British Standard Wire Gauge is a set of wire sizes given by BS

British Standards

British Standards are the standards produced by BSI Group which is incorporated under a Royal Charter...

 3737:1964 (now withdrawn), and is generally abbreviated to SWG.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 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.

SWG was fixed by Order of Council

Order of Council

An Order of Council is a form of legislation in the United Kingdom. It is made by the Lords of the Privy Council ....

 August 23, 1883. It was constructed by improving the Birmingham Wire Gauge. It was made legal standard March 1, 1884 by the British Board of Trade

Board of Trade

The Board of Trade is a committee of the Privy Council of the United Kingdom, originating as a committee of inquiry in the 17th century and evolving gradually into a government department with a diverse range of functions...

.

The basis of the system is the mil, or . No. 7/0, the largest size, is . (500 mils or ) in diameter, and the smallest, No. 50, is . ( or about ) in diameter. Between each gauge, the weight diminishes by approximately 20%. Because the weight per unit length is related to the area, and therefore the square of the diameter, the diameter diminishes by approximately 10.6%:

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A table of wire gauges and diameters is shown below. The relationship of diameter to gauge is piecewise linear, only approximating a (constant-ratio) exponential curve.

British SWG (Standard Wire Gauge) 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