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1U vs. 2U. What Does the 'U' Refer to When Buying a Server?

A question we sometimes get from customers is, "What is a '1U' server?"  "What is a '2U' server?  What is the difference?" 

The 'U' in any server description is short for "RU" which stands for Rack Unit -- this is the standardized designation for the form factor of the server:  

  • 1U = 1.75" in height or thickness.
  • 2U is 1.75" x2 = 3.5 inches.
  • All rackmount servers are 19" in width.

Additionally, since a U is 1.75", you can understand the dimensions of equipment noted as "3U," "4U," and so on, as illustrated by the table below: 

Rack Units

Height (in)

Height (cm)

1U

1.75″

4.4 cm

2U

3.5″

8.9 cm

3U

5.25″

13.3 cm

4U

7″

17.78 cm

 

Note: as this is a standard, these designations apply to other rack-mountable equipment, such as network switches, firewalls, or UPS units. 

Servers hard at work in their rack.  The highlighted 1U server is on top; the 2U is on the bottom.

 

Virtually all the HP and Dell servers that TechMikeNY resells are either 1U or 2U.  Here is a breakdown some of our popular models into their form factor bucket:

 

Dell 1U Servers:

PowerEdge R420, R430, R620, R630

Dell 2U Servers:

PowerEdge R720, R730, R820, R830

HP 1U Servers:

ProLiant DL360 G8, DL360 G9 

HP 2U Servers:

ProLiant DL380 G8, DL380 G9

 

A helpful tip: both Dell and HP keep a consistency of the form factors within their models.  For example, a Dell PowerEdge R600 series server will always be a 1U regardless of its generation (R610 vs. R620 vs. R630 = all 1U).   

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