Bavarian company Friedrich Richter has made measuring tapes for over 80 years. As such, they have an extensive range suitable for a variety of applications.
This product is their steel diameter tape with end hook. The user can use one side to read the diameter of a tree, and the other side to read circumference.
With tape, there are no calculations needed. Wrap the tape around the object you wish to measure, and read the graduations in millimeters and centimeters. If you are an arborist or professional that needs to measure the diameter of an object, this is the product for you.