Server Rack Unit (RU) Measurements

Rack units (RU) are the standard metric for measuring server enclosures and rack-mounted equipment. The rack unit system is used worldwide as part of the International Electrotechnical Commission (IEC) 60297 Mechanical structures for electronic equipment – Dimensions of mechanical structures, published in 2008. 

Whether you are comparing server rack sizes or compatibility for mounted equipment like a PDU, our guide to rack unit measurements can help you plan accurately for your requirements. 

Rack Unit Measurements

A rack unit or RU is defined as 1.75 inches or 44.45mm. Rack units are primarily used for server rack sizing - the most common size of a server rack is 42RU, alongside variations of 18RU and 22RU. Rack units may also be expressed as “U”, such as 1U, 2U, 4U. 

For example, a 42RU server rack would have a height of 73.5 inches or 186.69cm for equipment mounting, not counting the external housing of the rack itself and the gaps between mounting slots. 

While rack-mounted equipment also has its dimensions expressed in RU, the actual measurements are not exactly accurate to the defined value of RU. The actual height of rack-mounted equipment is typically smaller than its RU measurement to account for some clearance between devices. For instance, A 1 RU PDU may have an actual height of 1.73 inches or 44mm. 

The difference is negligible as the RU measurement is a more simplified way to determine how many slots the equipment will occupy. In other words, a 42RU server rack will be able to support any combination of equipment adding up to a total of 42 rack units. 

Rack Unit Spacing 

19-inch wide server racks have unevenly spaced holes, grouped in sets of three per rack unit. The distance between mounting holes within a rack unit is 0.625 inches/15.875mm, while the distance between each piece of mounted equipment should be 0.5 inches/12.7mm. 

23-inch wide server racks have evenly spaced mounting holes placed at 1-inch (25.4 mm) intervals.

Rack Unit Conversion Chart

Rack Units (RU)	Inches (in)	Millimetres (mm)
1	1.75	44.45
2	3.50	88.90
3	5.25	133.35
4	7.00	177.80
6	10.50	266.70
8	14.00	355.60
9	15.75	400.05
12	21.00	533.40
15	26.25	666.75
18	31.50	800.10
22	38.50	977.90
24	42.00	1,066.80
27	47.25	1,200.15
37	64.75	1,644.65
42	73.50	1,866.90
45	78.75	2,000.25
48	84.00	2,133.60

 

Rack Unit Sizing Considerations

Most types of server racks are available in the standard 42RU height, meaning that the choice of server rack will come down to the features required for operation. The key thing to keep in mind when searching for rack-mounted equipment is to ensure sufficient clearance between individual devices. 

Poorly-designed rack-mounted equipment from cheaper manufacturers may have inaccurate heights and take up more space in the rack than stated. To avoid this, always choose a manufacturer that adheres to the RU standard. As a local rack-mounted equipment manufacturer in Australia, RackLink offers a range of high-quality rack-mounted PDUs and MCB distribution panel boards that meet the IEC and Australian Electrical Standards.