An easy way to remember metric length is that one inch equals 2.54 centimeters,
or 25.4 millimeters-
so a half inch is ROUGHLY 12 mm (actually 12.7 mm)
a quarter inch is about 6mm
and each 1/8" is about 3 mm
so if something is 30 mm long, it is 5 mm longer than an inch, or roughly another 1/4" -total 1 1/4"
something 40 mm long is 15 mm greater than an inch or another 1/2" plus 1/8", or 5/8"- total 1 5/8"