Key gauge to imperial/metric conversion Gauge Inches (decimal) Millimeters (mm) 1 0.300 7.60 2 0.276 7.01 2 1/2 0.264 6.71 3 0.252 6.40 3 1/2 0.242 6.15 4 0.232 5.89 4 1/2 0.222 5.64 5 0.212 5.38 5 1/2 0.202 5.13 6 0.192 4.88 6 1/2 0.184 4.67 7 0.176 4.47 7 1/2 0.168 4.27 8 0.160 4.06 9 0.144 3.66 10 0.128 3.25 11 0.116 2.95 12 0.102 2.62
RST stem lengths are measured from the bottom of the key head to the very end of the stem.