Definition

# U (measurement)

## What is a U (measurement)?

A U is a standard unit of measure for designating the height in computer enclosures and server rack cabinets.

A rack unit is abbreviated as U or RU with a number prefix. A U equals 1.75 inches, so a 4U chassis would be 7 inches high. A 40U rack cabinet would be 70 inches high.

## How to calculate a U measurement

A rack unit of length is based on the Eurocard EIA-310 standard rack specification. It was adopted globally as IEC 60297 "Mechanical Structures for Electronic Equipment," which details standard sizes for the rack, sub-rack, pitch, circuit boards and telecommunications equipment.

A 1U front panel would be 123⁄32 inch or 43.66 millimeters (mm) tall in metric units. If n is the number of rack units, panel height is h = (1.75n - 0.031), or h = (44.45n - 0.794) in millimeters when using the metric system.

Typically, a full-size rack is 42U, which translates to over 6 feet or 180 centimeters of rack space for equipment such as servers, network switches or routers.

## What is rack threading?

While rack units are typically the same, the thread type used to connect them on mounting rails can vary. No. 10-32 tapped is the Unified Thread Standard. However, No. 12-24 tapped and metric M6 are also used.

This was last updated in November 2021