What size server rack do you need?
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Add up the U your equipment takes, then round up to a rack height. Count more than what bolts to the rails. The rails are the two vertical strips that equipment fixes to. A shelf takes U of its own, and a desktop NAS standing on it blocks the U above it. The Synology DS925+ is 6.5 inches (166 mm) tall and blocks 4U. Add the U you expect to add later.
A UPS, a PDU, a switch, a patch panel and a desktop NAS on a shelf take 10U. That is 6U on the rails and 4U above the shelf. A 12U rack holds them, with 2U free.
What takes U
A rack unit, or U, measures height in a rack. A rack's size is its height in U. So is each item's.
Equipment on the rails takes its own height. The CyberPower CP1500PFCRM2U UPS takes 2U. A rack server counts the same way: the Dell PowerEdge R240 takes 1U. The listing or the spec sheet gives the figure.
A shelf takes U too. The StarTech 1U cantilever shelf, 16in deep takes 1U on the rails.
What stands on a shelf blocks the U above it. Round the item's height up to whole U. The DS925+ is 6.5 inches (166 mm) tall, so it blocks 4U. Nothing else can go in those U while it stands there. Gearome's How it works page states the rule the planner applies.
Some equipment takes no U. A vertical PDU such as the StarTech 24-outlet vertical PDU fixes to the rack's frame, beside the rails. It takes no U, but this one needs a rack of 42U or taller.
Adding up one rack's equipment
| Item | On the rails | Blocked above the shelf |
|---|---|---|
| UPS, CyberPower CP1500PFCRM2U | 2U | |
| PDU, CyberPower CPS1215RM | 1U | |
| Shelf, StarTech 1U cantilever shelf, 16in deep | 1U | |
| NAS on the shelf, Synology DS925+ | 4U | |
| Switch, UniFi USW-Pro-24-PoE | 1U | |
| Patch panel, StarTech 24-port Cat6 | 1U | |
| This equipment | 6U | 4U |
- Add up what goes on the rails. Five items, 6U between them.
- Add what the shelf's load blocks. The NAS blocks 4U above the shelf.
- Compare the total with the rack. The equipment takes 10U. The cabinet is 12U, so it fits with 2U free.
Counting only what bolts to the rails gives 6U. A rack that height would not hold this equipment.
- Synology DS925+, 4-Bay NAS166 millimeters tall
The Gearome planner does this sum for your own equipment, blocked U included.
Server rack heights in our catalog
Our catalog holds 19-inch racks only, so height is the size you choose. The table lists each height we hold five or more racks at: 4U, 6U, 8U, 9U, 12U, 15U, 18U and 42U.
A rack that can go on a wall or the floor counts in both columns. Our catalog has 19 more racks at heights in between, out of 96 in all.
Round your total up to a height in the table, or browse every rack and filter by height. 5 of our 5 42U racks stand on the floor.
How much room to leave in a server rack
This equipment leaves 2U free in the 12U cabinet. StarTech sells the same cabinet design as a 15U, where it leaves 5U free.
| 12U | 15U | |
|---|---|---|
| Free U | 2U | 5U |
| Outside height | 25.1 inches (637 mm) | 30.3 inches (770 mm) |
| Its own weight | 63.7 lb (28.9 kg) | 87.3 lb (39.6 kg) |
| Weight capacity | 198.4 lb (90 kg) | 198.4 lb (90 kg) |
With too little room, the next item does not fit. A second DS925+ on its own shelf needs 1U for the StarTech 1U cantilever shelf, 10in deep and 4U above it. The 12U has 2U free. The 15U takes it, with 0U left.
More room makes the rack taller and heavier. The 15U is 30.3 inches (770 mm) tall, against 25.1 inches (637 mm). It weighs 87.3 lb (39.6 kg), against 63.7 lb (28.9 kg). The wall carries that as well as the equipment.
The table shows the same weight capacity for both. The 15U holds more U and no more weight. Our guide to a rack's weight capacity covers what counts against it.
Count the items you plan to add. If you can name the next one, leave its U free. If you cannot, the 12U is the shorter and lighter of the two on the wall. On a floor rack, the weight goes to the floor instead.
A free run of U
An item needs its U in one unbroken run, one above the other. In the 12U cabinet, this equipment leaves 2U empty together at the top, so an item of that height fits there. Gaps in different places do not add up. A 3U item cannot use two empty U at the top and one at the bottom. Keep the empty U together, where the next item will go.
Height is one part of fit
A rack that is tall enough can still be too shallow. Fit has three parts: rack units, depth and mounting. Your deepest equipment has to fit the rack's mountable depth, which our guide to server rack depth explains. Mounting is whether the rack's rails, frame or a shelf can hold each item.
The shelf, the PDU and the patch panel in this equipment are items a rack order can need. Our guide to server rack accessories goes through them.
The Gearome planner checks all three parts of fit for your own rack, along with weight and power. It tells you when an item has no room left, or when what stands on a shelf runs past the top of the rack. Equipment that is not in our catalog goes in as a custom item, with the height you measured.