Understanding Rack Mount System Sizing

The sizing of rack mount systems is defined by the term "U" or "Rack Unit (RU)". This unit has been defined by Electronic Industries Alliance (EIA) as the standard for sizing rack mount equipment. 

1 Rack Unit or 1U is 1.75 inches (44.45 mm) high.

Systems which are 2U are 2 x 1.75 inches i.e 3.5 inches high and so on. Some companies also use 1RU instead of just using 1U to represent the height of a rack mount system.

These heights are maximum dimensions. Manufacturers usually make every 1U height rack-mount system with a mechanical tolerance of 0.03 inches to provide some room for the passing of air. So the effective height of a 2U rack-mount is: 3.5 - (2 x 0.03) = 3.44 inches.

The width of most rack mount systems is 19". In some cases, these are 23". So a 19" 1U rack mount device would be 19 inches wide and 1.75 inches high.