Server rack units describe how tall a server is in a standard data-centre rack. Dedicated server specs and colocation prices often mention 1U, 2U or 4U, and the form factor affects how many drives, how much cooling and how many expansion cards a server can hold. This guide explains the terms.
Before you start
You only choose form factors when you buy hardware or rent colocation space. With a VPS, shared or reseller hosting, the provider handles all of this.
What is a rack unit (U)?
A standard server rack is 19 inches (482.6 mm) wide. Height is measured in rack units: 1U = 1.75 inches (44.45 mm). A full-height rack is commonly 42U, so it can hold up to 42 1U servers, although space is also used for switches, patch panels and power distribution.
Common rack server sizes
| Size | Typical drive bays | Strengths | Trade-offs |
|---|---|---|---|
| 1U | 4–10 small (2.5") or NVMe bays | Dense, lower colocation cost per server | Small, loud fans; few expansion slots |
| 2U | 8–24 small or up to 12 large (3.5") bays | Balance of storage, cooling and PCIe slots | Uses twice the rack space of 1U |
| 4U | Dozens of large drives, or several GPUs | Storage servers, GPU compute, best cooling | Heavy, expensive to house |
Other form factors
- Tower servers – look like a desktop PC; quiet and suited to offices, not data centres.
- Blade servers – thin compute modules that slide into a shared chassis providing power, cooling and networking. Very dense, but tied to one vendor's chassis.
- Multi-node servers – e.g. 2U4N: four independent servers sharing one 2U chassis and power supplies.
Inside a rack server
- Hot-swap drive bays – replace a failed disk without powering down, usually with RAID.
- Redundant power supplies – two PSUs connected to separate power feeds (A and B) so one feed or PSU can fail.
- Out-of-band management – a BMC (IPMI, iDRAC, iLO) for remote console and power control.
- Rails – slide the server out for service without unmounting it.
- Front-to-back airflow – racks are arranged in cold and hot aisles, so servers draw cool air from the front and exhaust at the back.
Colocation basics
With colocation you own the server and rent rack space, power and network in a data centre. Prices are usually based on rack units, power (amps or kW) and bandwidth. You handle hardware purchases and replacements, often with paid "remote hands" for physical tasks.
FAQ about server rack units
Is 1U or 2U better for a dedicated server?
1U suits compute-focused servers with a few NVMe drives. 2U is better if you need many drives, more cooling or extra PCIe cards.
How many servers fit in a 42U rack?
In theory 42 1U servers, but in practice fewer, because of switches, cable management and the rack's power limit.
Does form factor affect performance?
Indirectly: larger chassis can cool more powerful CPUs and GPUs and hold more drives, but the components determine speed.