Rack unit
A rack unit (U) is 1.75 inches of vertical rack height. Server chassis are sized in whole U, most commonly 1U, 2U, and 4U.
A rack unit, abbreviated U, is the standard measure of height for rack-mounted equipment. One U equals 1.75 inches, or 44.45 mm, and the figure comes from EIA-310, the standard that defines the 19 inch rack. Chassis height is always a whole number of U, so a 2U server measures 3.5 inches tall and a 4U server measures 7 inches.
A full-height cabinet gives 42U of usable space. That is forty-two 1U servers, twenty-one 2U servers, or any mix totaling 42U or less. A 1U chassis is dense and cheap per slot but takes only one or two low-profile cards and leaves little room for airflow. A 4U chassis trades that density for room: eight double-width GPUs, taller heatsinks, and larger fans that move the same air at lower speed.
More U buys room for GPUs, drives, and cooling, at the cost of rack space and colocation fees that are often billed per U. Match chassis height to GPU count and cooling before signing a rack contract. A cabinet full of 4U GPU nodes runs out of power long before it runs out of U.
Sources
Source | Publisher |
|---|---|
Dell | |
Racks and Panels (EIA Technical Standards Catalog, EIA/ECA-310-E) | ECIA (Electronic Components Industry Association) |
- Publisher
Dell
- Publisher
ECIA (Electronic Components Industry Association)
Last verified August 29, 2026.
- rack unit
- U height
- 1U server
- 2U server
- 4U server
- EIA-310
- rack height
- rack density
- rack space