In UPS and server rack batteries, “U” is commonly used as a unit of measurement for size. Understanding the meaning of “U” helps in choosing the right rack equipment.
What is a Rack Unit?
A Rack Unit (RU or U) is a standard unit for measuring the vertical height of rack-mounted equipment. One rack unit (1U) is equal to 1.75 inches (44.45 mm). A standard 19-inch rack is 42U in height. Common rack battery sizes range from 1U to 4U. The actual height of the equipment may be slightly less to allow installation space.
How to Calculate Rack Units?
As mentioned earlier, 1U equals 1.75 inches, so 2U = 3.5 inches, 3U = 5.25 inches, and so on.
Common Rack Unit Conversion
Most equipment sizes range from 1U to 42U. Below is a conversion chart for typical rack unit measurements:
| Rack Units | Height (in) | Height (ft) | Height (cm) |
|---|---|---|---|
| 1U | 1.75″ | 0.15′ | 4.4 cm |
| 2U | 3.5″ | 0.29′ | 8.9 cm |
| 3U | 5.25″ | 0.44′ | 13.3 cm |
| 4U | 7″ | 0.58′ | 17.8 cm |
| 5U | 8.75″ | 0.73′ | 22.2 cm |
| 6U | 10.5″ | 0.88′ | 26.7 cm |
| 7U | 12.25″ | 1.02′ | 31.1 cm |
| 8U | 14″ | 1.17′ | 35.6 cm |
| 9U | 15.75″ | 1.31′ | 40 cm |
| 10U | 17.5″ | 1.46′ | 44.5 cm |
| 11U | 19.25″ | 1.6′ | 48.9 cm |
| 12U | 21″ | 1.75′ | 53.3 cm |
| 13U | 22.75″ | 1.9′ | 57.8 cm |
| 14U | 24.5″ | 2.04′ | 62.2 cm |
| 15U | 26.25″ | 2.19′ | 66.7 cm |
| 16U | 28″ | 2.33′ | 71.1 cm |
| 17U | 29.75″ | 2.48′ | 75.6 cm |
| 18U | 31.5″ | 2.63′ | 80 cm |
| 19U | 33.25″ | 2.77′ | 84.5 cm |
| 20U | 35″ | 2.92′ | 88.9 cm |
| 21U | 36.75″ | 3.06′ | 93.3 cm |
| 22U | 38.5″ | 3.21′ | 97.8 cm |
| 23U | 40.25″ | 3.35′ | 102.2 cm |
| 24U | 42″ | 3.5′ | 106.7 cm |
| 25U | 43.75″ | 3.65′ | 111.1 cm |
| 26U | 45.5″ | 3.79′ | 115.6 cm |
| 27U | 47.25″ | 3.94′ | 120 cm |
| 28U | 49″ | 4.08′ | 124.5 cm |
| 29U | 50.75″ | 4.23′ | 128.9 cm |
| 30U | 52.5″ | 4.38′ | 133.4 cm |
| 31U | 54.25″ | 4.52′ | 137.8 cm |
| 32U | 56″ | 4.67′ | 142.2 cm |
| 33U | 57.75″ | 4.81′ | 146.7 cm |
| 34U | 59.5″ | 4.96′ | 151.1 cm |
| 35U | 61.25″ | 5.1′ | 155.6 cm |
| 36U | 63″ | 5.25′ | 160 cm |
| 37U | 64.75″ | 5.4′ | 164.5 cm |
| 38U | 66.5″ | 5.54′ | 168.9 cm |
| 39U | 68.25″ | 5.69′ | 173.4 cm |
| 40U | 70″ | 5.83′ | 177.8 cm |
| 41U | 71.75″ | 5.98′ | 182.2 cm |
| 42U | 73.5″ | 6.13′ | 186.7 cm |
Rack Hole and Rack Unit Relationship
According to the EIA standards, rack holes are grouped in sets of three, with each group corresponding to 1U of height. This allows for easy determination of rack height by counting the groups of rack holes.
Conclusion
Knowing what rack units mean helps you quickly determine the actual height when building a UPS or rack-mounted backup power project. Using standardized units also allows for better project management. If you are looking for standard rack batteries to support your project, you can check out PKNERGY’s 10 tips on how to choose the right rack-mount battery in more detail!
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