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Aspect Ratio
The concept of aspect ratio is central to chain maille. Different weaves require rings of different aspect ratios in order for the weave to hold together. Some weaves allow for a wide range of aspect ratios, while other weaves require precise aspect ratios in order to maintain their structure.
The aspect ratio is a measure of a particular ring. It's the ratio of the ring's inner diameter divided by the diameter of the ring's wire. The diameter of the ring's wire can be determined by that ring's material and its gauge. For example, a 1/8" inner diameter ring of 20-gauge (0.032" diameter) stainless steel would give us an aspect ratio of:
1/8" ÷ 0.032" = 0.125" ÷ 0.032" = 3.906
This metric describes how many straight wires of the same gauge could fit through that single ring. So in this example, almost 4 other 20-gauge stainless steel wires could fit through the 1/8" diameter jump ring.
There are a few important things to keep in mind about the aspect ratio. First, it's independent of the ring's material. If two rings that have the same inner diameter and wire diameter, they'll have the same aspect ratio even if one is gold and the other is aluminum. Second, make sure that the units match in both the numerator and denominator. In the example above, both units are in inches. However, some sellers describe their jump rings in millimeters. In that case you need to make sure that the wire diameter is also given in millimeters or you're going to end up with a very odd (and very wrong) aspect ratio. Third, the gauge of a wire changes depending on the material that the wire is made from. For instance, a 20-gauge stainless steel ring will typically have a different wire diameter than a 20-gauge sterling silver ring because the gauges used to measure base metals are different from those used to measure precious metals. Finally, make sure that the numerator represents in the inner diameter and not the outer diameter of the ring. If all you know is the outer diameter and the wire diameter, then you can calculate the inner diameter as:
Inner Diameter = Outer Diameter - (2 * Wire Diameter)