Home Technology 1U, 2U, or 4U? The Dummies Guide to Server Rack Sizes

1U, 2U, or 4U? The Dummies Guide to Server Rack Sizes

15 min read
1U, 2U, or 4U? The Dummies Guide to Server Rack Sizes

Let’s talk racks.

Not this kind.

Or this kind.

We are talking the sexy kind of rack. Your data server rack! If you’re new to data centers or IT-talk, you’ve probably heard “1U server” or “42U rack”, however, the meaning may be a little fuzzy.

So let’s start with the basics and work our way through together. A rack is simply an open-sided, standard size storage unit meant to securely hold computer servers and other telecommunications or electrical relay equipment.

The face of the unit has a vertical lip on each side to allow equipment attachment. Server rack sizes differ slightly by manufacturer, but the width is fixed at roughly 19″.

There’s more– keep reading.

What Is a “U”?

When you talk about server rack sizes, the key dimension is height. The “U” stands for a unit. This indicates the distance between mounting holes in a server rack. Visually, this is a horizontal “slice” of a tall, narrow cabinet.

A standard rack unit is 1.75″.  When you hear of equipment that is rack-mount, sub-rack, rack-mountable, or shelf-mount, you need to know the number of “U”. Most standard cabinets are 42U. However, it isn’t uncommon to find racks as tall as 60U.

What Is Server Rack Size?

Rack depth varies by manufacturer. A 1U chassis may be as shallow as 17.8″ or as deep as 26″ or more. The standard is 36″, but chassis vary widely.

Equipment manufacturers tend to make their equipment front panel height slightly smaller than the allotted “U”. A 1U rackmount server, for example, is  1.719″. Likewise, a 2U or 4U server is also smaller than the allotted space by about 1/32 inch.

This allowance gives a smidgen of wiggle room around an installation to allow for easy removal without interference from the immediate equipment.

If a piece of equipment is taller than 1.75″, it is identified in multiples of rack units. Many ISPs rent their rack space in 1U, 2U, or 5U. These options aren’t the only ones available.

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Colocation providers also offer half and whole rack rentals. A half rack is 20U or 24U, a full rack is 42U.

What Kind of Fasteners Go in a Server Rack?

A server rack is meant to hold equipment in self-contained modules. Square hole racks allow mounting without additional screws or bolts. The equipment simply inserts and slips over the lip of the square. The gravity and some small clips keep it in place.

Government and military installations use tapped hole racks. They use a specialized threaded bolt. Now, threaded hole racks are in use for older, rarely changed equipment. Frequent equipment changes can damage threads and render the mounting hole unusable.

Clearance hole racks use a cage nut to clip into an open mounting hole. If the nut is stripped out, the bolt is replaced.

How Is Heavy Equipment Mounted?

Most equipment front panels bolt or clip to the mounting holes. Structural support for the equipment rests on the front edge. Heavier equipment needs a pair of mounting strips on the back edge. Equipment depth varies.

Typical spacing between the front and rear mounting strip is 31.5″.  Although the most common standard is 36″ of rack depth, 39.4″ is now common.  Additional space at the back allows for cable routing.

For strength, mounting strips should not be flat strips. A wider (2mm) folded steel strip around the corners of the rack is preferred. Aluminum strips should be thicker still.

Rails, Shelves, and Drawers

For frequently serviced equipment or heavy equipment that poses a lifting hazard, attachment to four rails poses a problem. It is inefficient to dismount all four corners, work on the floor and then re-mount the unit.

A pair of sliding rails installed directly onto the rack, with the equipment attached to the rails is the answer. The equipment then slides along the supporting rails. When in place, the equipment bolts or locks in place.

The best type of rail pair supports the equipment clear of the rack at full extension.  This allows technical examination and regular maintenance. Specs for slides or mounting rails should be confirmed by the equipment producer. There is no standardization for equipment height and width including rails, nor for the rail mounting itself.

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Shelves provide solid support for equipment. They may be fixed or sliding. For safety, do not allow equipment to simply rest on shelves. Attach equipment to the shelf and attach the shelf to the rack front and back.

Drawers (with slides) or bins (without slides) attach to the rack and allow for handy organization and storage of cables and tools. Equipment does not go in drawers or bins.

Earthquake and Worker Safety

Secure racks to the floor or the building structure (not drywall). This is usually part of the building code in areas with seismic activity. Two-post racks have an uneven weight distribution and are particularly unwieldy.

Securing the racks also protects workers as they move equipment and move among the racks. According to the Bureau of Labor Statistics, nearly 50,000 injuries and 250 fatalities of workers result from falling objects.

A four-walled cabinet or other locking device adds stability to sensitive equipment and protects it from shaking loose during an earthquake.

Data Cages and Suites

These colocation arrangements allow for groups of racks and cabinets. A cage is a metal wire enclosure (chain-link fence is usual) for cooling airflow efficiency. A private cage holds only the servers and IT equipment of one company. Cage space is usually rented by square feet, not U.

Suites are secure and fully-enclosed. They have solid walls (walls can be glass) and may have their own independent climate control and fire suppression.  Some suites allow printers and office space as well as server rack colocation.

Data Server Talk De-Mystified

Both cages and suites accommodate a variety of server rack sizes. Now that you have worked your way through some basic vocabulary, you can check out our other articles right here on our blog about technology and data.

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