Understanding wire gauge will make purchasing and working with jewelry wire easier. Wire diameter is measured in gauges. There are two different wire gauge systems that are used; AWG (American Wire Gauge) and SWG (Standard Wire Gauge). Canada and Great Britain both use SWG primarily while The USA and The Rest-of-the-World use AWG primarily. Unkamen uses AWG. This chart provides a comparison of gauge thickness in both inches and millimeters. Note that the larger the gauge, the thinner the wire (28 gauge is much thinner than 14 gauge). For a point of reference, 20 gauge is the "standard" ear wire size. The most common shapes of wire are round, half round and square. I make jump rings from both round and square wire. Half Round wire is commonly used for wire wrapping in the thinner gauges and in bangle making in the thicker gauges.
I make jump rings in 10-24 gauge, with 18 gauge being the most popular. I sell wire in 4-34 gauges and I not only carry round, half round and square, but twisted square, twisted round and rectangle! I hope this chart helps you, I know a lot of the sizes off the top of my head, but I still refer to this exact chart daily.