Ring Sizing Issues Again

I am having some trouble finding a definitive source for ring sizes.
I only make titanium rings and they are precision machined from
solid bar stock to the exact inside diameter of the rings. Of
course, nobody measures rings via the inside diameter (or at least
few people do). For the most part the sizing charts all agree on the
inside diameter of various sizes and the different scales used for
American, British, Japanese, etc.

However, lately I have been having trouble with a jewelry store in
London, England. They have been ordering rings in British sizes and
the larger the size, the more off I am on the fit. Apparently the
charts I am using are incorrect, and they have been instructed to
order all rings via specifying the actual inside diameter to the
thousandth of an inch or hundredth of a millimeter. This works fine
for them. Now I have some orders for other British customers and
they have given me their British sizes. Here are two sizing charts
with two different values for a British size “X” inside diameter.

One has it at 0.830" and the other at 0.841", this is an
unacceptable variation to me. What is the “real” value for British

Daniel J. Statman, Statman Designs

[snip] Here are two sizing charts with two different values for a
British size "X" inside diameter.
http://www.niceice.com/ring_sizes.htm One has it at 0.830" and the
other at 0.841" 

Interesting. I wonder is there a translation problem inherent in
going from US sizes to Brit sizes? Perhaps it’s better to ascertain
the actual diameter in mm for each and compare that. I use a
spreadsheet to calculate the diameter in mm for all the British sizes

  • first assign a number sequence to each letter A-Z (A=1 etc) then:

11.6713 + 0.3937 x = mm diameter

According to this, British X = 21.12mm Translate that to inches.
Perhaps 0.8314943998?

Sorry I don’t have a source reference for my method, but Gary
Wooding wrote in rec.crafts.jewelry a similar formula:

rec.crafts.jewelry … If you want to use an approximate formula,
convert the UK letter size to a number (A=1, B=2, C=3 etc) and
substitute the number for E in the following: UK Size (inches) =
0.458 + 0.01552 * E As can be seen, the formula is not accurate
because there is no linear relationship, but its fairly close.

Brian Adam