The original gauge was a measure of how many dies the wire passed through to reach the correct measurement. There were a total of 44 sizes, beginning with 0000, 000, 00, and followed by a numbered system 0-40. The larger the gauge number, the smaller the wire.
Wires are measured by passing them through a measuring wheel. The wheel has notches for the wire to pass through, labeled with the gauge number. Most wheels give the inch-decimal equivalent on the rear side of the wheel. Using a wheel which gives both measurements is the simplest way to convert the wire gauge to decimal; however, charts which give the inch-decimal conversion are also available online.
The inch-decimal measure can be converted to millimeters by dividing the inch-decimal measurement by 0.03937.