Rackmount U Measurements - main image

This article defines and explains rack unit (U) measurements used in data centers. It covers the basics of U measurements, their standardization, and how they are applied to rack-mounted equipment.

Key Takeaways:

  • A “U” is a unit of measurement for vertical space in server racks.
  • 1U = 1.75 inches or 4.445 cm.
  • Rack-mounted devices are sized in U’s, with 1U devices requiring one unit of space, 2U devices requiring two units, and so on.

Q: What is a rack unit (U)?

A: A rack unit (U) is a unit of measurement for the vertical space available in a server rack or cabinet. It is standardized to 1.75 inches, allowing for consistent sizing of rack-mounted equipment.

When looking at any piece of rack-mounted server equipment, such as network switches, server racks, rackmount pdus, etc. you would have come across a descriptive term such as ‘1u’.

This refers to a unit of measurement of the space within a server rack or server cabinet. Rack-mountable hardware is manufactured according to the number of total ‘U’s, or rack units, that the equipment will require in the data rack/cabinet.

The IT industry’s use of ‘U’ as a unit of measure to designate racks and rack-mounted equipment stems from the Electronic Industries Alliance (EIA) standard unit for specifying rack height. It denotes the vertical space available for a rack-mounted device. Racks, cabinet spaces, and as a result the devices that fit in them are all measured in the space units, hence ‘U’. The size of a rack mountable device will often be specified in a numerical + ‘U’ value. A device that requires one-rack unit of space is designated as ‘1U’, while a device requiring two rack unit spaces will be designated ‘2U’, and so on.

How ‘U’ units are worked out is by the space between the shelves of a server rack that is available for the installation of a device. So ‘1U’ actually corresponds to a height of 1.75 inches or 4.445 cm.

Also, in some cases, the measurement of vertical rack space may be referred to as an ‘RU’, which simply means rack unit.

Server technician at server - sub image


‘U’ is a unit of measurement for the standardised amount of designated vertical space that either a rack itself contains, or that a rackmount devices will require. For example, a 30U server rack will provide 30 units (or rack units) of space, while a 3U device will occupy three units of designated rack chassis space.

If you have any questions, our sales team is more than happy to explain how exact dimensions should be considered when buying racks.


Related Products Available from Comms Express:

Data & Server Racks | Power Distribution Units (PDU) | NETGEAR Switches | Patch Panels


If you require any further information on these or any other products that we stock here at Comms Express, please do not hesitate to contact our team who will be only too happy to help.

Until next time…