.5 inches. Labels sold in the United States dose in inches.