Tree Diameter Calculator (DBH)
Enter the circumference of a tree trunk measured at breast height (1.37 m / 4.5 ft above ground) and instantly get the diameter at breast height (DBH). The calculator also estimates tree age from DBH using species-specific growth factors, computes basal area, and optionally strips bark to give diameter under-bark. Switch between metric and imperial units at any time.
What is diameter at breast height (DBH)?
Diameter at breast height is the standard forestry measurement of a tree trunk's cross-sectional size, taken at exactly 1.37 metres (4.5 feet) above ground on the uphill side of the tree. Because bark, butt swell, and root buttresses distort the base, DBH is measured partway up the trunk where the girth is most consistent. Every country and profession that works with trees uses DBH as the reference size: foresters to estimate timber volume, arborists to price removal and pruning, ecologists to track biomass and carbon, city planners to determine which trees need removal permits, and researchers to monitor forest growth over time.
How to measure tree circumference accurately
Wrap a flexible tape (or a string you will then measure against a ruler) snugly around the trunk at exactly 1.37 m (4.5 ft) above ground. On sloping ground, measure from the uphill side because gravity causes the tape to ride higher on the downhill side. If the tree leans, measure 4.5 ft along the underside of the trunk rather than as a vertical height. Avoid swellings, branch stubs, wounds, or ivy by measuring just below the deformity. For a forking tree that divides below 4.5 ft, treat each stem as a separate tree or use the multi-stem calculator, which applies the ISA quadratic mean method: each stem diameter is squared, the squares are summed, and the square root of that sum gives a single combined DBH. That combined value captures the actual basal area of all stems together, which is what planners and ecologists need.
Tree age estimation and why it is an approximation
The growth factor method estimates age as: Age = DBH (inches) x growth factor. The growth factor is the average number of years a given species adds one inch to its diameter in open-grown conditions, and it ranges from about 2 for a cottonwood or black willow to about 8 for a horse chestnut. Forest-grown trees tend to grow faster in youth but are suppressed by competition, so they often end up thicker per year than open-grown trees in some early decades and thinner in others. Climate, soil fertility, moisture, and disturbance history also shift the result. Treat the growth factor estimate as a reasonable approximation that is within 20-30 percent of the true ring count in most cases. The only exact method is core sampling and counting annual rings.
Basal area and its uses in forest management
Basal area is the cross-sectional area of a trunk at breast height. For a single tree it is simply the area of a circle: BA = pi x (DBH/2)^2. In imperial units, foresters use the shortcut BA (ft^2) = 0.005454 x DBH (in)^2 derived from dividing pi by 4 and converting square inches to square feet. Basal area per acre or per hectare is the sum of all individual tree basal areas in a stand, and it is the most widely used measure of stand density in North American forestry. A fully stocked hardwood forest typically carries 80-120 ft^2 per acre; a very dense stand can exceed 200 ft^2 per acre. Thinning prescriptions target a specific residual basal area to leave enough growing space for the remaining trees while maximising total growth.
Species growth factors and typical lifespan
| Species | Growth factor | Growth rate | Approx. lifespan |
|---|---|---|---|
| Cottonwood | 2 | Very fast | 70-100 yr |
| Black willow | 2 | Very fast | 30-70 yr |
| Quaking aspen | 2 | Very fast | 50-150 yr |
| Silver maple | 3 | Fast | 100-125 yr |
| Tulip poplar | 3 | Fast | 200-250 yr |
| Basswood | 3 | Fast | 200 yr |
| Pin oak | 3 | Fast | 200 yr |
| Green ash | 4 | Moderate | 150 yr |
| American elm | 4 | Moderate | 200 yr |
| Red oak | 4 | Moderate | 400-500 yr |
| Sweetgum | 4 | Moderate | 300 yr |
| Loblolly pine | 4 | Moderate | 150-200 yr |
| Sycamore | 4 | Moderate | 250-600 yr |
| Red maple | 4.5 | Moderate | 150 yr |
| Black walnut | 4.5 | Moderate | 250 yr |
| Douglas fir | 5 | Moderate | 500-1000 yr |
| Eastern white pine | 5 | Moderate | 200 yr |
| White oak | 5 | Moderate | 500-600 yr |
| Bur oak | 5 | Moderate | 300-400 yr |
| Yellow birch | 5 | Moderate | 150 yr |
| Black cherry | 5 | Moderate | 150-200 yr |
| Sugar maple | 5.5 | Slow | 300-400 yr |
| American beech | 6 | Slow | 300-400 yr |
| Shagbark hickory | 7.5 | Slow | 200-300 yr |
| Common horse chestnut | 8 | Very slow | 200-300 yr |
Growth factor (years per inch of DBH) from the International Society of Arboriculture. Multiply DBH in inches by the growth factor to estimate age.
Frequently asked questions
What does DBH stand for and why is 4.5 feet the standard?
DBH stands for diameter at breast height. The 4.5-foot standard (1.37 m) was adopted because it is roughly at the chest of a standing adult, making it easy to reach with a diameter tape, above the root flare and typical butt swell that would otherwise inflate the reading, and consistent enough that measurements taken by different people at different times are comparable. Some countries use 1.3 m (about 4.3 ft); the difference is minor in practice.
How do I measure DBH without a diameter tape?
Wrap any flexible tape or a length of string around the trunk at 4.5 feet, measure the circumference, then divide by 3.14159 (pi) to get the diameter. Alternatively, use the optical method: stand a known distance from the tree, hold a ruler at a fixed arm length, and read how wide the trunk appears. This calculator supports both methods. For rough work, a rope and a tape measure are all you need.
How do I calculate DBH for a multi-stem tree?
The International Society of Arboriculture recommends the basal area summation (quadratic mean) method: convert each stem circumference to a diameter, square each diameter, sum the squares, and take the square root of the total. For example, two stems of 10 in and 8 in give diameters of 10 and 8, squares of 100 and 64, a sum of 164, and a combined DBH of sqrt(164) = about 12.8 in. This combined DBH equals the diameter of a single circular trunk that would have the same total cross-sectional area as all the stems together.
What is diameter under bark (DUB) and when does it matter?
Diameter under bark (DUB) is the diameter of the wood itself, excluding the outer bark layer. It matters for timber valuation (buyers pay for wood, not bark), carbon estimation (wood density is what is measured), and some pest or disease assessments. DUB = diameter over bark (the DBH you measure) minus 2 times the single-side bark thickness. Bark thickness varies widely - eastern white pine might have 0.5 in of bark, while a large white oak could have 1-2 in.
What DBH triggers a tree removal permit?
Permit thresholds vary by jurisdiction. A common threshold for protected trees in U.S. cities is 6-15 in DBH, while heritage or significant tree designations often apply at 24-36 in. Some localities use circumference: Washington D.C. applies special rules above 44 in of circumference (about 14 in DBH) and heritage status above 100 in circumference (about 31.8 in DBH). Always check with your local urban forestry office before removing or significantly pruning a mature tree.
How accurate is the growth factor age estimate?
The growth factor method is a rough approximation, typically within 20-30 percent of the ring count for open-grown trees in average conditions. Trees growing in poor soil, under drought stress, or in dense forest competition may be older than the formula suggests; trees in fertile, irrigated, or recently disturbed sites may be younger. For legal, scientific, or appraisal purposes, confirm age with an increment core sample counted by a professional.