公制对一般人较易理解，10mm 等如 1cm; 100 等如 1m; 1000m 等如 1km, 小学的数学课已经教过, 但因为英、美两国还是用英制，而且像套筒、螺丝批批头接口等规格都是用英制，而且很多老一辈的师傅或五金零件行的店员也惯用英制, 所以学会看英制还是很有用的。
很多时到五金零件行买螺丝，除了有公制规格外，也有英制的 “分”，例如 1 分螺丝、2 分螺丝， 1 分大约等如 3.175mm；1 吋等如 2.54cm, 这个还可以很容量理解，但一些套筒或螺丝批批头，会用分数表示，例如 1/8, 3/8, 1/4 等。
1/8, 这个分数的意思是 1/8 寸，即 1 分。那么就是说 1 分等如 1/8 寸，如果是 2 分，会用 1/4 代表。即那个分数是多少寸的意思, 有点像小学数学课啊!
以下是英制尺寸常用换算表：
1 尺 ＝ 12 吋 = 30.48cm
1 吋 ＝ 8 分 = 2.54cm
1 分 = 3.175mm
1/64″ = 0.3969mm
1/32″ = 0.7938mm
3/64″ = 1.1906mm
1/16″ = 1.5875mm = 半分
5/64″ = 1.9844mm
3/32″ = 2.3813mm
7/64″ = 2.7781mm
1/8″ = 3.175mm = 1 分
9/64″ = 3.5719mm
5/32″ = 3.9688mm
11/64″ = 4.3656mm
3/16″ = 4.7625mm ＝ 1 分半
13/64″ = 5.1594mm
7/32″ = 5.5563mm
15/64″ = 5.9531mm
1/4″ = 6.35mm = 2分
17/64″ = 6.7469mm
9/32″ = 7.1438mm
19/64″ = 7.5406mm
5/16″ = 7.9375mm ＝ 2 分半
21/64″ = 8.3344mm
11/32″ = 8.7313mm
23/64″ = 9.1281mm
3/8″ = 9.525mm = 3 分
25/64″ = 9.9219mm
13/32″ = 10.3188mm
27/64″ = 10.7156mm
7/16″ = 11.1125mm ＝ 3 分半
29/64″ = 11.5094mm
15/32″ = 11.9063mm
31/64″ = 12.3031mm
1/2″ = 12.7mm = 4分
33/64″ = 13.0969mm
17/32″ = 13.4938mm
35/64″ = 13.8906mm
9/16″ = 14.2875mm ＝ 4 分半
37/64″ = 14.6844mm
19/32″ = 15.0813mm
39/64″ = 15.4781mm
5/8″ = 15.875mm = 5 分
41/64″ = 16.2719mm
21/32″ = 16.6688mm
43/64″ = 17.0656mm
11/16″ = 17.4625mm ＝ 5 分半
45/64″ = 17.8594mm
23/32″ = 18.2563mm
47/64″ = 18.6531mm
3/4″ = 19.05mm = 6 分
49/64″ = 19.4469mm
25/32″ = 19.8438mm
51/64″ = 20.2406mm
13/16″ = 20.6375mm = 6 分半
53/64″ = 21.0344mm
27/32″ = 21.4313mm
55/64″ = 21.8281mm
7/8″ = 22.225mm = 7 分
57/64″ = 22.6219mm
29/32″ = 23.0188mm
59/64″ = 23.4156mm
15/16″ = 23.8125mm ＝ 7 分半
61/64″ = 24.2094mm
31/32″ = 24.6063mm
63/64″ = 25.0031mm
1” = 25.4mm = 8 分 = 1 吋