The Diameter is a type of measurement used for circles (or other round objects). The diameter measurement is used in the piercing industry to determine the size of a ring used in piercings, to ensure that the correct size is worn.
There are two type of jewellery Diameter measurement.
The inner diameter is used when measuring jewellery such as Ball Closure Rings, Circular Barbells and Body Spirals. This is the width of the inside of the ring at it's widest point.
The outer diameter is used to measure jewellery such as flesh tunnels, plugs and top hats. This relates to the thickness of your piercing.