Understanding Rack Units: What They Are and Why They Matter for Equipment Racks


When it comes to describing sizes associated with 19-inch racking, a rack unit (or U) is the standard terminology. It is important to know how much rack unit space is required when installing 19-inch rack cabinets and enclosures. Fortunately, a unit calculator is available below to convert rack U's into cm, inches and feet, which is a helpful tool for installers or musicians who need to determine what equipment to purchase when building a 19-inch rack. For a complete table of rack U-to-inch/feet/cm values, please scroll down.

The rack unit, also known as U (or RU), is a unit of measurement used for equipment racks and the devices they contain, such as servers and disk drives. One U is equivalent to 1.75 inches (44.45mm), and the standard rack is 19 inches, which equals 42U.

Hardware designed to be mounted in racks is manufactured in multiples of 1.75 inches and specified in rack units, such as 1U, 2U, 3U, or 4U, and racks are designed to hold equipment of those sizes. The mounting flanges of racks have holes arranged in groups of three, which is also called a rack unit.

It is important to note that the rack unit dimensions for equipment are considered maximum dimensions. In reality, devices are often made slightly smaller than the specified U value to allow for some extra space. For example, a device specified as 2U may actually measure 3.44 inches in height instead of the exact 3.5 inches that is a multiple of 1.75.


Rack Unit Inches Centimeters
1U 1.75" 4.45 cm
2U 3.5" 8.89 cm
3U 5.25" 13.34 cm
4U 7" 17.78 cm
5U 8.75" 22.23 cm
6U 10.5" 26.67 cm
7U 12.25" 31.12 cm
8U 14" 35.56 cm
9U 15.75" 40.01 cm
10U 17.5" 44.45 cm
11U 19.25" 48.90 cm
12U 21" 53.34 cm
13U 22.75" 57.79 cm
14U 24.5" 62.23 cm
15U 26.25" 66.68 cm
16U 28" 71.12 cm
17U 29.75" 75.57 cm
18U 31.5" 80.01 cm
19U 33.25"