Comparative ring sizes

Hi Don,
Regarding European ring sizes and what “I” and “J” means:
US 3 3/4 = British "H"
US 4 = British "H 1/2"
US 4 1/4 = British "I"
US 4 1/2 = British "I 1/2"
US 4 5/8 = British "J"
US 5 = British “J 1/2”

Where the British system comes from is anybody’s guess. France,
Switzerland and Germany each have their own system. The American
system is based on one inch. Size 1 is just about 1/2 inch in
circumference (25.4 mm = 1 inch). Size 1 = 12.3 mm in diameter. The
difference between two whole sizes is 1/10 of an inch in diameter
or 2.54 mm.

A table of ring size conversions between US, British, French,
German, Japanese and Swiss appears in my forthecoming book:
Professional Jewelry Repair - Rings, due out at the end of the

I hope that helps you. Let me know if you need more
-Alan in sunny Marin.