This is a genuine question, not a riddle. Okay, so recently I bought a ring online, and normally, I wear a size 5. Well, they were out of that size, so I purchased a 5.5. In looking at a ring size chart online, a size 5 ring is 15.6 mm but a 5.5 is 16.0 Just by eyeballing the two, it looks like it should be fine, but my question is how do you figure out the inside diameter of a ring? (Edited, because I sorted out my original two-part question)
I'm not talking about the width of the ring, the thickness. The only different between the OD and the ID is the ring thickness.
Isn't the sizing based on the inside diameter? That's the bit that needs to fit on the finger, after all. The outside diameter is only relevant if you want to know whether wearing the thing is going to significantly impede your non-ring fingers.
Here's the formula for the inner diameter, $d$, (in mm), for a given (US) ring size, $s$: $d=0.8128s+11.63$ Checking, that gives a diameter of 15.7 mm for size 5, and 16.1 mm for size 5.5.
