Rack Unit (U) Space Planner
Count the rack units your kit needs, including the gaps you forgot.
What you provide
Full-height racks are usually 42U or 47U. Wall cabinets are 6U to 18U.
Result
PASS24U free in a 42U rack
Height is the easy constraint. The two that actually stop installations are depth and weight: short-depth wall cabinets will not take a full-length server no matter how many U are free, and a rack filled with disk shelves can pass what the floor is rated for. Check the door-to-door depth and the loaded weight before ordering.
- Rack height
- 42U
- Equipment
- 16U
- Airflow and cable gaps
- 2U
- Total occupied
- 18U
- Free
- 24U
- Filled
- 43%
What this cannot tell you
- Height arithmetic on what you enter. It does not know your equipment list or the rack in front of you.
- Height is rarely the constraint that stops an installation. Depth, weight and cooling are, and none of them are calculated here.
Take this with you
How this calculation works
One rack unit is 44.45 mm, and every piece of rack-mount equipment is built as a whole number of them. The calculator totals the height of what you are installing, adds the blanking and cable management gaps that get left off equipment lists, and subtracts the lot from the cabinet height. Those gaps are not padding: a rack with open gaps and no blanking panels lets hot exhaust air circulate straight back to the intakes, which raises inlet temperatures across everything above it.
What the results mean
- Equipment
- The rack units your actual hardware occupies, before anything is left for air or cables.
- Airflow and cable gaps
- Space set aside for cable management bars and blanking panels. Leaving it out is how a rack that fits on paper runs hot in practice.
- Free
- Unused units left over. Below about 15% of the cabinet, plan the next purchase around a second rack rather than squeezing.
Common problems and fixes
- The equipment fits on paper but not in the cabinet
- Measure depth, and measure it door to door rather than post to post. Wall cabinets are commonly 450 to 600 mm deep and a full-length server needs 750 to 1000 mm once cable bend radius at the back is allowed for. Rear cable managers, PDUs and door hardware all eat into the usable depth.
- The rack is full but the room is still hot
- Check the empty units first. Every unfilled gap without a blanking panel is a path for exhaust air to loop back to the front of the rack, and that recirculation raises inlet temperatures more than most people expect. Fit blanking panels in every gap, keep the hot and cold sides of the rack consistent, and make sure nothing is drawing air from behind another machine.
Frequently asked questions
How tall is 1U in millimetres?
44.45 mm, or 1.75 inches. The measurement is a standard from the days of relay racks and it has not changed, which is why nineteen-inch equipment from very different decades still bolts into the same holes.
How much weight can a rack hold?
It depends on the rack, and the number that matters is often the floor rather than the frame. Server racks are commonly rated between 500 and 1500 kg static, but a cabinet full of disk shelves can approach the point loading limit of a raised floor. Check both ratings before filling a rack from the bottom up with heavy equipment.
Should I leave gaps between servers for cooling?
No. Modern rack equipment draws air front to back and stacking units directly on top of each other is what the design expects. Gaps without blanking panels actively hurt by letting hot air recirculate. Leave space for cable management, and fill everything else you are not using with blanks.
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