公制對一般人較易理解，10mm 等如 1cm; 100 等如 1m; 1000m 等如 1km, 小學的數學課已經教過, 但因為英、美兩國還是用英制，而且像套筒、螺絲批批頭接口等規格都是用英制，而且很多老一輩的師傅或五金零件行的店員也慣用英制, 所以學會看英制還是很有用的。

很多時到五金零件行買螺絲，除了有公制規格外，也有英制的 “分”，例如 1 分螺絲、2 分螺絲， 1 分大約等如 3.175mm；1 吋等如 2.54cm, 這個還可以很容量理解，但一些套筒或螺絲批批頭，會用分數表示，例如 1/8, 3/8, 1/4 等。

1/8, 這個分數的意思是 1/8 寸，即 1 分。那麼就是說 1 分等如 1/8 寸，如果是 2 分，會用 1/4 代表。即那個分數是多少寸的意思, 有點像小學數學課啊!

以下是英制尺寸常用換算表：

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