Aspect Ratio is versatile tool to used to to help discuss and compare rings and to figure out what ring will work for what weave. The abreviation for aspect ratio is AR. Basically AR tells you how fat or loose the ring is. Some weaves require loose rings (high AR) and some require tight (fat) rings (low AR).
AR is the ratio of the inside diameter (ID) divided by the wire diameter AR=ID/wiredia
Once you are familiar with ring sizes you may wish to alter the ring size that a pattern calls for to use a larger or smaller ring. Aspect ratio is the easy way to do this.
Have a look at the following picture to illustrate some things about AR
First starting at the left you can see the first 2 rings are both 14g - the same wire thickness. The larger ring has an AR of 6.4 and is considered a loose ring. The smaller ring has an AR of 3.2 and is considered a tighter ring.
The middle 2 ring sizes actually have the same inside diameter. Both have a 1/4 inch ID. Notice how the AR's are different.
The last 2 ring sizes have the same AR. If you had a weave you were used to making with 18g 1/4" and you wanted to make it using a fatter ring, the 16g 5/16" in the same AR and could be used
Let do a sample calculation of AR for your bag of tricks.
Lets look at the first ring on this chart SXAB1212 12ga 1/2" ID
The AR is already calculated and shown here as 5.7
Remember AR = ID/wiredia so in this case the ID is listed as actual ID of 0.54" or 13.72mm and the wire diameter is listed as 0.094" or 2.4mm. You can use etiher mm or inches in your calculation as AR is a ratio - you just have to use the same ones ... inches OR mm.
AR = 0.54"/0.094" = 5.74
AR = 13.72mm/2.4mm = 5.72
In AR the 3rd digit is not significant - so this ring as you can see has an AR = 5.7
Another way to understand aspect ration is to think if I took a picture of a ring, and there was nothing in the picture for scale, the only size information you could guess is AR. I could take a picture of an 18g 1/4" ring and a 16g 5/16" ring in separate images and you would not be able to tell me which was 16g and which 18g.
Most weaves do not require the ring size to be precise! If you have a weave that you determine needs and AR of 5.3 but the ring on our site is AR 5.5 in most cases this is perfectly acceptable and it will work well. If you are deciding between 2 rings, you have to decide will tighter be ok or would looser be better? This depends on the weave and you. Tighter can be used but it depends on your experience with the weave - beginners tend to have more trouble closing rings in tight weaves. You can get a ring too tight. Most weaves work well enough in a range of about +/-0.3 on the AR. You can confirm by looking at the weaves samples page and seeing the recommended sizes. There are links at the top of that page as well for more resources on weaves and AR.