Multisizer ring sizer
According to the package, it's "World Famous". I don't know about all that, but this ring sizer is pretty neat. It's the easiest, most inexpensive way to make sure a ring you order from me will be the right size.
"The Multizer works like a belt. Simply push the end through the buckle to form a ring shape.
Slip the Multisizer onto your finger. Adjust to give a comfortable fit, checking that it just slips back over the knuckle.
When best fit is achieved, read off size indicated by the arrow! (Size 7 shown)"
My tips for sizing a stack of rings is also included.