Rack Unit
Also known as: U · RU · HE
A rack unit is the standardized measurement used for equipment height in a 19-inch server rack, where one rack unit equals 44.45 millimeters.
A rack unit, abbreviated U or RU, is the measurement used to specify the height of equipment in a 19-inch server rack; one rack unit equals 44.45 millimeters (1.75 inches). It exists so that server racks from different manufacturers stay compatible: a 42U rack offers a defined 42 mounting slots for 1U devices, or fewer slots for taller ones.
In practice, every rack-mountable device is labeled with the number of rack units it occupies, such as 1U for a simple switch or 2U to 4U for a server with several drives. Planning a rack is basic addition: a 42U rack holding a 4U server, two 1U switches, and a 2U UPS still leaves 35U free for future additions.
A common mistake is sizing a rack exactly for current needs without leaving room for future equipment. The next expansion then forces either a rebuild or a second rack, when a bit more spare capacity from the start would have been cheaper.
What it means in practice
For a business, it pays off to plan for more rack units than currently needed when buying a server rack, so a new switch or server doesn't immediately run into a lack of space.
Is this handled properly at your site?
We look at how it actually stands with you — and say honestly whether anything needs doing.
Related terms
All termsA term from your quote missing here?
Send us the passage you do not follow. We will explain it — with no obligation to order anything.