What Is a Rack Unit? 1U, 2U and Rack Sizes

Spec sheets say 1U, 2U or 3U. Rack listings say 12 space or 24 space. Before you can size a rack for a job, you need to know exactly what those numbers measure and what they leave out. This guide answers what is a rack unit, how the mounting holes line up with it, what 19 inch rack dimensions really cover, and how to count the units a system needs.

What is a rack unit?

A rack unit is a unit of height, and only height. The 19 inch racking standard, published as EIA-310 in the U.S. and as IEC 60297 internationally, fixes the width of the mounting rails and the vertical pitch of the holes. Equipment built to it fits any conforming rack.

The University of Wyoming’s data center guide puts it plainly: the height of rack mounted equipment is standardized in multiples of 1.75 inches, called a rack unit, rack mount unit, RU, RMU or U. All of those names mean the same thing.

AV rack listings often say “space” instead of U. A 20 space rack has 20U of mounting height. The AV equipment racks category lists a swing gate rack described as 20 space and 35 inches, which checks out: 20 x 1.75 = 35.

Rack unit height: 1U, 2U and common rack sizes

Multiply the U count by 1.75 to get inches of mounting space. The table shows common sizes; every figure is that multiplication.

The 1.75 inch figure is the pitch on the rails, not the exact height of the box. The same Wyoming guide notes that vendors make equipment just slightly shorter than the full unit height, so there is clearance between neighbors. A 1U device measures a little under 1.75 inches, and that is normal.

To find the U for a box with no rating, divide its height by 1.75 and round up. A 3.4 inch tall unit is 3.4 / 1.75 = 1.94, so it takes 2U. On standard rails, equipment takes whole units.

Mounting holes and the three-hole pattern

Each rack unit has three holes on each rail. Per the Wyoming guide, the three holes within a unit sit 0.625 inch apart, center to center, and the gap to the next unit’s holes is 0.5 inch. That adds back to the pitch: 0.625 + 0.625 + 0.5 = 1.75.

Because the spacing is uneven, equipment must start on a unit boundary. Mount a 1U panel half a unit off and its ears will not line up with the holes. Most rails are marked or numbered by unit; start counting from those marks.

• Tapped round holes: screws thread straight into the rail. The rack spec names the thread.
• Square holes: take cage nuts, which clip into the square and accept the screw.
• Mixed hardware: keep the screws that match the rack, and do not force a thread that does not fit.

19 inch rack dimensions: what the width covers

The 19 inch figure is the width of the equipment’s front panel, including the ears that bolt to the rails. The IEC standard for this family is titled for the 482.6 mm (19 in) series and covers front panels, subracks, chassis, racks and cabinets.

The clear opening between the rails is narrower than 19 inches, because the ears sit in front of the rails. The chassis behind the ears must fit through that opening. The outside width of a rack or cabinet is wider again and varies by model, so check the listing for wall space and door clearance.

A few systems use a 23 inch width, mostly in telecom. Equipment for one width will not mount in the other without adapters, so confirm before ordering.

How to count rack units: a worked example

Count every item that takes rails, not just the components with ears. Every figure below is an assumption for a sample media room rack; replace them with your own inventory.

1. Receiver on a vented shelf: 3U.
2. Two amplifiers at 2U each: 4U. The AV amplifiers category lists the amplifier options.
3. Network switch and patch panel: 1U each, 2U.
4. Horizontal cable manager: 1U. Vertical managers sit outside the rails and use no units; see network rack cable management.
5. UPS: 2U, sized as in our UPS for an AV rack guide.
6. Power conditioner: 1U; power conditioner vs surge protector explains which one fits.
7. Streaming players on a shelf: 2U.
8. Vent panels: 1U above each amplifier and above the receiver, 3U.

The total is 3 + 4 + 2 + 1 + 2 + 1 + 2 + 3 = 18U. Add growth room, here an assumed 25 percent: 18 x 1.25 = 22.5, rounded up to 23U. A 24 space rack, such as the residential rack in the racks category, covers that with one unit to spare.

Rack depth is not part of the rack unit

A rack unit says nothing about depth, and depth is where many racks fall short. Two depth figures matter:

• Rail depth: the distance from the front rail mounting surface to the rear rail. Rail kits and rear supports must attach at this distance.
• Usable or frame depth: how deep a device can be, including connectors and cables behind it, before it hits the back or stops a door closing.

Measure the deepest device, then add room for connectors, cable bend and air behind it. A wall-mounted rack is usually the shallowest option, so check its listed depth first. Our guide on how to choose an AV equipment rack covers depth, weight and airflow by rack type.

Finding racks, shelves and panels

The AV equipment racks category lists wall-mounted swing gate, rotating, residential and floor racks, each by its space count. Cable managers and straps are in the wire and cable management category, and the full range sits in the Bursma audio video division.

For pricing and availability, ask your sales representative through the Bursma contact page or call the Columbus, Ohio branch. Installers without an account can apply for a Bursma dealer account.

Frequently asked questions