Diameter
From BME Encyclopedia
Body jewelry in a ring configuration is always measured by inner diameter. In smaller gauges, this doesn't make a whole lot of difference, but when you consider a 2ga ring, the inner diameter may be 3/4", but because the metal itself is so thick, the outer diameter of the ring could possibly be over an inch and a quarter; keep this in mind when ordering jewelry -- it is not uncommon for jewelry companies to get angry calls from disgruntled customers who fail to consider the difference between inner and outer diameter.
It is important that ring diameter be chosen in a "piercing-appropriate" fashion -- for example, a common mistake for inexperienced piercers is to use rings with too small a diameter for nipple piercings (for example, 3/8" rings in a female nipple), failing to account for normal swelling during healing, and erection/contraction related to temperature, leading to rejection.
