There are 2 ways to measure a jump ring. The first is by it’s ID or Inside Diameter. This measures the internal space from the inside of the jump ring to its direct opposite inside. The second is OD or Outside Diameter. This measures the outside space that a jump ring will take up from the outside of the jump ring to the direct opposite outside. Here is a diagram that shows this.
The ID and OD are usually measured in mm but they are also be measured in inches or fraction of an inch such as 5/8″ or 3/16″. I measure all of my handmade rings by ID and all of my commercially made rings by OD. Here is another diagram:
If you purchase jump rings that just give a number and don’t give an ID or OD, they are almost always OD.