Basal area is the cross-sectional area of trees at breast height (1.3m or 4.5 ft above ground). It is a common way to describe stand density. In forest management, basal area usually refers to merchantable timber and is given on a per hectare or per acre basis. If one cut down all the merchantable trees on an acre at 4.5 feet (1.4 m) off the ground and measured the square inches on the top of each stump (πr*r), added them all together and divided by square feet (144 sq inches per square foot), that would be the basal area on that acre. In forest ecology, basal area is used as a relatively easily-measured surrogate of total forest biomass and structural complexity, and change in basal area over time is an important indicator of forest recovery during succession .

The basal area (BA) of a tree can be estimated from its diameter at breast height (DBH), the diameter of the trunk as measured 1.3m (4.5 ft) above the ground. DBH is converted to BA based on the formula for the area of a circle:

${\displaystyle BA=\pi \times (DBH/2)^{2}}$

If ${\displaystyle DBH}$ was measured in cm, ${\displaystyle BA}$ will be in cm². To convert to m², divide by 10,000:

${\displaystyle BA(m^{2})={\frac {\pi \times (DBH(cm)/2)^{2}}{10000}}}$

If ${\displaystyle DBH}$ is in inches, divide by 144 to convert to ft²:

${\displaystyle BA(ft^{2})={\frac {\pi \times (DBH(inches)/2)^{2}}{144}}}$

The formula for BA in ft² may also be simplified as: