Basal area is a calculation used in forest management. There are a number of applications for this calculation, ranging from assessments of sites where logging is proposed to management of natural sites in recovery from logging or natural disasters. There are a number of ways to calculate basal area, along with an assortment of tools which can be used to make the task easier.
Total basal area is measured by determining the area of individual trees in the stand and adding the measurements together. The area of an individual tree is the total area covered by the trunk in cross section at a set height. Many foresters use chest height as a measurement, aiming for around four and a half feet (1.5 meters) from the ground with each trunk to ensure a standardized measurement. The area is found by measuring the tree and using the measurement to plug in numbers into the formula for the area of a circle.
Once the basal areas of individual trees are known, the total basal area can be calculated. The higher the basal area, the more timber there is. Rather than measuring each individual tree, foresters often use tools which allow them to estimate this area from a distance, taking notes as they move through a stand of trees on how much trees of each size are present. This system may not be precise, but it is much easier than measuring the trees one by one.
When foresters generate reports on stand density and site quality, basal area is an important calculation. Stand density refers to how many trees there are in a given area, while site quality refers to how much timber is present. These calculations can be used to determine the value of the timber. Other information which impacts value includes the kind of timber present and the condition, and the size of individual trees. Larger trees are more valuable than smaller ones, so a stand with 30 really large trees would be more valuable than a site with 90 small trees.
Foresters use calculations like this to manage forests of all types and sizes. In parklands where trees are being maintained for the enjoyment of the public, such measurements are used to assess forest health, and to identify health problems early. On land being managed specifically for its timber, observations such as basal area calculations are used to determine when land should be logged, and to monitor logging practices and forest recovery.
wizup Post 2 
@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. 


Markus Post 1 
I can remember as a kid helping my father mark the trees for thinning out our forest area. I still to this day don't understand how to measure the area using the basal area formula, but that's okay because at least I have some good memories helping my dad do it.
He created a homemade basal area calculator using a yardstick with a one inch square cardboard attached to the end of it. We'd go out to the middle of the forest and my dad would place that tool between his eyes pointing out to me which trees I needed to mark for thinning.
After we worked in a complete circle, we'd walk another fifty feet or so and do the next area. It was an all day job but I had a lot of fun working with him. 