The size of a piercing, at least up to a particular size, is generally referred to by "gauge" just like the gauges defined by the
AWG system used to measure electrical conductors. As the diameter of a piercing and the jewellery increase the gauge decrease, thus an 8 gauge piercing is larger than an 18 gauge piercing. Most sources seem to indicate the sizes of piercing gauges correspond directly the the AWG sizes, though the
Wikipedia entry on stretching disagrees for some sizes. The
BMEzine Wiki has a fantastic
entry on sizes.
Here's a table of gauge sizes and their corresponding diameters (approximately) in inches and mm based on the AWG system. Sizes larger than listed are generally just specified by their diameter instead of continuation of the gauge system.
| Gauge | inches | mm |
| 18 | 0.04 | 1.0 |
| 16 ("typical" earring) | 0.05 | 1.3 |
| 14 | 0.06 | 1.6 |
| 12 | 0.08 | 2.1 |
| 10 | 0.10 | 2.6 |
| 8 | 0.13 | 3.3 |
| 6 | 0.16 | 4.1 |
| 4 | 0.20 | 5.2 |
| 2 | 0.26 | 6.5 |
| 0 | 0.32 | 8.3 |
| 00 | 0.36 | 9.3 |
