Wondering what that RU means on the side of the server box? Trying to figure out if you have enough space for the IT department? This primer on rack units will help you understand what a rack unit is and what you need to know about dimensions, capabilities and how to use rack units to your advantage.

1U spacers in a cabinet
Letters, Numbers, or Both?

1U measurements
When you see the word “rack unit,” two things probably come to mind: a metal shelving unit and units of measurement. You’re on the right track, because both of these are correct. While a rack unit (RU or simply U) is technically the term for a unit of measurement, it only pertains to the height of the space (and the equipment) on a shelving rack designed for electronic equipment.
The term is a standard measurement that comes from the Electronic Industries Alliance. It’s used on a variety of rack mount electronic equipment used for audio/visual, computing and information technology purposes. Servers and network switches are the most common devices measured in RUs.
The rack unit is the height of the equipment that will fit on a rack, 1.75 inches (44.45 mm) high to be exact. So to determine how high your equipment is, or how much space it will take up, in RUs, use basic algebra:
• 1U = 1.75 inches
• 2U = 3.5 inches
• 3U = 5.25 inches
And on and on. Why do you need to know this? That’s where the racks themselves come in. In order to plan your usage of space, you need to know how many rack units you need. Or if you already have racks in use and you’re expanding, you need to know how many rack units you have left on your existing racks.
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