So you need some ring terminals or copper lugs to complete your project. You find the type you are looking for, then realize they are available in a huge variety of dimensions. In addition to finding one that utlizes the gauge of wire you have, you must determine what size the hole needs to be in the ring.
Ring terminals often utilize the US Custom Stud Size format for the smaller rings before switching to a more conventional fractional inch system. So how is a person to know if they need a #8 or #10 stud size ring terminal if they don't know what size the stud is? Or for that matter 1/4" or 3/16"?
First, be aware that the dimension listed means the ring terminal can work for that corresponding stud or bolt. For instance if you are attaching the terminal to a 1/4" stud, you use a 1/4" terminal. The opening of a ring terminal is sightly oversized so it will fit easily over the corresponding bolt.
So if you don't know what the stud is, you'll have to measure. A set of calipers is ideal for this task and they can be had for under $20. Calipers are one of those tools many people do without, but once you have a set, you'll find you use it more frequently than you imagined.
If you are in doubt which ring size to get, go for the larger option. Although it is best to use a terminal that fits rather snug over the stud, a slightly oversized one will usually work just fine.
Here is a chart showing the actual ring terminal sizing:
