1U, 2U, 3U Rack Mount Heights – Just What Are “U” Talking About?

Blank Rack Mount Panels
Blank Rack Mount Panels

Just what does the “U” mean when talking about rack equipment?

As you can see from my Ham Radio console, it is setup using rack mount cabinets. I am using the older rack cabinets that use round holes instead of the newer ‘rectangle” holes. But, whichever style cabinet you are using, the equipment that mounts in there will be pretty much the same as far as the mounting sizes. Most all standard rack mount gear and even blank rack panels are 19 inches wide. There are some other “standard” widths out there, but most you will find, today, will mostly be the garden variety 19″ racks. This is especially true in today’s server racks and Ethernet hubs and switch racks.

What you will find is that equipment rack height specifications are often measured in U’s. Like 1U, 2U, 3U, etc. But without being in the know, the term “U” can be a mystery. Well, we’re about to solve that mystery right now if you’re ready.

First of all the “U” stands for the term “Unit.” In “Rack-speak” unit is the smallest size, in this case “height” since the width is fixed, that a piece of equipment or blank panel can be. The “U” usually is prefixed with a number like a 1 or a 2 and so on. So in “rack-speak” you can have a 1U, 2U, 3U, 4U, 5U, 6U, 7U and up to describe the height of the gear and how much precious rack space that equipment is going to consume in the rack.

Each “U” is a multiple of one and three quarter inches or 1 3/4″. That’s 1.75″ per “U”. So if you had a rack panel that was 3U, then that panel should be 3 X 1.75 inches or 5.25 inches. However, don’t go running out to get your yardstick and prove me wrong because you will find that manufacturers cut the panel a bit short (about 0.03″ per U) to allow for tolerances. So, taking these tolerances into account, that same panel that should be 5.25 inches tall may be actually only 5.16 inches high. { 5.25″ – (3 x 0.03″) = 5.16″ }

This is actually very good as the result allows for air to move in and out of the front of the cabinet and keeps you from cussing so much when one manufacturer cuts the panel a little differently than another and the panels don’t get all hung up on each other when you are trying to mount them. This is especially true if you are trying to mount some heavy equipment all by yourself. (Trust me on that one.)

Another thing you might notice is that the equipment manufacturer will often make the actual chassis of their equipment a tiny bit shorter to account for any top or bottom panels for their gear. This also helps prevent the equipment hanging on other gear as you are mounting or dismounting the equipment. Also, most manufacturers will use countersunk screws so they mount flush with the bottom or top panels.