Rack Unit
A rack unit, written as U or RU, is the standard measure of vertical space in a server rack, equal to 1.75 inches. A standard full-height data center cabinet is 42U, with 45U, 48U, and 52U cabinets increasingly common in higher density facilities.
The rack unit is the standard vertical increment for equipment mounted in a nineteen inch rack, defined as 1.75 inches. Hardware is described by how many units it occupies: a 1U pizza box server, a 2U storage node, a 4U chassis.
Cabinet sizing
The traditional full cabinet is 42U. Facilities designed for higher density increasingly deploy taller cabinets at 45U, 48U, or 52U to fit more equipment within the same floor footprint.
The power constraint
Physical space is rarely the binding limit. A cabinet sold with a fixed power allocation will typically hit that ceiling with the cabinet only partly filled, which is why kilowatts per cabinet is the more meaningful number in a colocation quote than the U count.
Why it matters for marketing
This is a foundational term that belongs in the glossary precisely because it is assumed knowledge in every colocation quote a prospect receives.
It also sets up the more important point: rack units are a space measure and space is rarely the constraint. Any facility page that quotes cabinet size without quoting the power allocation per cabinet is leaving the buyer to guess at the number that actually determines whether the deployment fits.
Common questions
How many rack units are in a standard cabinet?
A standard full-height cabinet is 42U, though 45U, 48U, and 52U cabinets are increasingly common in facilities designed for higher density. Half cabinets are typically sold as 20U or 22U.
Can you always fill every rack unit in a cabinet?
Not usually. Most deployments exhaust their power and cooling allocation long before they run out of physical rack units, particularly with dense modern hardware. A cabinet sold as 42U with a ten kilowatt power cap may only ever be half populated.