8.0-inch

8.0-inch drive bays were found in early IBM computers, CP/M computers, and the TRS-80 Model II. They were 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, and were used for hard disk drives and floppy disk drives. This form factor is obsolete.

5.25-inch

A 5.25-inch DVD drive

5.25-inch drive bays are divided into two height specifications, full-height and half-height.

Full-height bays were found in old PCs in the early to mid-1980s. They were 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, used mainly for hard disk drives and floppy disk drives. This is the size of the internal (screwed) part of the bay, as the front side is actually 5+7⁄8 inches (149.2 mm). The difference between those widths and the name of the bay size is because it is named after the size of floppy that would fit in those drives, a 5.25-inch-wide square.

Half-height drive bays are 1+5⁄8 inches (41.3 mm) high by 5+3⁄4 inches (146.1 mm) wide, and are the standard housing for CD and DVD drives in modern computers. They were sometimes used for other things in the past, including hard disk drives (roughly between 10 and 100 MB) and floppy disk drives. As the name indicates, two half-height devices can fit in one full-height bay. Often represented as 5.25-inch, these floppy disk drives are obsolete.

The dimensions of a 5.25-inch floppy drive are specified in the SFF standard specifications which were incorporated into the EIA-741 "Specification for Small Form Factor 133.35 mm (5.25 in) Disk Drives" by the Electronic Industries Association (EIA).^[1] Dimensions of 5.25 optical drives are specified in the SFF standard (they are somewhat shorter and not only size of the body is standardized, but also size of the bezel).

3.5-inch

A 3.5-inch floppy drive

3.5-inch bays, like their larger counterparts, are named after diskette dimensions; their actual dimensions are 4 inches (101.6 mm) wide by 1.028 inches (26.1 mm) high. Those with an opening in the front of the case are generally used for floppy or Zip drives. Hard drives in modern computers are typically mounted in fully internal 4″ (nominally 3.5″) bays. Most modern computers do not come with a floppy drive at all, and may lack any externally accessible 3.5″ bays. There are adapters, sometimes called a "sled", which can be used to mount a 3.5″ device in a 5.25″ bay.

More recently, it is becoming common to use 3.5″ bays for smart card and memory card readers, or for panels with additional USB ports.

The dimensions of a 3.5″ drive are specified in the SFF standard specifications SFF-8300 and SFF-8301 which were incorporated into the EIA-740 specification by the Electronic Industries Association (EIA).^[2]

2.5-inch

A 2.5-inch solid-state drive (SSD)

For 2.5-inch bays, actual dimensions are 2+3⁄4 inches (69.9 mm) wide, between 5 millimetres (0.20 in) and 3⁄4 inch (19.1 mm) high, and 3.955 inches (100.5 mm) deep. However, most laptops have drive bays smaller than the 15 mm specification. 2.5-inch hard drives may range from 7 mm to 15 mm in height, there are two sizes that appear to be prominent. 9.51 mm size drives are predominantly used by laptop manufacturers, however at present 2.5-inch Velociraptor^[3] and some higher capacity drives (above 1 TB), are 15 mm in height. The greater height of the 15 mm drives allow more platters and therefore greater data capacities. Many laptop drive bays are designed to be removable trays in which the drives are mounted, to ease removal and replacement.

The dimensions of a 2.5-inch drive are specified in the SFF standard specifications SFF-8200 and SFF-8201 which were incorporated into the EIA-720 specification by the Electronic Industries Association (EIA).

1.8-inch

1.8-inch bays have two specifications, a 60 mm × 70 mm form factor, and a 54 mm × 78 mm form factor. The actual dimensions of the 60 mm × 70 mm are 2.75 in wide by 0.276–0.374 in high and 2.362 in deep (69.85 mm × 7–9.5 mm × 60 mm). The actual dimensions of the 54 mm × 78 mm are 2.126 in wide by 0.197 or 0.315 in high and 3.091 in deep (54 mm × 5 or 8 mm × 78.5 mm). These drives have been used in small devices including as add-ons to game systems.

The dimensions of a 1.8-inch drive are specified in the SFF standard specifications SFF-8111 and SFF-8120 which were incorporated into the EIA-720 specification by the Electronic Industries Association (EIA).