If you’ve spent any time in data centers or around IT people, you’ve probably heard talk of something being a “2U server” or a “1U switch”. In my most recent video I built a really cool 1U server. I was a little surprised by how many people were not aware of what this “1U” thing actually meant, and quickly followed the questions of “What does 1U mean?” So in this article we’re going to talk about Rack Units!

What is a Rack Unit?

A “U” or rack unit is a measurement of the height of a piece of computer or network gear that is designed to fit into a standard 19″ or 23″ rack.  A single rack unit is exactly 1.75″ (44.45 MM) in height, although many manufacturers will make their equipment slightly smaller than this to ease installation into racks where things were not spaced as accurately as they should be.

Some other gear such as stage and music gear (pro audio and video units, like mixers and amplifiers) are also designed to fit within standard rack units.

When a piece of equipment is designed to be larger than a single rack unit (1U), it will be referred to as a multiple of rack units and written such s 2U, or 6U. This would mean an 6U server is 10.5″ or 266.7 mm in height.

Sometimes people refer to rack units as RU (ex: 1RU, 2RU, 6RU), but this has mostly been phased out over time in favor of the simpler “U”.

Rack Units on 19″ Racks

Rack Units on 19″ Racks

On most modern racks, the rack units will be visibly labeled with numbers and lines (generally white on black racks; black on beige racks). These lines indicate where the top and bottom of the rack unit starts.  the number is generally in the center of the rack unit.

Older racks without silk-screened numbers and lines are usually denoted by a small hole or dimple drilled  into the rack every 1.75″.

The most common racks contain 42U worth of rack space, although 48U racks are quickly becoming the new standard. In addition to standing floor racks, racks are available in wall mounts, such as the wall mount 8U rack I installed in my home theater room and vertical (hanging) racks like I used to mount my Synology RS2416+ in the same closet. You’ll want to consult your equipment vendor before mounting gear vertically, as not all equipment is designed to work in vertical racks.

Who came up with Rack Units?

Rack Units were created by the Electronic Industries Alliance (“EIA”) to help standardize equipment used my telecommunications carriers.  The specification is known as EIA-310. Although rack units got a birth in telecom, they are found literally everywhere today due to the proliferation of server and network equipment in businesses and even homes (commonly in theater room gear and dedicated network closets).

Rack Hole Spacing for Rack Unit Mounts

EIA-310 also sets the specifications for the hole spacing, both vertically and horizontally of rack units within racks and cabinets.

Horizontal Spacing

Horizontal spacing of rack units depend on whether or not the holes are drilled and tapped in the rack, versus the use of cage nuts.

Vertical Spacing

Vertical spacing of rack units is defined by EIA-310 as 18 5/16″ or 465.1mm.

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