Question
I just have a very basic question which I hope you won't find too boring.  I want to build a tree ring using curved 16" stones with the scalloped top.  Is there a formula to determine how large it should be, or how many blocks make a circle, etc.?

Answer
Your question was posted in the "question pool" and after reading it, I began to notice these pre-cast stones being used everywhere. There is no "fomula" I am aware of, other than basic circular geometry. (http://math2.org/math/geometry/circles.htm)  

I usually use poured concrete curbing or pre-cast metal tree rings, installed on a metal/concrete frame.  There are formulas for those.

It looks like three of these pre-cast blocks create an approximate 20-inch (inside) diameter circle.  It you use four units, it looks strange.  For a tree, a minimum opening is probably closer to three feet diameter, as you have to consider the size of the tree & trunk at maturity.  These little blocks are not going to stop the tree from growing beyond this nice little border, if it is a larger variety.

I even saw these units the other day, stacked upside down and poured solid with concrete to make a bollard!  Not pretty, but functional.

~M