To accurately size rings, you measure the finger with an standard
ring sizer, and to get the actual length you need a table of ring
sizes available from Rio Grande Technical Bulletins, also I have seen
the table in Rio’s catalogue. Take the diameter listed and add the
thickness of the metal to be bent and then multiply by Pi ( 3.14), and
get the length to be cut. The reason for this is that metals bend
around the neutral axis, which is the center of the ring. The metal
on the outside of the axis expands and the metal on the inside of the
axis contracts, so when you add the metal thickness to the diameter
shown in the table you get the neutral axis. The formula is:
circumference = Pi x diameter. Split ring shanks are a little tricky
but if you draw it up and remember the above calculation you will get
close. John Burton - Mech Eng.