Rack Mount U Size: What is a Rack Unit?

You’re likely to hear phrases like “1U switch” or “2U server” in conversations among IT specialists. What does the ‘U’ mean in server racks? This article explains it.

What is U in Server Rack Size?

Server cabinets and racks are designed to house high-precision hardware that supports business operations. One of the key considerations in selecting the right rack is determining its optimal size. For this, a special unit of measurement, U, is used.

A “U” rack dimension stands for a rack unit and refers to the height of a device that can be mounted in the server rack. One U equals 1.75 inches (or 44.45 mm). This measurement is standard for all rack-mounted hardware.

For larger hardware, sizes greater than 1U are used. Common values include 2U, 3U, 4U, 6U, 7U, 12U, etc. These sizes accommodate either multiple smaller devices or larger single pieces of equipment, offering more flexibility in storage.

What Does “U” Mean in Server Racks as a Standard?

The term “U” in server racks refers to a rack unit, a standardized measurement used to define the height of equipment that can be mounted in a 19-inch rack. Server rack sizes in U have become a universal way to describe the height of devices, ensuring compatibility across different manufacturers. This standardization originated from the need to create uniformity in the dimensions of racks used in telecommunications and radio equipment. The Electronic Industries Association (EIA), which was a group of trade associations for electronics manufacturers in the United States, developed the EIA-310-D standard in 1992 to standardize features of 19-inch rack enclosures.

Prior to this standardization, equipment sizes varied widely, leading to compatibility issues. The introduction of the rack unit measurement allowed for consistent hole spacing and equipment dimensions, ensuring that devices from different manufacturers could be mounted in the same racks. 

The EIA ceased operations in 2011, and its functions were transferred to the Electronic Components Industry Association (ECIA). 

Today, the rack unit measurement remains a fundamental standard in the IT industry, facilitating the design and installation of equipment in data centers and server rooms worldwide.

How to Calculate Rack Units?

To calculate rack units for your equipment, you need to know the height of the device you plan to mount. The standard 1U rack size equals 1.75 inches (44.45 mm). To determine the number of rack units (U) a device will occupy, divide its height by 1.75 inches. For example, if a device is 3.5 inches tall, it will occupy 2U (3.5 / 1.75 = 2). For equipment with non-standard heights, round up to the next whole number to determine the required rack units. This calculation ensures proper fitting and efficient use of space in your server rack.

Popular Sizes

Common Sizes: 1U, 2U, and 4U

• 1U models mean that the height of hardware accommodated inside is 1.75 inches.
• 2U is used for bigger devices. The height of such models is calculated by multiplying 1.75 by 2. Thus, 2U = 3.5 inches. Such cabinets are suitable for more powerful computing systems that require more space. 
• 4U is designed for even bigger electronic devices, the height of which is 7 inches (4 * 1.75). However, this is not the maximum size. There are also 9U, 12U, and larger solutions.

Half Rack

This dimension is applicable to the width of networking hardware. Furniture pieces have a standard frame width equal to 19 inches. Half-rack models mean that the width of the equipment is 9.5 inches.

Full Rack

When determining the number of racks needed for a server, note that servers can vary in size. Full rack models take up the entire 19 inches of width, meaning a single unit occupies the entire rack or cabinet.

Note: Although the height and width of the products are standardized, the depth may vary. Models are produced in several depths to accommodate different equipment.

Table of Units Sizes

In conclusion, understanding what “U” means in server racks is crucial when selecting the appropriate cabinet size, as it directly impacts the system’s performance and efficiency. The “U” in rack size represents a unit of measurement used to define the required space. Proper organization of the internal components within the server cabinet is essential, ensuring adherence to the requirements established for data centers established for data centers.