How to size a cabinet correctly, the first time
An undersized cabinet is one of the most common and expensive mistakes in a network installation. It forces a return trip, a second purchase, or worse, equipment crammed in with no airflow and cabling under strain. This guide gives you the actual method, not a guess, for working out the right cabinet size before you order.
Quick answer
- 1 RU equals 44.45mm (1.75 inches) of vertical rack space.
- Add up the RU height of every piece of equipment you are installing to get your minimum total.
- Size up from that total, do not size to it exactly. Keep planned fill to around 70 to 80 percent of the cabinet's rated capacity to leave room for airflow, cabling, and future growth.
- Check load rating separately from RU count. A cabinet can have plenty of spare RU space and still be at its weight limit, the two are not the same constraint.
What a rack unit actually is
A rack unit, RU or U, is the standard measure of vertical space in a 19-inch rack cabinet. One RU equals 44.45mm, or 1.75 inches. Every piece of rack-mountable equipment, switches, patch panels, UPS units, servers, is manufactured in multiples of this measurement, which is what allows equipment from different manufacturers to fit predictably into the same cabinet.
| RU | Height (mm) | Height (inches) |
|---|---|---|
| 1RU | 44.45mm | 1.75" |
| 2RU | 88.9mm | 3.5" |
| 4RU | 177.8mm | 7" |
| 9RU | 400mm | 15.75" |
| 18RU | 800mm | 31.5" |
| 42RU | 1867mm | 73.5" |
The 19-inch measurement itself refers to the width between the mounting rails, not the overall external width of the cabinet. This is a common point of confusion, a cabinet's outer width, typically 600mm or 800mm, is wider than the 19-inch usable rail space, since the extra width houses cable management and airflow space either side of the mounted equipment.
Working out your minimum RU requirement
List every piece of equipment going into the cabinet along with its RU height, then add them together. For example:
- Switch, 1RU
- Patch panel, 1RU
- Small UPS, 2RU
- Cable management panel, 1RU
That totals 5RU as your minimum requirement. This is the number most guides stop at, but it is only the starting point, not the answer.
Why you should never size to the exact total
Sizing a cabinet to fit exactly the equipment going in on day one is the single most common planning mistake. Using the 5RU example above, a 6RU cabinet technically fits, but leaves no room for a second switch, a larger UPS, or any additional patch capacity added later. As a working rule, keep your planned fill to roughly 70 to 80 percent of the cabinet's total RU capacity. Applied to the example above, that points to a 9RU cabinet rather than 6RU, giving genuine headroom rather than a cabinet that is full the day it is installed.
This spare capacity also matters for airflow. Equipment packed with no gaps traps heat, particularly around UPS units and switches under load. Leaving a small gap, or fitting a blanking panel into unused RU space, helps maintain proper airflow direction through the cabinet rather than letting air leak through open gaps in an uncontrolled way.
RU count and load rating are two separate constraints
A cabinet with plenty of spare RU space is not automatically within its weight limit, and this is where a second sizing mistake commonly happens. RU count tells you if equipment physically fits, load rating tells you if the cabinet and its mounting can safely bear the weight.
| Cabinet type | RU range | Load rating |
|---|---|---|
| Indoor wall mount | 4RU to 18RU | 90kg |
| Indoor swing frame wall mount | 6RU to 18RU | 60kg |
| Indoor floor standing | 22RU to 45RU | 500kg |
| Outdoor IP55, wall or floor | 6RU to 27RU | 300kg |
A small UPS alone can weigh 15 to 25kg, and a fully populated 18RU wall mount cabinet with switches, patch panels, and a UPS can approach the 90kg indoor wall mount rating well before the RU space itself is full. Always check both numbers separately, never assume spare RU space means spare load capacity.
Depth planning alongside height
RU planning covers vertical space, but depth needs the same attention. Measure the deepest single piece of equipment you are installing, including any rear-mounted power module or cabling that extends past the equipment's own body, then add clearance for bend radius and airflow. Refer to our wall mount vs floor standing guide for indoor depth options, or the outdoor IP55 cabinet guide for exterior installations.
Common sizing mistakes
- Sizing to the exact RU total with no spare capacity. Leaves no room for growth and packs equipment too tightly for good airflow.
- Confusing RU space with load capacity. A cabinet can have spare RU height and still be at or near its weight limit.
- Forgetting to include cable management panels in the RU total. These take up real rack space and are frequently left out of the initial calculation.
- Leaving unused RU space unblanked. Open gaps disrupt directed airflow, a blanking panel maintains it properly.
- Measuring equipment depth without accounting for rear cabling clearance. The equipment's stated depth is rarely the true clearance required.
Frequently asked questions
How much spare RU space should I actually leave?
Around 20 to 30 percent of total capacity is a reasonable working target for most commercial installations, more if the site is expected to scale significantly.
Do half-populated cabinets need blanking panels?
Yes, wherever there are gaps in the mounted equipment. Blanking panels maintain proper front-to-back airflow direction rather than letting air bypass equipment through open spaces.
Is the 19-inch rack width the same as the cabinet's external width?
No, 19 inches refers to the mounting rail spacing only. The external cabinet width, typically 600mm or 800mm, is wider to accommodate cable management either side of the mounted rails.
Can I mix RU sizes from different manufacturers in one cabinet?
Yes, RU is a standardised measurement under the EIA-310 standard, so correctly rated equipment from any manufacturer will mount to the same rail spacing regardless of brand.
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