Disk drive and drive bay form factors

Whether it be hard disk or floppy disk drives, the size of the drive, it's form factor, is determined by where it will be installed, the drive bay of a computer or rack enclosure. Commonly used disk formats come from historic pseudo-standards arbitrarily introduced by single manufacturers and were later formally standardized in industry wide standards. Form-factors are named by their width. Note that this name-giving width isn't the actual outder dimension but the size of the internal platter, magnetic disk or removable diskette. Some form factors seem to follow a nestable scheme, where one larger form factor would hold multiple instances of the next smaller form factor, similar to how ISO paper sizes are multiples in size of the next smaller sized sheet. But that's more an impression and only true for full-height / half-height versions of a respective form-factor.

8.0-Inch

H/W/D: 4.624 in × 9.5 in × 14.25 in (117.5 mm × 241.3 mm × 362 mm) 4+5⁄8 inches (117.5 mm) high, 9+1⁄2 inches (241.3 mm) wide, and approximately 14+1⁄4 inches (361.9 mm) deep
The 8.0" drive and drive bay format was introduced by IBM and Tandy. When third-party manufacturers began to offer form-factor and interface compatible drives, hard-disk-drives and floppy-disk-drives, the format began to "stick". Shugart Associates, which was later renamed to Shugart Technology and ultimately became Seagate Technology, adopted the format with the SA1000 model and offered later compatible drives like the ST506. Micropolis as well offered early rigid disk drives in this format, drives from the 1220 and 1240 series.

5.25-Inch

Full-height:

H/W/D: 3.25 in × 5.75 in × 8 in (82.55 mm × 146.1 mm × 203 mm)
3+1⁄4 inches (82.6 mm) high, 5+3⁄4 inches (146.1 mm) wide, and up to 8 inches (203.2 mm) deep
This drive bay format is named after the size of a 5.25-inch floppy-disk that would fit in a floppy-disk-drive made for such drive bays. The actual bay is 5.75-inches (146.1 mm) wide and the front opening even 5.875-inches (149.2 mm) wide. By coincidence, this drive bay is more or less 8 inches deep, resembling the next larger form-factor. Note that these dimensions are the full-height drive dimensions, which were used until mid 1980. Micropolis manufactured a number of full-height 5.25-inch hard-disk-drives. In our HDD support section they are tagged with the "FHT" abbreviation.

Half-height:

H/W/D: 3.25 in × 5.75 in × 8 in (41.3 mm × 146.1 mm × 203.2 mm)
1+5⁄8 inches (41.3 mm) high by 5+3⁄4 inches (146.1 mm) wide, and up to 8 inches (203.2 mm) deep
Half-height is, just as the name implies, a 5.25-inch drive bay utilized only half, so that two half-height devices can usually fit in one full-height bay. Half-height used to be the standard housing dimensions for more modern 5.25-inch floppy disk drives and "slimline" or "low-profile" hard-disk-drives of the late 1980s. Today, CD and DVD drives usually follow this form-factor in width, but come in even shallower height factors, which in turn are then called slimline optical drives.

3.5-Inch

H/W/D: 1.028 in × 4 in × 5.75 in (26.1 mm × 101.6 mm × 146 mm)
1.028 inches (26.1 mm) high, 4 inches (101.6 mm) wide, and up to 5.75 inches (146 mm) deep
3.5 inch drive form factor used to have a "full-height" version as well and manufacturer Rodime offered a 3.5 inch full-height hard-drive in the early 1980s, but most drives, floppy and rigid disk, usually use the 3.5 inch half-height format which is 1 inch high. While full-height drives were still common with HDDs during the 1990s, today the "low profile" 3.5-inch form-factor is the most common one for 3.5-inch floppy disk drives and hard-drives - with hard-disks slowly fading towards 2.5-inch or smaller form factors with solid-state memory.
Micropolis manufactured a number of drives fitting the 3.5-inch form-factor, in full height for example the large-capacity Tomahawk drives from the 3391 series but also in half-height, like the Micropolis Aries 2 or lower capacity Tomahawks.

Note on abbeviations

Above, H/W/D means "Height, Width, Depth". In Micropolis technical documentation you may also find the L/W/H abbreviation for "Length, Width, Height" which is more common in engineering, while H/W/D is more common in sales/product descriptions.

More information (external links)