19" rack sizes
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tl;dr:
- the width you slide between the posts is 445mm (~17.8")
- the 19 inch (483mm) is including the mounting ears
- device height is a multiple of a unit of 4.45 cm (1.75")
- not always precise but racks give out space this way (relates to the screws as well)
- depth is not standardized,
- varied devices may be under under 610mm (historically 23" or 24")
- and when buying racks that may be filled with other's devices, consider ensuring there is space for 812mm / 32" devices, plus space for comfortable wiring
Two or four posts
Two-post racks fixing devices only by their front panel. This is plenty for relatively light and/or shallow devices, such as patch bays, short-depth switches, various devices in telephony and audio, etc.
You wouldn't want it for full blown computers, because they're heavier and it'd put a lot of torque on the panel (and posts, but they're usually quite study).
Some devices have depth adapter brackets, so that a device can be attached at its center (of mass, this means a little less force on the front panel, though isn't really a huge difference).
If you must put a heavier device in a two-post rack, mount it on the center so it won't be a bendy breaky thing some time later.
Not everything has the extra screwholes for that, in which case you probably want a shelf.
Four-post racks are a much better idea for heavier devices, which includes most servers.
Since four-post racks are often enclosed, and often for computers, it's common to be able to slide them out for easier maintenance.
There is no real standard for slides, so they come with the server.
There is no hard standard for depth, so (sliding/fixed) mounts tend to be adjustible in length.
Device width
The 19.0" (483mm) width includes the mounting ears.
To fit between the posts, a device must be at most 451mm (17.8")
Device height
The height is measured in rack units, where 1U is 1.75" / 4.45 cm.
Most things are 1U through 4U, but you can go higher:
- 1U (~4.5cm) and 2U (~8.9cm) devices include various switches, and some slim servers
- 3U (~13.4cm) are often servers
- 4U (~17.8cm) servers may be consumer-tower-like cases that are also rack-mountable
- larger devices (6U, even 8U and 10U) include things like many-disk bays, blade enclosures, large UPSes
When computers stands its PCIe-style cards upright (e.g. relevant now that GPU nodes are popular):
- 3U us the minimum for full-height cards (~12cm) - but only if designed to fit that way. 4U is more comfortable.
- 2U is the minimum for half-height cards (~8cm, consider case and spacer and motherboard add a few cm)
Device/mount depth
Mount depth
In four-post (square-hole) racks, you typically mount the front panel on outside of the post (relevant because the posts usually have depth, rather than being strips with holes).
The distance between front and back posts is not standard,
and has increased over the years, starting at 19" (~483mm) and now usually at least 600mm (23.6").
This is why servers often come with mounting brackets (often sliding in some way) that are extendable, to meet with the back post at varied distances.
Some racks allow moving the posts / mount rails depth-wise.
AOTW currently typical is around 29" / ~740mm, mostly because modern server depths are around there.
Exact measurements vary per manufacturer, for example Dell is 28 7/8″, HP is 29 1/8″, IBM is 28 1/2″ (verify)
Device depth
There is no strong standard for
- the depth of the equipment
- the inner depth available for devices
- the outer depth (or width) of the rack itself
Historically many devices kept under 23" or 24" in depth (so under 610mm),
In modern servers you see things like
- around 712mm / 28"
- around 740mm / 29"
- around 812mm / 32"
Your typical large airy 42U datacenter racks are on the large and spacious side, to avoid any problems.
Convention makes many racks 800mm or ~1000mm deep (inner depth?(verify)),
which historically meant plenty of space for cabling,
and also allowed devices to be a little deeper without problem.