Rack space is vertically divided in "rack units", conventionally referred to as "U" (sometimes also as HU or RU).
Each U corresponds to 1.75 inches, or 44.45 mm
So, 2U correspond to 88.9 mm, 3U correspond to 133.35 mm, and so on
The equipment to be mounted in a rack normally have thickness and the front panel of slightly smaller dimensions, so as to allow a little tolerance.
