Stepping into a server room or even just looking at a rack can initially seem confusing. How does all that gear fit? That’s where the Rack Unit, or “U,” comes in. It’s the standard measure for vertical space in server racks. Understanding ‘U’ is key for anyone handling server hardware – it tells you exactly how much vertical room equipment is needed. It simplifies fitting everything. Stick with me, and we’ll nail down what a rack unit is, how you can figure out your own gear’s size, run through the useful conversions, and look at how those rack holes work.
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What Does the Rack Unit U Mean?
A Rack Unit (U or RU) is basically how we measure how tall equipment is when it’s built to go into a server rack.
- It’s a Set Height: Listen, one Rack Unit (1U) is always 1.75 inches tall. That’s the absolute core measurement everything else in the rack world is based on. (Yep, that also comes out to 44.45 mm in metric).
- It’s Official: This isn’t just some size someone decided on randomly. This standard measurement comes from the Electronic Industries Alliance (EIA-310 standard). That’s key because it’s what guarantees gear made by different companies will physically fit into those standard racks built to the same spec.
- It Sizes Gear: Your equipment – things like servers, network switches, batteries, you name it – all come in heights that are exact multiples of this U size (so you get 1U, 2U, 4U, and so on). They’re designed to stack up perfectly.
- It Sizes Racks: And the racks themselves? They’re sized too, based on the total U count they can hold vertically (like those big 42U racks you see everywhere).
How to Calculate Rack Units
Do you have a device and need its U size? Use this quick method. It helps plan rack space without guessing.
| Étape | What to Do |
|---|---|
| 1 | Measure Height: Find the device’s total vertical height. Don’t include extra bumps or handles. |
| 2 | Use Inches: Convert that height to inches if needed (1 inch = 25.4 mm). |
| 3 | Divide: Divide the height in inches by 1.75 (inches per U). |
Formula:
U = Device Height (in inches) ÷ 1.75
Exemple :
- A server measures 5.25 inches tall.
- Calculation: 5.25 ÷ 1.75 = 3U.
- It’s a 3U server.
This simple math helps confirm the device size for correct placement in your rack.
Server Rack Unit Conversion
Need to switch between U, inches, millimeters, or centimeters? These conversions are helpful.
The core factors are: 1U = 1.75 inches = 44.45 mm = 4.445 cm.
These simple conversions save time when checking specs or dimensions.
Rack U to Inches
One rack unit is always 1.75 inches.
Formula:
1U = 1.75 inches
Exemple :
- A 3U device = 3 × 1.75 = 5.25 inches tall.
1U to Inches
Exactly 1U height is 1.75 inches.
Formula:
1U = 1.75 inches
This is the fundamental conversion you need to know.
Rack Unit to Millimeters
To convert U to millimeters, multiply the number of U’s by 44.45 mm.
Formula:
Height (mm) = U × 44.45
Exemple :
- A 4U device = 4 × 44.45 = 177.8 mm tall.
Rack Unit to Centimeters
To convert U to centimeters, multiply the number of U’s by 4.445 cm.
Formula:
Height (cm) = U × 4.445
Exemple :
- A 5U device = 5 × 4.445 = 22.225 cm tall.
