SWG wire gauge

A table of the gauge numbers and wire diameters is shown below.^[1][2] The basis of the system is the thou (or mil in US English), or 0.001 in. Sizes are specified as wire diameters, stated in thou and tenths of a thou (mils and tenths). The wire diameter diminishes with increasing size number. No. 7/0, the largest size, is 0.50 in. (500 thou or 12.7 mm) dia., No. 1 is 0.30 in. (300 thou), and the smallest, No. 50, is 0.001 in. (1 thou or 25.4 µm).

The system as a whole approximates an exponential curve, plotting diameter against gauge-number (each size is a approximately a constant multiple of the previous size). The weight per unit length diminishes by an average of approximately 20% at each step. Because the weight per unit length is related to the cross sectional area, and therefore to the square of the diameter, the diameter diminishes by approximately 10.6%:

Diameter Ratio=1−1−0.2≈10.6%

However, the system is piecewise linear, only approximating the exponential curve loosely. Thus, it runs in constant steps of 0.4 thou (0.4 mil) through the range No. 49 - No. 39 and of 0.8 thou (0.8 mil) through No. 39 - No. 30.

                                                                   British Standard Wire Gauge (SWG)

                                                                                      diameters