This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
Most jewelry manufactured in the US is gauged according to the Brown &
Sharpe system. Most jewelry from the UK and Europe is manufactured by
metric gauge.
A visual representation of gauges and diameters is available at
http://www.cf.ac.uk/uwcc/psych/stevensonwc/bodyart/gauge.html
Brown and Sharpe Gauges (used by most American manufacturers)
-------------------------- gauge inches millimeters --------------------------- 20 0.032 0.812 18 0.040 1.024 16 0.051 1.291 14 0.064 1.628 12 0.081 2.053 10 0.102 2.588 8 0.128 3.264 6 0.162 4.115 4 0.204 5.189 2 0.258 6.544 0 0.325 8.251 00 0.365 9.266
 
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