The unit to measure the thickness of the steel (or) sheet metal is called as the gauge. Most used steel sheet metals have gauge values between 30 and 7. In this conversion calculator, you can convert gauge to inches and millimeters from the known gauge number.
If the Birmingham wire gauge has a gauge number of 8, the diameter of the wire will be 0.165 inches and 4.191 mm. For a US standard gauge, it would be 0.1719 inches and 4.37 mm.