Many people use imperial sizes for their jump rings instead of millimeter. If that is what you are used to, I've made up a handy ruler/chart to see what inch size corresponds to a millimeter diameter. For example, if you are used to using a 5/32 diameter ring, then you would want to choose a 4.0mm size. Please note that my rings are in the American Wire Gauge (AWG) system and factor that into your choice. Aluminum, steel, and some other metals usually use Standard Wire Gauge (SWG) system which may have a different wire diameter than AWG.