A rack unit, written U, is the vertical pitch of the mounting holes in a 19 inch rack. One U is exactly 1.75 inches, which is 44.45 mm. Every piece of rack-mount equipment is built as a whole number of those, so a 1U switch is 44.45 mm tall and a 42U cabinet has room for 42 of them stacked.
Quick answer
- 1U is 44.45 mm, or 1.75 inches. Multiply by the U figure for usable mounting height.
- 42U gives 1,867 mm. A 12U wall box gives 534 mm.
- The 19 inch in the name is the equipment width, not the cabinet width or depth.
- Depth catches more people out than height. Measure your deepest device first.
What a rack unit actually measures
The U is a hole pitch, not a box size. Mounting holes run up both front rails in a repeating group of three, and one full group is one U. Because the pitch is fixed by the EIA-310 standard, a server from one maker bolts into a cabinet from another without anyone having to agree on anything else.
The 19 inch figure in the name is the width of the equipment front panel, which is 482.6 mm. It says nothing about the cabinet, which is typically 600 mm wide externally and can be 800 mm if it has cable channels down the sides. It says nothing about depth either, and depth is where sizing usually goes wrong.
Common server rack sizes
Cabinets cluster into a handful of familiar heights. Wall boxes are small and shallow, mid-height cabinets suit a comms room, and full-height cabinets are what a server room or data centre uses. The usable mounting height is just the U figure times 44.45 mm.
| Height | Usable mounting height | Typical use |
|---|---|---|
| 4U | 178 mm | Wall box for a switch and a patch panel |
| 6U | 267 mm | Small wall-mounted comms point |
| 12U | 534 mm | Office comms cabinet, AV rack |
| 18U | 800 mm | Branch office or edge cabinet |
| 24U | 1,067 mm | Small server room cabinet |
| 32U | 1,422 mm | Half-height cabinet for low ceilings |
| 42U | 1,867 mm | Standard full-height data centre cabinet |
| 47U | 2,089 mm | Tall cabinet where ceiling height allows |
Working out the height you need
Add up the equipment in U, then add everything people forget: patch panels, cable management bars, a shelf for anything that is not rack-mount, and a rack-mount power strip if it is horizontal. Then add spare capacity, because a cabinet filled to the last slot has nowhere for the next switch.
Buy the next size up from your figure. The price gap between a 22U and a 32U cabinet is small next to the cost of replacing one and moving live kit into it. Height is also the cheapest dimension to over-buy, unlike depth, which costs floor space.
Fit blanking panels in the empty U. Without them, hot exhaust air loops back through the gaps to the front of the cabinet and gets pulled straight back into the equipment. It is the cheapest cooling fix there is.
