As you might imagine ring diameter is something that I think about often. Here at UnkamenSupplies we measure our rings by their inner diameter ( ID). Elsewhere rings are sometimes measured by their outside diameter ( OD).
I prefer measuring by ID because of the greater precision it allows me. When you buy a 3 mm ring from me, you buy it with the knowledge that it will have a 3 mm inner diameter. My calipers are as much an extension of my body as a teenager’s cell phone. I take pride in knowing that our rings are some of the most precisely cut and sized rings on the market.