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Extension > Environment > Trees and woodlands > Forest management practices fact sheet: Managing water series > Making and using measurement tools - basal area

Making and using measurement tools - basal area

Best Management Practices (BMPs) can prevent or minimize the impact of forestry activities on rivers, lakes, streams, groundwater, wetlands, and visual quality.


There are a variety of measurement tools that can help assess a woodland. The tools can be sophisticated pieces of equipment (such as prisms) that are purchased from vendors or they can be homemade devices. This fact sheet will describe how to make and use a homemade tool for estimating basal area.

Basal area is the cross-sectional area of a tree 4.5 feet above ground. The basal area of all trees in a given land area describes the degree to which an area is occupied by trees and is generally expressed in square feet per acre (ft2/acre). Basal area is useful in determining whether enough trees remain to provide adequate shade within shade strips. (Shade strips are bands of vegetation along streams, lakes, and wetlands. They help moderate water temperatures.)


Basal area of a land area can be estimated by holding an object such as a washer, penny, or thumb a fixed distance from your eye. Use the following formula to calculate the distance to hold the object from your eye:

Distance from eye = width of object x 33


For example, a thumb with a width of 0.75 inches should be held 24.75 inches away from your eye (0.75 x 33 = 24.75). You can maintain that distance while using the tool by stretching a string of the appropriate length between your eye and the object.

To estimate basal area, stand in the center of a randomly selected location or plot. Hold the object the appropriate length from one eye and close the other eye. Remember these key points:

Basal area/acre = average number of trees counted x 10

As an example, assume that a total of 30 "in" trees were counted in 5 sample plots. The average number of "in" trees per plot is 6. The basal area/acre is 6 x 10, or 60 square feet/acre.


Homemade tools for estimating basal area are less expensive and easier to use than commercially available tools. They provide accurate enough estimates of basal area to make decisions regarding shade strips.


Homemade tools are less accurate than commercially available ones.

University of Minnesota Extension Service, Minnesota Department of Natural Resources, Minnesota Logger Education Program, Michigan Department of Natural Resources, Michigan State University Extension, and Wisconsin Department of Natural Resources.

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