The height of 19” rack mountable products is commonly measured in ‘U’ (Units) or ‘RU’ (Rack Units).
Conversion between U, inches and millimetres is as follows:
1U = 1.75” = 44.45mm
More information can be found here:
http://en.wikipedia.org/wiki/Rack_unit
Online rack unit calculator:
https://www.pennelcomonline.com/en/Rack-Unit-Calculator/cc-118.aspx