42U rack: the units you pay for and will never be able to fill
On the quote, a rack is 42U. In the room, once the cabling is in, 32 to 36 are often left for your servers. The difference does not come from your machines. It comes from the cabinet, and first from its width: when nothing can run down along the rails, all the cabling comes back to the front, into accessories that occupy slots you pay for every month. Here is where those lost units come from, what they cost, and the questions that reveal them before signature.
Estimate my cost →What a U measures, and what “42U” leaves unsaid #
The U is the height unit of equipment: 44.45 mm, or 1.75 inch. The EIA/ECA-310 standard sets that value and the 19-inch spacing (482.6 mm) between the mounting rails. A 42U rack therefore offers 1,867 mm of mounting height. The standard stops there. It says nothing about the cabinet's width, nothing about its depth, and nothing about what will occupy those 42U before your servers do. “42U” describes a mounting height, not a hosting capacity.
600 or 800 mm: the width decides the rest #
The 19-inch rails take the same width in every cabinet. What changes is the room left around them. At 600 mm, two to three centimetres remain on each side: enough for a few patch cords, not for a bundle or a duct. All the cabling then comes back to the front. At 800 mm, about fifteen centimetres per side suffice for cables and for vertical power strips, which occupy no slot at all. Operators often choose 600 mm: it fits more racks in a row. The arithmetic is theirs, the loss is yours, and the quote says nothing about it.
| 600 mm cabinet | 800 mm cabinet | |
|---|---|---|
| Free space on each side of the rails | 2 to 3 cm | about 15 cm |
| Vertical cable runs | not as a bundle | along the rails |
| Power strips | horizontal, 1U each | vertical, 0U |
| Patching and cable managers | at the front, on your units | moved to the sides |
| Units typically left for servers | 32 | 37 |
A “0U” strip needs clearance, not only width #
A power strip described as “0U” does not mount on the 19-inch rails. It goes beside them, or at the back of the cabinet, so it needs free space, whether that comes from width or from depth. A 600 mm cabinet 1,200 mm deep can take one, provided the rails and servers leave the rear clearance and the strip does not end up in the cable path. This is verified cabinet by cabinet, drawing in hand. Do not assume it: that is precisely the question to ask.
Where the units go: the inventory of what is not a server #
- Horizontal cable managers: 1U each. Usually one per patch panel or per switch, as soon as nothing can run down the sides.
- Horizontal power strips, the PDUs that distribute power inside the rack: 1U each, and you need two for the A and B feeds.
- Patch panels, where the network cords land: 1U for 24 ports, 2U for 48. Needed in every case, and each one calls for its cable manager when the sides are closed.
- Brush panel for cable entry: 1U, top or bottom, when the cabinet has no dedicated passage.
- The operator's demarcation panel, often a fibre tray: 1 to 2U the data center keeps at the top of the rack to deliver your cross-connects. Sometimes without mentioning it.
- Switches, console, shelf: those are yours, but they count too. On a small estate, the network and its accessories take up to a third of the slots.
The count, on a real rack #
Take a 42U rack in a 600 mm cabinet with no side passage, cabled by the book: two switches, two patch panels, two cable managers, two horizontal power strips, one brush panel and one console. Each takes 1U, so 10U before the first server. Thirty-two remain. The same rack at 800 mm, with vertical strips and side ducts, sacrifices only the switches, the patching and the console: 5U, and 37U for your machines. Five slots apart, for the same rent and the same figure on the quote. In a quarter rack the effect is harsher: of 10U rented, a patch panel and a cable manager take 2, a fifth of the lease.
What it costs, in euros #
A rack is paid monthly, whatever its usable height. With 32U of servers out of 42 rented, each unit costs you a third more than the apparent per-U price. The calculation rests on the assumption stated just above, ten slots of passive equipment: redo it with your own inventory, it is one division. The real bill comes later. When the estate grows, space overflows before power does, and the second rack is rented at the price of a full rack. That is what the Digital Architect, our sizing tool, has counted from the start: its requirement sheets reserve the space for network, patching and cable managers, and keep only 33 usable units out of 42 for a standard estate, 38 for GPU chassis where the network weighs less. If an operator sells you 42 usable units, one of you is wrong, and the rack is not the one that will grow.
Is it a scam? #
The word is strong, the feeling is legitimate. Selling “42U” is not a lie: the height is exact, it is standardised, it can be measured. What is missing is the rest of the sheet: the cabinet's width, its depth, the slots the operator keeps, the format of the strips supplied. Two 42U offers can mean 32 usable units on one side and 37 on the other, and nothing on the quote lets you tell them apart. The problem is not the figure, it is the silence around it: you are sold a height, you are not described a volume. No contract writes “usable U”. Until the question is asked, it does not exist.
The questions that reveal the lost units before signature #
- What cabinet width, 600 or 800 mm? What depth, 1,000 or 1,200 mm? Cable-management arms and vertical strips live in the depth.
- Can cables run down along the rails without consuming a slot? Are there vertical ducts, or room to fit some?
- Are power strips supplied, and in which format: vertical, so 0U, or horizontal, so 1U each?
- How many slots does the operator keep for its demarcation and cross-connects, and at which position in the rack?
- Where do cables enter: top, bottom, a dedicated passage, or a brush panel that costs 1U?
- Does a 47U or 48U cabinet fit on the same floor footprint? That is the most direct way to recover what cabling takes.
- In a quarter or half rack, are the patch panel and cable manager counted inside my units, or outside?
Recovering units when the rack is already there #
If the cabinet is narrow and changing it is not an option, four moves limit the loss. Mount the switches at the rear, ports facing the servers: patching stops crossing the rack and the front cable manager becomes unnecessary. Order patch cords at the right length rather than two-metre cords to coil. Consolidate patching on 48-port panels, which halves the associated cable managers. Finally, check whether the depth allows a vertical strip, as explained above. And keep the blanking panels: they cost no slot and stop hot air from returning to the front. That is another subject, but the same money.
Comparing racks on an identical basis #
Two 42U offers only become comparable once brought back to usable units, the way power comes back to the kilowatt and connectivity to the cross-connect. Our guide on comparing offers on an identical basis gives the method, and the one on rack pricing lists the costs missing from the first line of the quote. One admission that binds us: the DataColoc comparator does not know cabinet widths site by site. The data is not in our listings, and we prefer to say so rather than assume it. Ask the question, write the answer next to the price, and the comparison becomes honest again.
FAQ #
Between 32 and 38 depending on the cabinet and the estate. In a 600 mm cabinet with no side passage, cabling by the book (switches, patching, cable managers, horizontal strips, brush panel, console) takes about ten units. In an 800 mm cabinet with vertical strips, the loss drops to about 5U.
Because the 19-inch rails leave barely two centimetres on each side: cables cannot run down along the rails and power strips cannot mount vertically. Everything comes back to the front into cable managers and horizontal strips, each taking a 1U slot.
A vertical strip fixed in the side space or at the back of the cabinet, occupying no 19-inch slot. It needs room beside the rails, so in practice an 800 mm cabinet or sufficient depth.
Yes, often 1 to 2U at the top of the rack, for the panel where it delivers your cross-connects. It is common and legitimate, but it must be written in the offer, with the position and the number of slots.
The cabinet's width and depth, whether cables can run down the sides, the format of the supplied power strips, the units the operator keeps, the cable entry, and the availability of a 47U or 48U cabinet on the same floor footprint.
Written on 12 September 2026.
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