Standard wire gauge

British Standard Wire Gauge is a set of wire sizes given by BS 3737:1964, and is generally abbreviated to SWG. Use of SWG sizes has fallen greatly in popularity, but is still in use. Cross sectional area in square millimetres is now a more popular size measurement.

Also known as: Imperial wire gauge, British Standard Gauge.

SWG was fixed by Order of Council August 23 1883. It was constructed by improving the Birmingham Wire Gauge. Made legal standard March 1 1884 by the British Board of Trade.

The basis of the system is the mil, or 0.001in. No. 7/0, the largest size, is 0.50 in. (500 mils or 12.7 mm) in diameter, and the smallest, No. 50, is 0.001 in. (1 mil or about 25 µm) in diameter. Between each gauge, the weight diminishes by 20%. Because the weight per unit length is related to the area, and therefore the square of the diameter, the diameter diminishes by 10.557%:


 * $$\mbox{Diameter Ratio} = 1-(1-0.2)^{\frac{1}{2}} \approx 10.557%$$

A table of wire gauges and diameters is shown below