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Rack Units (U)

A rack unit (abbreviated U or RU) is the standard unit of measurement for the height of equipment designed to mount in a 19-inch server rack.

What is a rack unit?

Section titled “What is a rack unit?”

One rack unit equals 1.75 inches (44.45mm) in height. This standard was established by the Electronic Industries Alliance (EIA-310) and is used globally.

Units Height (inches) Height (mm) Height (cm)
1U 1.75“ 44.45mm 4.45cm
2U 3.5“ 88.9mm 8.89cm
3U 5.25“ 133.35mm 13.34cm
4U 7“ 177.8mm 17.78cm

Common equipment sizes

Section titled “Common equipment sizes”
Equipment type Typical size Notes
1U server 1U Most common server form factor
Standard switch 1U Network switches, patch panels
2U server 2U More drive bays, better cooling
4U server 4U High-performance, storage-heavy
UPS 2U–6U Varies by capacity
PDU 0U–2U Often mounted vertically (0U)

U position numbering

Section titled “U position numbering”

Rack positions are numbered sequentially, with each number representing one U of space.

Bottom-up numbering (most common)

Section titled “Bottom-up numbering (most common)”

The most widely used convention numbers from the bottom:

┌─────────────┐
│ U42         │ ← Top of rack
│ U41         │
│ ...         │
│ U2          │
│ U1          │ ← Bottom of rack
└─────────────┘

Advantages:

  • U1 is always at floor level
  • Consistent reference point regardless of rack height
  • Heavy equipment naturally placed at the bottom

Top-down numbering

Section titled “Top-down numbering”

Some data centres number from the top:

┌─────────────┐
│ U1          │ ← Top of rack
│ U2          │
│ ...         │
│ U41         │
│ U42         │ ← Bottom of rack
└─────────────┘

Device placement

Section titled “Device placement”

When placing a multi-U device, the starting position is the lowest U it occupies.

For example, a 2U server at position U10:

  • Occupies U10 and U11
  • U10 is the “mounting position”
  • U11 is covered by the device
┌─────────────┐
│ U12         │ (empty)
│ U11         │ ┐
│ U10         │ ┘ 2U Server (position: U10)
│ U9          │ (empty)
└─────────────┘

Half-rack units

Section titled “Half-rack units”

Some equipment uses half-rack heights:

Size Height Examples
0.5U 0.875“ (22.2mm) Thin switches, KVM
1.5U 2.625“ (66.7mm) Some Dell servers

Rackula supports half-U devices. Set the device height to 0.5, 1.5, etc.

Rack height standards

Section titled “Rack height standards”

Common full-rack heights:

Height Use case
42U Standard data centre rack
45U Extended height rack
48U Tall data centre rack
25U Half-height rack
22U Audio/video rack
12U Wall-mount rack
6U Desktop/portable rack
4U Mini rack
Section titled “Related concepts”