To help with a visual reference, review a few jewelry designs in the Gallery of Designs and the materials lists. Notice the size relationships for findings such as jumprings and split rings to the overall design, the type of design as well as the scale between the findings and components, such as the beads and other findings like the clasp.
One specific example is to see what size jumpring or split ring is suggested for a lobster claw clasp. Findings are measured in millimeters, and jumprings and split rings will be measured for their outside diameter ("OD") or inside diameter ("ID").
For findings you may now have and wish to measure accurately, a millimeter gauge or digital caliper is a good tool. A digital caliper can exactly measure the OD of rings (and the inside diameters "ID" of rings) and any style wire.
Chainmaille patterns will call for specific sizes, especially in relationship to each other to make the pattern work. With a little practice in measuring with your eye or with a tool, you can judge what size is best for which application.
- Sandra Lupo, Jewelry Designer and Instructor and Swarovski crystal Ambassador