 Body piercing jewellery is measured by thickness and diameter / length . Most countries use millimeters . In the USA , the Brown & Sharpe AWG gauging system is used , which assigns lower numbers to thicker middles . 00 gauge is 9 @.@ 246 millimetres ( 0 @.@ 3640 in ) , while 20 gauge is 0 @.@ 813 millimetres ( 0 @.@ 0320 in ) . 
