What Is a Rack Unit (U)? Calculating Rack Size | SistemDC

The first sticking point for teams that decide to rent a rack is choosing the size: is 10U enough, or should you take 21U? A wrong calculation is costly in both directions. Choose too small and you face moving costs six months later; choose too big and you pay for space you do not use. This guide explains the rack unit (U) concept, standard rack dimensions and how to correctly calculate the number of U you need.

What Is a Rack Unit (U)?

U is the abbreviation of "rack unit" and is sometimes written as RU. It is the standard unit for measuring the vertical height of data center equipment. According to the EIA-310 standard, 1U = 1.75 inches = 44.45 mm.

Thanks to this standard, servers, switches and storage units from different manufacturers mount in the same rack without problems. If a server's box says "1U rackmount", you know it will fit any 19-inch rack in the world.

Width and Depth: The Two Dimensions U Does Not Tell You

U expresses height only. The other two axes are what actually cause problems in planning.

• Width: In the industry-standard 19-inch rack, the distance between the mounting rails is 482.6 mm. The equipment body itself is usually around 448–450 mm. The external width of the rack is 600 mm (standard) or 800 mm (with side cable channels).
• Depth: Rack depths are manufactured as 600, 800, 1000 and 1200 mm. Modern 1U servers are 650–800 mm deep. At the back you need to leave at least 100–150 mm of clearance for power cables and cable management; that is why most production racks are 1000 mm deep.

How Many U Do You Need? A Step-by-Step Calculation

1. List the active equipment. Write down the U value of every server, storage unit and GPU chassis.
2. Add the network layer. At least a 1U switch, and in most setups a 1U patch panel as well. If redundant networking is required, plan for two switches.
3. Set aside a cable management allowance. One 1U horizontal cable organiser for every 4–6 active devices keeps the rack maintainable.
4. Think about console access. If you plan on-site interventions, a 1U sliding KVM console is useful. Because SistemDC plans come with free remote KVM, this line is usually unnecessary.
5. Count the PDUs correctly. Vertical (0U) PDUs sit in the rear channel of the rack and consume no U. If you prefer horizontal PDUs, each takes up 1U.
6. Add a 20% growth allowance. Leave room for the hardware you will add within a year; changing racks means moving.

Example: 10U rack layout

As you can see, a 10U rack is a realistic choice for five servers: network and cabling consume three U and two U are left for growth. Trying to fit the same five servers into a 6U shared plan is not possible in terms of either space or power budget. For a price comparison, see the rack rental prices guide for details.

Why Should You Close Off Empty U?

Cooling in racks works front to back: air taken from the cold aisle passes through the server and is expelled, heated, from the rear. Empty U disrupt this pattern; hot air recirculates to the front of the rack and the inlet temperature rises.

The solution is simple: every unused U gets a blanking panel (blind panel). The cost is low and the effect is big — by lowering the inlet temperature a few degrees it reduces fan speeds and therefore power consumption. Because power is billed on a metered basis this shows up directly on the bill; we explained how the calculation works in the power cost guide article.

Hardware That Does Not Fit a Rack: The ATX Shelf Option

Not all hardware is rackmount. Custom-built game servers, high-clock desktop platforms and large air-cooled GPU systems mostly come in ATX tower cases. These cases cannot be mounted on 19-inch rails; they have to be placed on a shelf.

For this need, the ATX shelf rental plan is used: 20 ATX shelf spaces, a 10 Gbps uplink and the same 13₺/kWh metered power model for 25,000₺/mo. If your hardware is in a tower case, consider this plan instead of a rackmount rack.

Weight and Power Distribution

A fully loaded 42U rack easily weighs hundreds of kilos. Two practical rules:

• Heavy equipment at the bottom: Storage chassis and UPSs are placed in the lower part of the rack. This both reduces the risk of tipping and makes installation easier.
• Power balance: On servers with dual power supplies, the first feed is connected to side A and the second to side B. That is the purpose of dual PDUs: when one side is taken into maintenance the system keeps running. Connecting all servers to the same side makes the redundancy meaningless.

Frequently Asked Questions

How many cm is 1U?
1U is exactly 44.45 mm, or about 4.4 cm. 10U is about 44.5 cm and 42U about 186.7 cm.

Why is the 42U rack considered standard?
42U corresponds to an external height of about 2 metres. In a standard data center hall this is a suitable size for both service access and the overhead cable tray.

Does a PDU consume U?
Vertical (0U) PDUs fit in the rear vertical channel of the rack and do not reduce usable U. Horizontal PDUs take up 1U.

What should I do when the rack is full?
Scaling is designed to go from 10U to 21U and from there to 42U. Talking to the sales team in advance about space planning shortens the migration window.

I have a single server — do I need a rack?
No. For one or a few servers, 1U–4U / ATX shared rack colocation is both cheaper and faster to bring into service.

Related SistemDC Guides

• What Is Colocation? Server Colocation Guide
• Rack Rental Prices 2026
• Data Center Energy Costs and PDU Calculation
• What Is a Tier 3 Data Center?
• Moving a Server to a Data Center: Migration Checklist
• Private cage or shared rack?