A rack unit (U, sometimes written RU) is the standard unit of height for equipment mounted in a server rack or colocation cabinet. One rack unit is 1.75 inches (44.45 millimeters). Servers, switches, storage arrays and other rack-mounted devices are described by how many units tall they are, so a “1U server” takes one unit of vertical space and a “4U storage array” takes four. Cabinets are described the same way, such as a 42U or 48U cabinet. The measurement comes from the widely used 19-inch rack standard, which also sets the mounting width.
At a glance
- 1U = 1.75 inches (44.45 mm) of vertical space; device heights are whole multiples (1U, 2U, 4U and so on).
- Measures height only; rack width is standardized separately and depth varies.
- Full colocation cabinets are commonly around 42 to 48U, depending on provider.
- Space counted in rack units is only half the sizing picture; power and cooling are often the real limit.
What problem it solves
Without a common unit, every server, switch and cabinet would need custom mounting and fitting. The rack unit lets equipment from different manufacturers share the same cabinet, lets buyers check quickly whether gear will fit, and gives colocation providers and buyers a shared way to describe space.
For buyers, it turns a pile of equipment into a number. Add up the rack units of everything you plan to install, allow for cable management, patch panels and growth, and you know roughly how many cabinets you need. Small deployments can also be priced by the rack unit, so a few devices do not require renting a whole cabinet.
How it works
The measurement. Standard 19-inch racks have vertical mounting rails with holes in a repeating pattern of three per rack unit. Equipment is usually built to whole multiples of 1.75 inches tall, with faceplates usually a little shorter so adjacent devices do not bind. Rack positions are often numbered on the rails so installers can record exactly where each device sits.
Width and depth. In the most common standard, 19 inches is the nominal width of equipment front panels and mounting ears; the spacing of the mounting holes and the clear opening between the rails are narrower. Some telecom environments also use 23-inch racks. Depth is not standardized, and long servers may need deep cabinets. Check both against your equipment.
Planning a cabinet. Usable rack units are reduced by items that share the space: patch panels, cable organizers, horizontal power distribution units, shelves and blanking panels used to keep cooling airflow right. Leaving empty units between hot devices without blanking panels can hurt cooling.
Space versus power. A cabinet may have plenty of empty rack units yet no remaining power or cooling allocation. Dense equipment such as GPU servers can use up a cabinet’s rack power density limit while filling only part of it, so plan in kilowatts as well as rack units.
Pricing. Colocation is commonly sold by the cabinet, partial cabinet or cage, and sometimes by the rack unit in a shared cabinet, with power priced separately or bundled. Bare metal and dedicated server offers may describe server sizes in rack units too.
For help sizing space for owned or rented servers, see our Colocation solution page.
When it matters for buyers
- Moving into colocation. Count the rack units of every device, then add room for cabling, PDUs and growth before choosing a cabinet size.
- Refreshing hardware. New models may be shorter or taller, deeper or more power-hungry than the ones they replace.
- Comparing small-footprint offers. Per-U pricing in a shared cabinet can be cheaper for a few devices but limits power, access and growth.
- Renting servers instead. With bare metal services, the provider handles rack space, so rack units matter mainly when comparing hardware options.
Questions to ask vendors
- How many usable rack units does the cabinet offer, after any provider-installed equipment?
- What are the cabinet’s internal depth and rail type, and will our longest devices fit?
- If priced per rack unit, what power, network ports and access come with each unit?
- Do you supply blanking panels and cable management, or should we bring our own?
- Can we add rack units or cabinets next to our existing space later?
How it differs from a colocation cabinet
A rack unit is a measurement; a colocation cabinet is the physical, lockable enclosure you rent. A cabinet is described by how many rack units it holds, along with its power allocation, depth and security features. Knowing your equipment’s total rack units tells you how much space you need; the cabinet and its power terms tell you whether that space will actually work.