In the body piercing industry everyone measures the thickness of body jewelry using the American Wire Gauge system and it was introduced back in the 19th century as the electrical wire measuring standard.
The bigger is the gauge number, the smaller is the piercing diameter. The smallest body piercings have the body piercing gauge of 20, for example, the smallest nose rings, but standard piercings like navel rings have 14 gauge. Most of eyebrow rings are 16 gauge, so the body piercing gauge really depends on the body piercing type.
Oh yes, I didn’t include ear piercing into the previous paragraph when I said the smallest gauges are around 20. Typical ladies hanging earrings are dangling at the end of a small metallic hook, and its gauge is definitely very small! But such piercings usually aren’t considered ‘real’ body jewelry, and I think I wasn’t completely wrong with my statement above.
Here’s a body piercing gauge chart and you can see the real life body jewelry shaft size and the corresponding gauge size:
But if you need to find the exact sizing in the metric and US units as well, here’s a comprehensive chart starting from the smallest and ending with the largest body piercing gauges: