Tree Diameter Calculator (DBH)
Measure a tree like a forester: convert your tape reading to diameter at breast height, get basal area, combine multi-stem trunks, and estimate stand basal area per acre.
Forked trees: measure each stem's DBH below the fork and combine with d = √(d₁² + d₂² + …). Never add the diameters — that wildly overstates the cross-section.
Quick stand check (average-DBH method): enter a fixed plot's tree count, average DBH, and plot area.
How to measure DBH
- Wrap the tape 4.5 ft (1.37 m) above the ground on the uphill side, snug but not digging into bark.
- On a slope, find the 4.5 ft point vertically — on a 20° slope that is 4.7888 ft (145.96 cm) measured along the uphill ground (4.5 ÷ cos 20°).
- Measure below any swell, fork, or branch; on leaning trees measure on the side of the lean.
- Below the fork for multi-stem trees — then use the Multi-Stem tab.
Worked examples
| Tape reading | Diameter | Basal area |
|---|---|---|
| 75.4 in circumference | 24.0006 in = 60.9614 cm | 452.4107 in² = 3.1417 ft² |
| 40 in circumference | 12.7324 in | 127.3250 in² |
| 60 cm diameter tape | 60 cm = 23.6220 in | 2827.4334 cm² |
| Stems 10, 8, 12 in | 17.5499 in = 44.5768 cm | 241.9026 in² |
- tree-age — estimate age from DBH × growth factor
- tree-value — estimate amenity or timber value
- tree-spacing — plan planting distance
- basal-area — full stand analysis (TPH, Dq, stocking)
Frequently asked questions
What is DBH (diameter at breast height)?
DBH is the standard forestry measurement of a tree trunk's diameter, taken 4.5 ft (1.37 m) above the ground on the uphill side of the tree. Foresters use it for timber volume tables, growth estimates, and basal-area calculations. This tool measures and converts DBH — for estimating a tree's age from DBH see tree-age, for valuing it see tree-value.
Why is DBH measured at 4.5 feet / 1.37 meters?
The 4.5 ft (1.37 m) standard puts the measurement above the root flare at the base, where the trunk is approximately cylindrical. Measuring higher or lower would not be comparable with standard volume tables and growth models. On slopes, measure from the uphill side of the base.
What is the difference between a diameter tape and a regular tape?
A diameter tape (D-tape) has its scale pre-divided by pi, so the reading on the tape is the diameter directly. A regular tape reads circumference, which you must divide by pi yourself: for example, a 75.4 in wrap gives 75.4 / π = 24.0006 in diameter. Use the Calculator tab for the conversion.
How do I correct DBH on a slope?
Stand on the uphill side and measure 4.5 ft (137 cm) vertically above the ground, not along the slope surface. If you must measure along the uphill ground, divide by cos(slope angle): on a 20-degree slope that is 4.5 / cos(20°) = 4.7888 ft (145.96 cm) along the ground. Measuring straight along the slope without correction puts the tape too low and overstates diameter.
What is basal area and how is it computed?
Basal area is the cross-sectional area of the trunk at breast height: BA = pi x (d/2)². A 24.0006 in diameter tree has 452.4107 in² = 3.1417 ft² of basal area; a 60 cm tree has 2827.4334 cm². In forestry, stand basal area per acre is summed over a plot and expressed as ft² per acre (metric: m² per ha).
How do I get the combined DBH of a multi-stem tree?
Do not add the stems — measure each stem's DBH and use d_eq = sqrt(d1² + d2² + ...). For stems of 10, 8, and 12 in, d_eq = sqrt(100 + 64 + 144) = 17.5499 in (44.5768 cm). A plain sum (30 in) would overstate the cross-section almost threefold.
Example: 75.4 in around a tree — what is the diameter?
24.0006 in. Diameter = circumference / pi, so 75.4 / π = 24.0006 in = 60.9614 cm. The tree's basal area is 452.4107 in² (3.1417 ft²).
Example: plot of 15 trees averaging 14 in DBH on a 0.25-acre plot?
64.1409 ft² per acre (14.7247 m²/ha). Each 14 in tree has 153.938 in² of basal area; 15 trees give 2309.07 in² = 16.035 ft² total, divided by 0.25 acre. This average-DBH shortcut is fine for quick checks — for full stand analysis (TPH, quadratic mean diameter, stocking classes) use the basal-area tool.
Does bark thickness affect the measurement?
Slightly. A tape wraps the bark, so the reading is diameter over bark; timber tables sometimes require inside-bark diameter, which means subtracting twice the bark thickness. Bark varies by species and age, so this tool reports the tape (over-bark) measurement honestly — it does not guess a bark deduction.
Tree measurement vs tree age vs tree value — which tool do I need?
This tool is the measurement layer: it converts tape readings to diameter, computes basal area, combines multi-stem trunks, and estimates plot basal area. Once you have the DBH, tree-age (#367) estimates age from DBH x growth factor, tree-value (#369) estimates amenity or timber value, tree-spacing (#368) plans planting distance, and basal-area (#1387) analyzes a full stand. Start here, then branch out.
Free tool by NRTechStudio