Server Rack U Size: What Does a Rack Unit Mean?

U is the vertical unit of the 19-inch rack system, written as rack unit or RU. The standard behind it is EIA-310, with EIA/ECA-310-E the revision in common use today. The international equivalent is IEC 60297, known as the 19 in (482.6 mm) series. The unit dates to AT&T around 1922, when telephone central offices needed one height for repeater and termination gear so equipment from different shops would stack in the same frame.

A U figure is a maximum, not an exact size. A device counts as 1U when its front panel is no taller than 1.75 in. Panels are built slightly under nominal, about 1/32 in (0.79 mm) under, to leave clearance between stacked units. That is why a device sold as 1U often measures around 1.72 in (43.7 mm) on a spec sheet, and why a 1U server can come in at 1.69 in.

The vertical unit is the same in 19-inch and 10-inch racks. Only the width changes between the two formats.

Read the height straight off the table, or multiply the U number by 1.75 in (44.45 mm) for any figure that is not listed. These are usable mounting heights, not the outside height of a cabinet.

Each U on the vertical mounting flange is a group of three holes. Inside a group the holes sit 5/8 in (0.625 in) apart, then 5/8 in again, and then a 1/2 in gap separates one group from the next. The arithmetic is the whole story: 0.625 + 0.625 + 0.5 = 1.75 in. That repeating 5/8 in, 5/8 in, 1/2 in pattern is what makes the unit line up from rack to rack.

Across the rack, the hole centers sit 18.3125 in (18 5/16 in) apart. That distance, not the 19-inch figure, is what the ears of a rack device are drilled for.

Rails come in four common styles, and the style decides what hardware holds the gear:

• Square holes take cage nuts, which clip into the square and give you a threaded point wherever you need one. This is the most common style on new racks and the most flexible when gear moves around.
• 10-32 tapped rails have the thread cut into the metal, so the screw goes straight in with no nut behind it. Common on AV gear and lighter racks.
• 12-24 tapped rails use a coarser US thread and are widespread in data center racks.
• M6 tapped rails use a metric thread, mostly on racks built to the international standard.

On a tapped rail, a stripped thread means the hole is gone. On a square-hole rail, a worn cage nut is a cheap part to swap, which matters in a rack where servers get slid out and re-racked.

The U figure measures the rail height inside the rack. The cabinet around those rails is taller, because the frame, top panel, base, casters and leveling feet all add height. On a floor-standing cabinet that extra runs roughly 4 to 6 in.

A 42U rack gives 73.5 in (1866.9 mm) of usable height but typically stands around 78 to 80 in overall. A 48U rack gives 84 in (2133.6 mm) of usable space and stands taller still. The gap is the reason a rack that looked fine on paper can foul a ceiling or a door frame on delivery.

Measure the room, not just the rack. Check ceiling height, door openings, stairwells and elevator dimensions before ordering, and leave a few extra inches for the carton and the pallet the rack arrives on.

Wall-mount cabinets are commonly sold in 6U, 9U, 12U, 15U, 18U, 22U and 27U. Floor-standing racks come in 22U, 27U, 37U, 42U, 45U, 47U and 48U, and open frames run from a few U up to around 70U.

Run through these checks before the order goes in, because a rack that does not fit the room or the gear means building twice.

• Ceiling height. A 42U cabinet stands around 78 to 80 in, and a 48U is taller.
• Doorways, stairs and elevators. A typical cabinet 24 in wide, 40 in deep and 78 in high clears a standard 36 in door and an 80 in opening, but measure every part of the route.
• Packaging. Leave a few extra inches for the pallet and carton on delivery.
• Casters vs static load. A rack's rating on casters is lower than its static rating, so size a rolling setup against the caster figure.
• Depth and rail mounting plane. Match the deepest device plus clearance to the internal depth, and the rail kit's mounting-plane distance to what the rails can reach.
• Wall substrate. Use studs, not drywall anchors, for a wall cabinet.