What Do 1U, 2U, 4U and 42U Mean in Server Racks? A Complete Guide

When renting a server or a rack cabinet you often hear terms like 1U, 2U, 4U or 42U. These designations refer to the height of the equipment measured in rack units (U).

What Is a "U"?

U stands for "unit" and is a standard that defines the external height of rack‑mount equipment. One U equals 1.75 inches (44.45 mm). The width of a rack‑mount server is fixed at 19 inches (48.26 cm), while the height is a multiple of 4.445 cm.

1U, 2U, 4U Servers

Common rack‑mount servers are 1U, 2U or 4U tall:

1U = 4.445 cm

2U = 8.89 cm

4U = 17.78 cm

1U servers are the most space‑efficient and cheapest because providers charge based on the rack space occupied. However, 1U servers have limited expansion: up to four 2.5" drives and typically two PCI slots, whereas 2U servers can hold eight drives and up to six PCI slots.

Origin of the U Standard

The U measurement is not proprietary to servers; it originated in telecommunications rack equipment. The dimensions are defined by the Electronic Industries Alliance (EIA), an industry standards organization founded in 1924.

19‑Inch Rack Standard

A "19‑inch" rack refers to the width of the mounting rails (48.26 cm). Standard rack dimensions include:

Width: 600 mm or 800 mm

Height: typically 1.6 m or 2 m (range 0.7 m–2.4 m)

Depth: 450 mm–1000 mm, depending on installed equipment

Equipment height is expressed in U, where 1U = 44.45 mm. Devices are manufactured to match an integer number of U (nU).

42U Rack

42U racks are classified by width (600 mm or 800 mm) and depth (600 mm, 800 mm, 900 mm, 960 mm, 1000 mm, 1100 mm, 1200 mm, etc.). Custom dimensions are also possible. Although a 42U rack can theoretically hold 42 × 1U servers, practical capacity is lower because space must be reserved for cooling, cable management, switches, firewalls, and other devices.

Source: Architecture Engineer Technical Alliance