Standard professional practices dictate that trees are measured in this same fashion. The steps are as follows:
Take any form of tape measure and measure around the trunk of a tree (the tree's circumference), approximately four and a half feet above the ground. This is also known as dbh (diameter at base height).
Then take the measurement of the circumference of tree and divide it by 3.1415 (Pi). The resulting number will be the diameter of the tree trunk, also known as caliper inch.