# Standard wire gauge

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British Standard Wire Gauge is a set of wire sizes given by BS 3737:1964 (now withdrawn), and is generally abbreviated to SWG. 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 the more usual size measurement for wires used in electrical installation cables. The current British Standard for metallic materials such as wire and sheet is BS 6722:1986, which is a solely metric standard.

A standard wire gauge.

SWG was fixed by Order of Council August 23, 1883. It was constructed by improving the Birmingham Wire Gauge. It was made a legal standard on March 1, 1884 by the British Board of Trade. SWG is not to be confused with American Wire Gauge which has a similar but not interchangeable numbering scheme.

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 about 25 µm).

The system as a whole approximates a (constant-ratio) exponential curve. 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%:

${\displaystyle {\mbox{Diameter Ratio}}=1-{\sqrt {1-0.2}}\approx 10.6\%}$

But 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 0.8 thou (0.8 mil) through No. 39 - No. 30.

British Standard Wire Gauge (SWG) diameters
SWG (in) (mm) Step
7/0 0.500 12.700 0.036"/gauge
6/0 0.464 11.786 0.032"/gauge
5/0 0.432 10.973
4/0 0.400 10.160 0.028"/gauge
3/0 0.372 9.449 0.024"/gauge
2/0 0.348 8.839
0 0.324 8.230
1 0.300 7.620
2 0.276 7.010
3 0.252 6.401 0.020"/gauge
4 0.232 5.893
5 0.212 5.385
6 0.192 4.877 0.016"/gauge
7 0.176 4.470
8 0.160 4.064
9 0.144 3.658
10 0.128 3.251 0.012"/gauge
11 0.116 2.946
12 0.104 2.642
13 0.092 2.337
14 0.080 2.032 0.008"/gauge
15 0.072 1.829
16 0.064 1.626
17 0.056 1.422
18 0.048 1.219
19 0.040 1.016 0.004"/gauge
20 0.036 0.914
21 0.032 0.813
22 0.028 0.711
23 0.024 0.610 0.002"/gauge
24 0.022 0.559
25 0.020 0.5080
26 0.018 0.4572 0.0016"/gauge
27 0.0164 0.4166
28 0.0148 0.3759 0.0012"/gauge
29 0.0136 0.3454
30 0.0124 0.3150 0.0008"/gauge
31 0.0116 0.2946
32 0.0108 0.2743
33 0.0100 0.2540
34 0.0092 0.2337
35 0.0084 0.2134
36 0.0076 0.1930
37 0.0068 0.1727
38 0.0060 0.1524
39 0.0052 0.1321 0.0004"/gauge
40 0.0048 0.1219
41 0.0044 0.1118
42 0.004 0.1016
43 0.0036 0.0914
44 0.0032 0.0813
45 0.0028 0.0711
46 0.0024 0.0610
47 0.0020 0.0508
48 0.0016 0.0406
49 0.0012 0.0305 0.0002"/gauge
50 0.0010 0.0254