Rack Unit Calculator
Plan rack space in seconds: enter your device mix, pick a rack size, and see used units, free units, and the physical height: with U-to-inches and millimeters conversion built in.
Used
30U
Free
12U
Height (in)
52.50"
Height (mm)
1334
1U = 1.75 inches = 44.45 mm. Remember to reserve space for switches, PDUs, cable management, and blanking panels.
Convert Millimeters or Inches to Rack Units
Enter any height to convert between rack units, millimeters, and inches. Equipment must fit within whole rack units, so the whole-U figure is what actually matters when you check whether something mounts.
Rack units
44.99U
fits in 44U
Millimeters
2000.0
Inches
78.74"
Rack Unit to Millimeter and Inch Chart
Mounting height for common rack unit values. These are usable U heights, not a cabinet's external dimensions: a 42U or 45U enclosure stands taller once frame, castors, and roof panel are counted.
| Rack units | Millimeters | Inches |
|---|---|---|
| 1U | 44.45 mm | 1.75 in |
| 2U | 88.90 mm | 3.50 in |
| 3U | 133.35 mm | 5.25 in |
| 4U | 177.80 mm | 7.00 in |
| 5U | 222.25 mm | 8.75 in |
| 6U | 266.70 mm | 10.50 in |
| 8U | 355.60 mm | 14.00 in |
| 10U | 444.50 mm | 17.50 in |
| 12U | 533.40 mm | 21.00 in |
| 15U | 666.75 mm | 26.25 in |
| 18U | 800.10 mm | 31.50 in |
| 20U | 889.00 mm | 35.00 in |
| 24U | 1066.80 mm | 42.00 in |
| 27U | 1200.15 mm | 47.25 in |
| 30U | 1333.50 mm | 52.50 in |
| 36U | 1600.20 mm | 63.00 in |
| 42U | 1866.90 mm | 73.50 in |
| 45U | 2000.25 mm | 78.75 in |
| 47U | 2089.15 mm | 82.25 in |
| 48U | 2133.60 mm | 84.00 in |
Space Is Rarely What Fills First
A 42U rack has room for 42U of gear: but its power circuit and cooling envelope usually saturate long before the space does. Real capacity planning weighs all three, per rack, from measured data. That's the difference between a rack diagram and rack management.
Two companion tools cover the limits that bite first. The rack circuit capacity calculator turns volts, amps and phase into the usable kW a rack's feeds actually deliver under N, N+1 or 2N redundancy. The rack load calculator checks the loaded weight against your rack's UDL and point-load ratings, and against the floor underneath it.
Rack Unit Questions
How tall is 1U in inches and millimeters?
1U (one rack unit) is 1.75 inches, or 44.45 millimeters. A 2U server is 3.5 inches (88.9 mm), and a 4U chassis is 7 inches (177.8 mm).
How do I convert millimeters to rack units?
Divide the height in millimeters by 44.45. For example, 2500 mm ÷ 44.45 = 56.2U, and 44.196 mm is just under 1U. Round down to stay within the available mounting height.
How tall is a 45U rack in mm?
45U of mounting height is 2,000.25 mm (about 2 m), or 78.75 inches. A 45U cabinet's overall external height is greater once the frame, castors, and top panel are included: typically around 2.1 to 2.2 m.
How tall is a 42U rack?
42U of usable space is 73.5 inches (about 1.87 m) of mounting height. With frame, casters, and top panel, a typical 42U cabinet stands roughly 78–80 inches (2 m) tall overall.
How many servers fit in a 42U rack?
Up to 42 1U servers in pure space terms: but power and cooling usually run out first. With switches, PDUs, and cable management taking a few U, and per-rack power budgets, real-world racks often hold 20–35 servers.
When Free U-Space Is Not Free Capacity
U is the easy constraint. Sensaka DCOS records the verified U position of every device it discovers and puts it next to the cabinet's measured power draw and inlet temperature, so a rack with empty slots and no electrical or thermal headroom reads as full. That distinction between nominal and usable capacity is what decides where the next server actually goes. See data center capacity management.