If you're a ring maker, you need to know your customers' exact size before you make their ring! This is a great way to figure it out with them, and since these are low-cost you don't have to worry about sending them to your customers either.
The ring sizer is incredibly intuitive to use. It measures in half sizes up to size 17. To get your size, form a circle by wrapping the end through the hole. Place the sizer on the finger you wish to measure and tighten it by pulling on the end until you find a comfortable size that fits you. Be sure the sizer can be slid on and off your finger comfortably!