What is basal area in forestry?

Basal area is the cross-sectional area of a tree's stem measured at breast height (1.4 m above ground in New Zealand; some countries use 1.3 m and the United States uses 4.5 ft, about 1.37 m) and is expressed in square metres (m2) or square centimetres (cm2). Stand basal area is the total cross-sectional area of all trees in a defined area, usually expressed in square metres per hectare (m2/ha). It is a standard measure used by foresters to describe stand density, estimate timber volume, and guide thinning decisions. A stand basal area of 20 to 40 m2/ha is typical for productive commercial forests.

How do you measure DBH?

DBH stands for diameter at breast height. In New Zealand and most countries it is measured at 1.4 m above ground (some countries use 1.3 m). You can measure it directly with tree calipers or a diameter tape (D-tape). If you use a circumference tape, divide the circumference by pi (3.14159) to get the diameter. The measurement should be taken on the uphill side of the tree on sloping ground. On trees with buttresses, measure above the buttress.

What is a typical stand basal area for New Zealand plantation forests?

In New Zealand radiata pine plantations, stand basal area varies with age and management regime. Young stands may have a basal area of 5 to 10 m2/ha, while older stands approaching final harvest typically reach 50 to 70 m2/ha. Post-thinning residual basal area targets are commonly 15 to 25 m2/ha, depending on the intended end product. Native forests such as kauri or beech can have stand basal areas of 40 to 80 m2/ha. The basal area per hectare figure is also used to set stocking levels and assess competition for light and resources.

Basal Area Calculator

This calculator works out the basal area of a single tree from its diameter at breast height (DBH), then scales that up to a stand basal area per hectare so you can gauge how densely a plot of forest or farmland is stocked. Basal area is the cross-sectional area of a tree's trunk at breast height, defined as 1.4 m above ground, and is a standard forestry measure used to plan thinning, estimate timber volume and track stand density. You enter a DBH figure and its unit (millimetres, centimetres, metres or inches), then choose the unit for the single tree result (square metres, square centimetres or square feet). For the stand figure, enter a plot area with its unit (hectares, square metres or acres) and the number of trees in that plot, assuming they share the same average DBH. The calculator returns the single tree basal area in your chosen unit and in square centimetres, the stand basal area in square metres per hectare, and the stocking rate in stems per hectare, alongside a breakdown of the radius, formula and working, plus a verdict on whether the stand density is low, moderate, typical or high. Because it assumes one uniform DBH across the plot rather than measuring every tree, treat the stand figure as a planning estimate, not a substitute for a full plot survey.

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Standard Method Formula: BA = π × (DBH ÷ 2)². DBH measured at 1.4 m above ground (NZ/international standard).

1. Single Tree

Diameter at breast height (DBH) at 1.4 m height

DBH unit

Result unit

2. Stand Basal Area

Plot area for per-hectare calculation

Plot area unit

Number of trees in plot for stocking estimate

trees

Basal Area Results

Single Tree BA
-
m²

Single Tree BA

cm²

Stand Basal Area
-
m²/ha

Stocking

stems/ha

Single Tree Breakdown

DBH entered-

DBH in metres-

Radius (m)-

FormulaBA = π × r²

Basal area (m²)-

Stand Basal Area Breakdown

Plot area entered-

Plot area (ha)-

Trees in plot-

Stocking (stems/ha)-

Total BA in plot (m²)-

Stand basal area-

Result: Enter a DBH above to calculate basal area.

What is Basal Area?

Basal area is the cross-sectional area of a tree's stem (trunk) measured at breast height, which is defined as 1.4 m above ground in New Zealand and most international forestry standards (some countries use 1.3 m). It represents the space a tree trunk occupies at that height and is one of the most widely used metrics in silviculture, forest inventory, and ecological surveys.

Stand basal area, expressed in square metres per hectare (m2/ha), is the sum of all individual tree basal areas within a given area scaled to a per-hectare rate. It provides a measure of how densely stocked a forest stand is and correlates closely with timber volume, light competition, and site productivity.

Basal Area Formula

The basal area of a single tree is calculated from the diameter at breast height (DBH) using the formula for the area of a circle:

BA = π × (DBH / 2)²

where DBH is expressed in metres to obtain BA in square metres. Alternatively, using DBH in centimetres:

BA (m²) = π × (DBH_cm / 200)²

A shortcut used in practice is the basal area factor (BAF) method with a wedge prism or angle gauge, but direct measurement via DBH and this formula gives the most precise result for individual trees.

Stand Basal Area per Hectare

Stand basal area (also written G or BA/ha) is calculated by measuring the DBH of all trees on a sample plot, computing each tree's basal area, summing those values, and then scaling the total to a per-hectare figure based on the plot size:

Stand BA (m²/ha) = Sum of individual tree BAs (m²) / Plot area (ha)

If all trees are assumed to have the same average DBH (as in this calculator), the stand basal area equals the single tree BA multiplied by the number of trees, divided by the plot area in hectares.

Typical Basal Area Values

Forest Type / Condition	Typical Stand BA (m²/ha)
Young plantation (year 1-5)	0 to 5
Radiata pine, pre-thinning	20 to 40
Radiata pine, post-thinning residual	15 to 25
Radiata pine, final harvest (age 25-30)	50 to 70
NZ native beech forest	30 to 60
Old growth kauri	50 to 90
Managed sheep/beef farmland with trees	2 to 10

Worked Example

A radiata pine tree with a DBH of 30 cm has a radius of 15 cm (0.15 m). Its basal area is:

BA = π × (0.15)² = 3.14159 × 0.0225 = 0.07069 m²

A plot of 1 hectare containing 400 trees, all with this DBH, has a stand basal area of:

Stand BA = 400 × 0.07069 / 1 = 28.27 m²/ha

This matches the default output of the calculator above: DBH = 30 cm, 400 trees, 1 ha plot.

Related Calculators

Sources and method: Formula BA = π × (DBH/2)² per standard silvicultural practice. DBH measurement height 1.4 m per New Zealand Forest Research Institute and FAO Forestry Paper 118 (Forest Resources Assessment). Stand basal area method per Ministry for Primary Industries NZ Forest Growing Guide.

This calculator provides single-tree basal area and a stand basal area estimate based on a uniform average DBH. In practice, stand basal area is measured by summing the individual basal areas of all trees on a sample plot, or by using a wedge prism. Results are for indicative purposes only.