Standard wire gauge

British Standard Wire Gauge (SWG) is a set of wire sizes given by British_Standards 3737:1964 (now withdrawn). It is also known as: Imperial Wire Gauge or British Standard Gauge. Use of SWG sizes has fallen greatly in popularity, but is still used as a measure of thickness in guitar strings and some electrical wire. Cross sectional area in square millimetres is now a more popular size measurement. The current British Standard for metallic materials such as wire and sheet is BS 6722:1986, which is a solely metric standard. American wire gauge (AWG) is a similar but not quite equivalent numbering scheme.

The basis of the system is the thou or mil in US English which is 0.001 inch and is different to the millimetre (mm). No. 7/0, the largest size, is 0.50 inch, 500 thou or 12.7 mm in diameter, and the smallest, No. 50, is 0.001 inch, 1 thou or about 25 µm in diameter.