What is Cabinet and U space (rack unit)?
A rack unit measures height inside a rack or cabinet. One U is 1.75 inches (44.45 mm), and rack mount equipment is sold in multiples of it, such as 1U servers and 2U servers. The 19 inch figure refers to the width of the front panel, as set out in EIA-310 and related standards.
Cabinet height is given in U. A common size is 42U, but cabinets come in other heights, and some data centers use taller ones. Usable space is often less than the total, because a few units may be needed for cable management, power distribution or blanking panels.
Space in U is not the only limit. Power, cooling, weight and cabinet depth also decide what fits. Data center operators sell and measure space in different ways, so check whether a quote is for U space, a whole cabinet or a cage.
How it shows up in practice
- Check the depth of the cabinet and the equipment before ordering, because some servers do not fit shallow cabinets.
- Plan rack elevations in U, with positions for heavy items at the bottom, and leave space for airflow and cabling.
- Use blanking panels in empty spaces where the design requires them, to keep airflow under control.
- Number the units on the cabinet or in the drawing so the position in U is used in tickets and records.