On one of the sites "26 interesting facts about the member":
6 is How to measure the circle of a member? We take a white thread, wrap a member with it along the trunk, then measure the resulting section with a school line.
7 is Knowing the circle of the member, it is easy to calculate its diameter and radius. The radius is half the diameter. So, if the diameter of your penis is 10 centimeters, then the radius will be 5.
If the diameter of your member is 10 cm...O_O