丝经SWG（Wire Gage Swg）gage=gauge 是铁丝粗细的单位。标号越大，铁丝直径越小。规号与长度单位的换算表如下：
Gauge 英制标准 美国标准
(S.W.G.) (A.W.G.)
7/0 .500" ---
6/0 .464" .5800"
5/0 .432" .5165"
4/0 .400" .4600"
3/0 .372" .4096"
2/0 .348" .3648"
1/0 .324" .3249"
1 .300" .2893"
2 .276" .2576"
3 .252" .2294"
4 .232" .2043"
5 .212" .1819"
6 .192" .1620"
7 .176" .1442"
8 .160" .1284"
9 .144" .1144"
10 .128" .1018"
11 .116" .0907"
12 .104" .0808"
13 .092" .0719"
14 .080" .0640"
15 .072" .0570"
16 .064" .0508"
17 .056" .0452"
18 .048" .0403"
19 .040" .0358"
20 .036" .0319"
21 .032" .0284"
22 .028" .0253"
23 .024" .0225"
24 .022" .0201"
25 .020" .0179"
26 .018" .0159"
27 .0164" .0141"
28 .0148" .0126"
29 .0136" .0112"
30 .0124" .0100"
31 .0116" .0089"
32 .0108" .0079"
33 .0100" .0070"
34 .0092" .0063"
35 .0084" .0056"
36 .0076" .0050"
37 .0068" .0044"
38 .0060" .0039"
39 .0052" .0035"
40 .0048" .0031"
41 .0044" .00280"
42 .0040" .00249"
43 .0036" .00222"
44 .0032" .00198"
45 .0028" .00176"
46 .0024" .00157"
47 .0020" .00140"
48 .0016" .00124"
49 .0012" .00111"
50 .0010" .00099"
铁丝的号数与直径,是按照英国标准线来说的 。号码越大，铁丝直径就越细 。比如 ，8号铁丝 ，直径4.064毫米；12号铁丝，直径2.642毫米；20号铁丝 ，直径0.914毫米 ，等等。1毫米铁丝最接近0.914，所以应该是20号铁丝 。