Rack Units (1U, 2U)
What a Rack Unit Is and How It Is Measured
A rack unit, written as 1U, is a standard measure of height for equipment mounted in a server rack. One unit is 1.75 inches (44.45 mm) high, so a device that occupies three units is described as 3U.
Racks are called 19-inch racks because the front panel of the equipment is 19 inches wide. A common full-height cabinet holds 42U. Sizing equipment in units means a device from any compliant manufacturer fits any compliant rack.
Common Server Sizes: 1U, 2U and Beyond
- 1U servers are compact and dense, but they have limited room for fans, drives and expansion cards, so they often run hotter.
- 2U servers offer more room for components and cooling, and are common for general-purpose work.
- 4U and larger units suit storage arrays or systems with many accelerator cards.
Other equipment, such as network switches, firewalls and power distribution units, also comes in U sizes. Because a rack's height is fixed, every unit one device uses is a unit that another device cannot.
How Rack Units Relate to Colocation Space
Colocation providers often sell space by rack unit, for example a set number of U in a shared rack. Larger customers may rent a quarter, half or full rack, which are blocks of space defined by the provider. Check the terms for depth and cable space as well as height.
Space is rarely the only limit. Power and weight often run out long before the racks are full, so a rack with many free units may still be unable to hold more high-power servers. Power limits are covered in rack power density.
Frequently asked questions
Is a 1U server always the best choice for colocation?
No. A 1U server saves space, which matters when space is charged per unit, but its tight design can limit expansion and cooling. Many organisations choose 2U servers for more room to grow and easier cooling. The right size depends on the workload, the power budget and the expansion you expect.
Do all server racks use the same size?
Most data center and office racks follow the 19-inch standard, so units of the same height fit together. Some specialised equipment, such as certain telecom gear, uses other widths or depths, and older or non-standard cabinets may not match. Measure the depth and mounting details of your equipment, not just its height, before buying a rack or renting space.