[header] How to measure mm [title] Look at the unmarked lines on a metric ruler. [step] The numbered lines represent centimeters, but the unmarked lines represent millimeters. [substeps] If a metric ruler is too small, you could also use a meter stick. 
The small, unnumbered lines on a meter stick that fall in between the numbered lines should still be millimeters. Note that each 1 cm also equals 10 mm.