A rack unit (U) is the standard height increment for 19-inch equipment racks: 1U = 1.75 in = 44.45 mm (EIA-310). Enter the total U your equipment occupies and pick your rack frame to get the mounting height and the free space that remains.
How to convert rack units
Rack height is a fixed multiple, so the math is simple: multiply the number of units by 1.75 for inches or by 44.45 for millimeters. To plan a rack, add the U rating stamped on each device (a matrix switcher might be 1U to 4U, an amplifier 1U to 2U, a power conditioner 1U), then compare the total against your frame size and leave a little headroom for airflow and cabling.
Rack unit conversion table
| Rack units | Inches | Millimeters |
|---|---|---|
| 1U | 1.75 in | 44.45 mm |
| 2U | 3.5 in | 88.9 mm |
| 4U | 7 in | 177.8 mm |
| 12U | 21 in | 533.4 mm |
| 24U | 42 in | 1,066.8 mm |
| 42U | 73.5 in | 1,866.9 mm |
Frequently asked questions
How tall is one rack unit?
One rack unit (1U) is 1.75 inches, exactly 44.45 millimeters (4.445 cm). It is defined by the EIA-310 standard for 19-inch racks.
How do I convert rack units to inches or millimeters?
Multiply the number of rack units by 1.75 for inches or by 44.45 for millimeters. So 4U is 7 inches (177.8 mm), and a full 42U rack is 73.5 inches (1,866.9 mm).
How much free space should I leave in a rack?
Leave headroom for airflow, cabling, and future gear. Many integrators keep a rack around 70 to 80 percent full and add 1U to 2U ventilation gaps between hot devices such as amplifiers and matrix switchers.
What are common rack frame sizes?
Full-height cabinets are usually 42U, 45U, or 48U. Wall-mount and portable racks are commonly 4U, 6U, 9U, 12U, or 24U. All use 19-inch rails.
Is rack unit height the same as device width?
No. U describes height only. The mounting width is a standard 19 inches. A 1U device is one rack unit tall and fits the 19-inch rail spacing, whatever its depth.
Speccing a rack? Key Digital matrix switchers and extenders are built for clean rack integration. Find a dealer to spec your project.