On the website and in the catalog, for jumprings, you will find the outside diameter (OD) measurement along with the gauge of the jumpring. The gauge is the measurement of the thickness of the wire used to make the ring. The smaller the gauge number the thicker the wire, i.e. 26-gauge is fairly thin, and 18-gauge is very thick. (The inside diameter (ID) measurement of jumprings is more commonly used when making chainmaille jewelry.) When using jumprings to attach a clasp to a necklace or bracelet, it is good to use a size that is slightly larger than the ring on the clasp, and the thickest gauge that will still fit into the loop on the chain or beading wire that you are using to string with.
If you need to determine what size jumpring will complement your design as well as work with the clasp you have chosen, you can use the slide gauge to measure the loop on the toggle and then select a jumpring accordingly.