Brian Cryer's Web Resources


Rack unit. A standard measurement for the height of rack mounted equipment.

The capacity of server racks and data cabinets are normally measured in "U". A standard rack has an internal usable rack width of 19 inches, the usable height is typically expressed in rack units ('U'). Where one rack unit ('U') is 1.75inches or 44.45mm.

A given rack might be said to have a usable height or capacity of 30U. Each item that can be rack mounted will also have a height expressed in U. So a 30U cabinet could house 30x1U items, 15x2U items etc or a combination thereof.

An rack mountable item which is said to be a half-rack unit will only occupy half the width of the rack.

Rack units are sometimes written as RU, but this not common.

A typical tower server or desktop PC would be 4U (or perhaps 5U), whereas a server specifically design to be rack-mountable might be a small as 1U.

For more information see: