How to Calculate the Diameter of a Tree — DBH
The diameter at breast height (DBH) is the standard way foresters, arborists, and ecologists measure the size of a tree trunk. It is measured at a standardized height above the ground — typically 4.5 ft (1.37 m) in the United States — so results can be compared consistently across studies, surveys, and records.
Use our tree diameter calculator to instantly convert circumference to diameter (and vice versa), compute the basal area, and find the equivalent DBH for multi-stemmed trees — in both imperial and metric units.
How to Use the Tree Diameter Calculator
- Choose measurement type — single stem (standard) or multi-stemmed tree.
- Select your unit system — imperial (inches) or metric (centimeters).
- Choose what you measured — circumference at breast height (CBH) or diameter directly.
- Enter your measurement(s) and click Calculate.
- The calculator instantly shows you DBH, CBH, radius, and basal area in both unit systems.
How to Find the Diameter of a Tree from Its Circumference
The relationship between circumference and diameter is simply:
DBH = CBH ÷ π (where π ≈ 3.14159)
For example, if the circumference at breast height (CBH) is 48 inches:
- DBH = 48 ÷ 3.14159 ≈ 15.28 inches (≈ 38.8 cm)
- Radius = 15.28 ÷ 2 ≈ 7.64 inches
- Basal area = π × (7.64)² ≈ 183.4 in² (≈ 1,183 cm²)
How Do You Find the Diameter of the Wood Inside the Bark?
The standard DBH measurement includes the bark. If you need the inside-bark diameter (also called the diameter inside bark, or DIB), you must subtract twice the bark thickness:
DIB = DBH − 2 × bark thickness
Bark thickness varies greatly by species — from a few millimeters in birch and beech to several centimeters in old-growth Douglas fir or cork oak. Foresters use bark gauges or species-specific bark thickness equations to estimate this value. For most rough estimates, standard DBH (outside bark) is sufficient.
How Do I Calculate the Diameter of a Tree on a Slope?
On a slope, "breast height" is measured differently depending on which side of the tree you stand on:
- Uphill side: measure at exactly the standard height (4.5 ft / 1.37 m) above the ground on the uphill side.
- Downhill side: the measurement point will be higher above the ground on the downhill face, which is correct — this keeps the measurement horizontal through the tree.
The key rule: always measure from the uphill side at the standard breast height, keeping the tape horizontal around the trunk. This produces a consistent DBH regardless of slope angle. Never measure at the mid-point between uphill and downhill ground levels — always use the uphill reference point.
How Do I Measure the Circumference of a Leaning Tree on Level Ground?
For a leaning tree, breast height is measured along the axis of the lean (the center of the trunk), not vertically from the ground:
- Stand at the side of the tree perpendicular to the direction of lean.
- Measure 4.5 ft (1.37 m) up along the center of the trunk from the base of the tree.
- Wrap the tape measure around the trunk at that point, keeping it perpendicular to the trunk axis.
This gives a true CBH reading independent of the lean angle. If you measure vertically from the ground instead, you will get a larger distance along the trunk on the underside and a shorter one on the top side, resulting in an inconsistent and typically inflated measurement.
How Do I Calculate the Diameter of a Multi-Stemmed Tree?
Multi-stemmed trees (trees with two or more main trunks arising from near the base) require a special formula to calculate their equivalent DBH — a single number representing the combined basal area of all stems:
DBHequiv = √(DBH₁² + DBH₂² + … + DBHₙ²)
This formula preserves the total basal area of all stems combined. For example, a tree with two stems of 10 in and 8 in:
- DBHequiv = √(10² + 8²) = √(100 + 64) = √164 ≈ 12.8 inches
Our calculator supports up to 4 stems and automatically applies this formula when you select Multi-Stem mode.
Breast Height Standards by Country
| Country / Region | Breast Height (Metric) | Breast Height (Imperial) |
|---|---|---|
| United States | 1.37 m | 4.5 ft |
| Europe, Canada, Australia | 1.3 m | 4.27 ft |
| New Zealand | 1.4 m | 4.59 ft |
| Japan, Korea | 1.2 m | 3.93 ft |
FAQs
What does DBH stand for?
DBH stands for Diameter at Breast Height. It is the outside-bark diameter of a tree trunk measured at 4.5 feet (1.37 m) above the ground in the United States, or at a similar standardized height in other countries.
What does CBH stand for?
CBH stands for Circumference at Breast Height. It is simply the circumference of the trunk at breast height. Because circumference is easier to measure with a tape measure than diameter, foresters often measure CBH and then divide by π to get DBH.
Why is DBH measured at breast height?
Breast height is a practical, reproducible reference point. Measuring at the same height on every tree removes the variability caused by root flare, buttresses, or irregular trunk bases near the ground. It also allows comparisons across studies, national forest inventories, and decades of historical data.
What is basal area?
Basal area is the cross-sectional area of a tree trunk at breast height, calculated as π × (DBH/2)². For a single tree, it indicates trunk size. Summed across all trees per acre or hectare, stand basal area is a key measure of forest density and timber volume.
How accurate is measuring DBH with a tape measure?
A standard flexible tape measure gives excellent accuracy for field work — typically within ±0.5 cm or ±0.2 in when technique is consistent. For research-grade measurements, foresters use a diameter tape (D-tape) that is pre-scaled in circumference units but reads out directly in diameter, eliminating the need to divide by π manually.
Why do we use the equivalent DBH for multi-stemmed trees?
The equivalent DBH formula preserves the total basal area of all stems. This is more ecologically and economically meaningful than simply averaging the individual stem diameters, because basal area is directly related to stem volume, biomass, and carbon storage.