Rack Unit Measurement

One rack unit measurement is equal to a height of 1.75 inches (44.45 mm). The rack unit is also known as a U or RU. The measurement is used to determine the height of equipment to be set on a 23 inch or 19 inch rack. 
More Details about Rack Units 
The dimensions specified in the racks denote the equipment width that can be placed in it. The dimensions of the equipment set on the rack are depicted with the number in U. For example, a piece of equipment that is two rack units high is measured as 2U. If it is three rack units high, it is called 3U and so on. 
Half Racks 
When discussing rack unit measurements, the term half rack will be encountered. This is used mainly with AV and computer equipment. With audio visual equipment, the half rack refers to units that fit a specific number of RU but only take up half the width of the standard rack (19 in). That is, they only take up 9.5 in or 241 mm inches. 
Application of Half Rack Units 
The half rack is frequently utilized when a piece of equipment does not need the full U but needs more than 1U. For instance, a 4U half-rack DVCAM deck takes up 4U (7 in) height x 9.5 in width. It means that a couple of 4U half-rack decks can be mounted and take up the space. 
But when it comes to IT and computer technology, half rack connotes a measurement 1U high. The depth is half that of the standard four post rack. This can refer to a server, KVM switch, router or a network switch. It means that two units can be set in a single U space. One will be at the back and the other on the front. 
Other Information about Rack Units 
The track filler panel or front panel is not a precise multiple of the RU. To permit space between nearby rack-mounted equipment, the panel is 1/32 inch (0.031 inch or 0.79 mm) shorter than the full U number would suggest. 
So a 1U front panel is 1.719 inches (43.66 mm) tall. Assuming N is the rack number unit, the formula would be h = (1.750n – 0.031) inch = (44.45n – 0.79) mm. 
The rack unit measurement is based on the EIA-3 standards. While there are many types of racks, the dimensions specified above are considered the standards. 

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