@Markus - The basal area factor is not hard to calculate once you understand the formula used.
The formula for calculating basal area is (3.1416 x (DBH/2)2)/(144). DBH stands for diameter at breast height and 144 converts from square inches to square feet. The simplified basal area formula is 0.005454 x DBH/2 with the result being ft2.
In order to calculate the basal area per acre or one hectare of land, add all the trees in the area then divide that number by the area of land where the trees were measured.