Knowing the number of trees per acre is important in a number of circumstances including the application of pesticides, the determination of the number of available trees for harvest and just to keep track of the trees on the property. Actually marking off an acre and counting the trees is one option. A more efficient method involves mathematical calculations and works well if the trees are spaced evenly in both the distance between the trees within the row and the spacing between rows. In some situations the distances can be averaged if the tree spacing is not consistent.
Things You'll Need
 Tape measure

Measure the distance between trees in the row. This spacing is often uniform in planted woodlots. State this measurement in feet with a single decimal position as in "trees are spaced 10.5 feet apart." If the trees aren't evenly planted, measure several and divide the sum the measurements by the number of trees.

Measure the distance between the rows of trees. Again, state this in feet with a single decimal position. Average the row spacing if it's inconsistent.

Multiply the distance between the trees and the rows of trees to determine the square feet of space allowed for each tree in the woodlot. For example, if the trees are spaced 10 feet apart in rows that are 15 feet apart each tree has a 150 sq. ft. area within the woodlot.

Divide the number of sq. ft. in an acre, 43,560, by the number of square feet allotted each tree. For example, if each tree was allotted 150 sq. ft., the number of trees per acre is 290.4.
Tips & Warnings
 A number of websites, such as resources.cas.psu.edu and warnell.forestry.uga.edu provide tables where you can crossreference the tree spacing and row spacing to determine the number of trees per acre. Other tree rates per acre include the number of trees of a certain size. This is often determined by counting the trees or by measuring the average distance between trees of the proper size. You can then use the standard formula.