One rack unit is commonly designated as "1U"; similarly, 2 rack units are "2U" and so on. The size of a piece of rack mounted equipment is usually described as a number in "U". One rack unit is also sometimes referred to as "1RU".
Professional audio and video gear often comes with rack mount options and use the same measurement specifications.
Half-rack units typically describe units that fit in a certain number of RU, but occupy only half the width of the rack (9.5 inches / 241 mm). For example, a "4U half-rack" DVCAM deck would occupy 4U space (4 × 1.75 inches) height × 9.5 inches width. In theory, two half-rack decks could occupy the 4U space.
Coincidentally, a rack unit is equal to a vershok, which is an obsolete Russian length unit.
As in 1U, 2U, 4U etc
A "Rack Unit" or "U" is an Electronic Industries Alliance or more commonly "EIA" standard measuring unit for rack mount type equipment. This term has become more prevalent in recent times due to the proliferation of rack mount products showing up in a wide range of commercial, industrial and military markets. A "Rack Unit" is equal to 1.75" in height. To calculate the internal useable space of a rack enclosure you would simply multiply the total amount of Rack Units by 1.75". For example, a 44U rack enclosure would have 77" of internal usable space (44 x 1.75).
Stealth manufactures computers and peripherals that are designed to fit into a standard EIA size rack enclosures. Stealth's Rackmount PCs, LCD Monitors and Keyboards are available in many sizes and configurations. The slim space saving series rack products are available in 1U (1.75") and 2U (3.5") in overall height. Since rack space is at a premium these slim products represent significant cost savings to the end user. Standard rackmount products are available in 1U, 2U, 4U, 5U and 6U configurations.