Drive Bay Sizes: 3.5" and 5.25"

Not surprisingly, these sizes correspond to the two common sizes of floppy disks in use today. (Actually, the 5.25" disks aren't really that common any more, but they're still out there.) They also correspond to two of the most common hard disk form factors in the history of the PC (which were created from the floppy disk standards). The bays are not actually this size, because this is the approximate size of the media and the drive obviously must be larger, but they are almost always referred to by those names. While these bays were designed to fit the two sizes of floppy drive mentioned, most modern drives have all been designed to fit into the same dimensions. This table summarizes the dimensions of the standard drive bays on a PC (servers and other large systems have larger 3.5" bays to accomodate larger 3.5" form factor hard disks):

Note: The depth numbers shown are the minimum required for drives to fit the bay. The depth of actual drive bays will vary, but will typically be greater than these numbers to allow for access to the drives, cabling, etc.

Some devices require the larger size bay. Obviously, a 5.25" floppy drive needs one, and so do all CD-ROM and DVD drives (CDs and DVDs are 4.75 inches wide). Many tape drives do as well, as do many removable storage drives. A 3.5" floppy drive will of course fit in a 3.5" bay. So will most modern hard disk drives (in fact, most internal drive bays are 3.5" for that reason). The height of the different bay types is pretty much standard; a 3.5" bay is about 1" in height, and a 5.25" bay is about 1.63" in height. (This is sometimes called a "half-height" bay in reference to the old (enormous) full-height bays that were 3.5" in height in the earliest PCs.)

This 3.5" floppy disk drive, mounted in an adapter, is being inserted into
a 5.25" drive bay. The adapter includes the necessary wider metal
mounting rails, plus the plastic portion for the faceplate of the drive.
Note that normally, one would only bother doing this if all of the case's 3.5"
drive bays were already full, or there were some other compelling reason,
such as needing to relocate a drive to compensate for a short cable.

Original image � Kamco Services
Image used with permission.

It is possible to buy adapters that will make a 3.5" device fit into a 5.25" bay, if you need to do this. It is easier to do with a hard disk than a floppy disk or other device with a faceplate, because in addition to the mounting rails for the side of the drive, you need an adapter for the faceplate as well in the latter case. See this procedure for more details. Keep in mind when planning your system that nobody has yet developed an adapter to allow a 5.25" drive to fit into a 3.5" bay. :^)