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Understanding server rack size

42U. 18U. 24U. Have you ever come across these numbers, and wondered – exactly how big is this server rack that I’m buying? Will my hardware fit, and will it fit in my server room? Let us help you understand.

Where did ‘rack units’ come from?

The term ‘rack unit’ dates back to at least 1992, when the Electronic Industries Alliance (EIA) published a ‘standards’ document titled EIA-310.

1 rack unit, or 1U (sometimes written as 1RU) measures 1.75″ in height. No matter the width and depth of your server rack, a 42U server rack will measure:

42 × 1.75 = 73.5 inches

This is the internal height of the rack. You can select the right size depending on the number of servers and other hardware that you need to accommodate in your rack, and their dimensions.

1U is defined as the height of three consecutive holes built into the rack, to which the hardware can be secured. The holes could be round or square or threaded – the size of each hole, and the gap between them, is standardized across companies by the rack unit.

The rack unit was created to simplify the user experience. There was a risk that each major player would create a proprietary size requirement for their hardware, demanding brand-specific racks and locking consumers into their own ecosystem. With cross-industry standardization, users find it easier to plan for future expansion without being locked down to a single brand.

Selecting the right rack size

Servers usually come with a notation specifying height in U or RU (rack unit), but other hardware such as patch panels, switches and routers don’t always mention height in U. This is because these hardware do not always use the rack’s holes to be fastened to the rack, and usually are simply placed on a shelf (and not all shelves are standardized in terms of size). However, it’s easy enough to convert the height from inches to U and determine the required internal height of your server rack.

Account for some space for potential expansion. If you currently require 38U to fit all your hardware, consider opting for a 42U rack. Also remember that the unit must fit the room you’re putting it in! Confirm that it will pass through the door of the room, and that the space will accommodate the rack, before making your purchase.

Do note that the width and depth of the server rack are NOT specified in the rack unit. Make your choice in terms of width and depth based on the specifications of the hardware that you’re purchasing.

At Hardy Racks, we offer high-quality data center racks ranging from 18U to 42U. Do check out our product range here.

If you’re still unsure or need any help understanding server rack sizes, rack units and how to select the right rack size, do reach out to us. Our team would be happy to help.


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