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 |
Related concepts
Section titled “Related concepts”- Rack Types — Different rack form factors
- Rack Configuration — Configure rack height in Rackula