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What does 1U, 2U, 3U Mean? Rack Units Explained

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? What does 1U mean?

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?

So as we move along answering the question of “What does 1U mean?” let’s move on to who came up with this stuff in the first place! 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.

So now we have hopefully answered the question “What does 1U mean?” It’s a very important data center and server closet terminology.

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4 Responses

  1. Mike, as one who apprenticed in the broadcast industry, I can tell you that standard equipment racks got their start in the early 1900’s by the railroads for their relay boxes and signaling equipment and were standardized by 1911. This is why they were called “Relay Racks”. By 1922 the standard was used by AT&T and the rest of the telecom industry (as you stated), and again used for relays and switching equipment, thus reinforcing their being called relay racks. By 1934 it was the standard for broadcast equipment and recording and movie studios, and later in the 1930’s most all military equipment and avionics used the standard 19″ rack. By the 1940’s the ENIAC computer was mounted in 19″ racks. IBM & Sperry mounted their computers and data equipment in the 1940’s and 1950’s in 23″ racks to accommodate larger tape drives and disk packs.

  2. Thanks for explaining this.
    Under the heading “Who came up with rack units” change “equipment used my” to “equipment used by”

  3. Thanks for this very useful information. Just a picky point though, the headings for Horizontal spacing and Vertical spacing appear to have been transposed.

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