Step 1
Slip your measuring string around the trunk of a tree you might like to cut. If your string is marked at 3-inch intervals, it's easy to determine if it's large enough for your fencing project. As a tree's circumference is roughly three times its diameter, as an example, a tree measuring 18 inches around has a diameter of nearly 6 inches.
Step 2
Measure the height of the tree trunk, using the measuring tape. Keep in mind that a fence post should have roughly 1/3 of its length buried, if you want to your fence posts to stand 6 feet above the ground line, you'll need a tree of suitable girth, with a trunk at least 9 feet high.
Step 3
Turn on your chainsaw. Remove any of the lower branches from the tree. This will give you unobstructed access to the base of the trunk.
Step 4
Cut the tree trunk off flush with the ground line. On a smaller softwood tree such as a cedar, it will not be necessary to make a wedge cut on one side. Make a horizontal cut across the trunk and drop the tree.
Step 5
Measure the trunk to the length you require, and cut away the top of the tree. Use your chainsaw to remove any other branches on the portion that will become your fence post. Repeat the process until you have harvested enough trees to complete your fencing project.