Jewelers have two different ways to identify the size of a person's ring finger. There is the ring sizer and the graduated ring mandrel. The purpose of both is to find the perfect ring size. A perfectly fitted ring is one that the person wearing it is completely unaware of. Figuring out the ideal ring size takes very little time and anybody can do it.
Take the steel metal ring that 30 individual rings are strung on and start trying on different size rings.
Look for a good fit—one where the ring slides over the knuckle of the finger with little resistance. Check for the feel of the ring once it's in place. Is it too loose, too tight, or just right? Fingers and hands do swell as the day wears on. It is better to pick a ring size that's a half a size too large than to pick a size the fits too snugly.
Take the steel ring off the ring finger and read what size the ring is on the top or side of it.
Select an existing ring that fits the ring finger perfectly.
Slip the ring onto the end of the cone shaped, graduated ring mandrel. Slide the ring all the way down the steel mandrel until it can't slide down any further. Don't force the ring beyond the point where it makes first full contact with the mandrel.
Read the size marked on the mandrel where the ring made first contact with it. This is the ring size.
If you own a ring sizer, check your ring size in the morning and then again in the evening to see how much your fingers swell. Take this into consideration when the final ring size is chosen.
If you are planning on purchasing a ring with a very thick band, you must increase the ring size because more skin is trapped under a wide band then a thin one.