Rack Unit Conversion Formula
Here are the core conversions:
1U = 1.75 inches = 44.45 millimeters = 4.445 centimeters
Rack U Size Chart
| Rack Unit（u） | RU Height（mm） | RU Height（inch） | RU Height（cm） |
|---|---|---|---|
| 1U | 44.45mm | 1.75″ | 4.445cm |
| 2U | 88.90mm | 3.50″ | 8.890cm |
| 3U | 133.35mm | 5.25″ | 13.335cm |
| 4U | 177.80mm | 7.00″ | 17.780cm |
| 5U | 222.25mm | 8.75″ | 22.225cm |
| 6U | 266.70mm | 10.50″ | 26.670cm |
| 7U | 311.15mm | 12.25″ | 31.115cm |
| 8U | 355.60mm | 14.00″ | 35.560cm |
| 9U | 400.05mm | 15.75″ | 40.005cm |
| 10U | 444.50mm | 17.50″ | 44.450cm |
| 11U | 488.95mm | 19.25″ | 48.895cm |
| 12U | 533.40mm | 21.00″ | 53.340cm |
| 13U | 577.85mm | 22.75″ | 57.785cm |
| 14U | 622.30mm | 24.50″ | 62.230cm |
| 15U | 666.75mm | 26.25″ | 66.675cm |
| 16U | 711.20mm | 28.00″ | 71.120cm |
| 17U | 755.65mm | 29.75″ | 75.565cm |
| 18U | 800.10mm | 31.50″ | 80.010cm |
| 19U | 844.55mm | 33.25″ | 84.455cm |
| 20U | 889.00mm | 35.00″ | 88.900cm |
| 21U | 933.45mm | 36.75″ | 93.345cm |
| 22U | 977.90mm | 38.50″ | 97.790cm |
| 23U | 1022.35mm | 40.25″ | 102.235cm |
| 24U | 1066.80mm | 42.00″ | 106.680cm |
| 25U | 1111.25mm | 43.75″ | 111.125cm |
| 26U | 1155.70mm | 45.50″ | 115.570cm |
| 27U | 1200.15mm | 47.25″ | 120.015cm |
| 28U | 1244.60mm | 49.00″ | 124.460cm |
| 29U | 1289.05mm | 50.75″ | 128.905cm |
| 30U | 1333.50mm | 52.50″ | 133.350cm |
| 31U | 1377.95mm | 54.25″ | 137.795cm |
| 32U | 1422.40mm | 56.00″ | 142.240cm |
| 33U | 1466.85mm | 57.75″ | 146.685cm |
| 34U | 1511.30mm | 59.50″ | 151.130cm |
| 35U | 1555.75mm | 61.25″ | 155.575cm |
| 36U | 1600.20mm | 63.00″ | 160.020cm |
| 37U | 1644.65mm | 64.75″ | 164.465cm |
| 38U | 1689.10mm | 66.50″ | 168.910cm |
| 39U | 1733.55mm | 68.25″ | 173.355cm |
| 40U | 1778.00mm | 70.00″ | 177.800cm |
| 41U | 1822.45mm | 71.75″ | 182.245cm |
| 42U | 1866.90mm | 73.50″ | 186.690cm |
| 43U | 1911.35mm | 75.25″ | 191.135cm |
| 44U | 1955.80mm | 77.00″ | 195.580cm |
| 45U | 2000.25mm | 78.75″ | 200.025cm |
| 46U | 2044.70mm | 80.50″ | 204.470cm |
| 47U | 2089.15mm | 82.25″ | 208.915cm |
| 48U | 2133.60mm | 84.00″ | 213.360cm |
Rack Hole and Rack Unit Relationship
Take a real close look at the vertical rails inside any standard server rack, and you’re gonna see a bunch of holes running up and down. These aren’t random! These rack holes are exactly where you bolt your gear in, whether you’re using screws or cage nuts. And here’s the neat part: they line up perfectly with the Rack Unit system we’ve been talking about. Understanding their pattern makes working with racks much clearer.
What is a Rack Unit (U)?
Just as a quick recap: Think of a Rack Unit (U) as the standard way we measure the vertical height for equipment that goes into a server rack. One 1U space is equal to 1.75 inches (that’s about 44.45 mm).
What is a Rack Hole?
So, these rack holes are the specific spots drilled into the rack rails where you make your connections for mounting equipment. They aren’t just scattered randomly, though. The key thing is they come in specific groups along the rail, and these groups are spaced out to exactly match the standard U sizes. One of these hole groups marks out the height of one U.
How Do Rack Holes Relate to Rack Units?
- Pour 1U of equipment, there are three holes on the rack.
- The holes are spaced consistently:
- Top-to-middle gap: 0.625 inches (15.875 mm)
- Middle-to-bottom gap: 0.625 inches
- Bottom-to-next top gap: 0.5 inches (12.7 mm)
Common Rack Unit Applications
Different gear usually comes in certain U sizes based on needs like space or cooling. Knowing these helps plan your rack.
| U Size | Typical Gear You’ll See |
|---|---|
| 1U | Network switches, firewalls, small servers, patch panels. Good for saving vertical space. |
| 2U | Standard servers, storage servers, common UPS batteries. Need more internal space. |
| 3U | Older or specialized gear like tape libraries, and dense storage. Less common for new servers. |
| 4U | Big storage arrays, GPU servers, large UPS. Need significant height. |
| 5U+ | Very large systems, modular batteries, custom gear. |
This gives you a quick idea of how much space different types of IT equipment usually need.
Conclusion
Getting a handle on Rack Units (U) is key to managing server racks and batterie pour rack de serveur. Understanding U means you can pick gear that fits, plan your space right, and keep everything organized. It simplifies installation and helps maximize your vertical space efficiently
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