When we refer to the size of a computer or component, we might be talking about its capacity, speed or connectors. Or we could be talking about its dimensions—how much physical space it occupies on the desktop, inside the PC box, in a server rack or in a briefcase. We apply the term form factor to the latter interpretation, to describe size and packaging.
Like much computer terminology, form factor began as technical shorthand but later was adopted as a marketing term as well. Thus, in many cases, form factor refers to a strict, technological definition while in others it is a vague and nonstandard promotional term.
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QuickStudies
Here, we offer a quick rundown of common form factors (highlighted in italics) for various types of products.
Disk Drives
For disk storage, form factor is pretty much synonymous with the diameter of the disk platter. Nowadays, the standard form factor for optical drives is 5.25 in. (in a 1.75-in.-thick package that used to be called half-height but is now the standard), and for desktop hard drives it's 3.5 in. (but starting to move to 2.5 in.). For notebook computers, for many years the standard form factor has been a 2.5-in. drive, but small notebooks are moving to smaller drives—1.8 in., 1.0 in. and now even 0.85 in. Drives for notebooks are also characterized by their thickness or height, usually 17, 12.5 or 9mm.
Flash Memory/Disk Drives
Memory cards used in notebook computers, digital cameras, handheld devices, music players and other portable devices are described based on their capacity, in megabytes, and their configuration: In order of decreasing physical size, these form factors include PCMCIA (also PC Card and CardBus); Compact Flash Type III and the thinner Type II; Memory Stick and Memory Stick Pro; SmartMedia; the physically identical MMC (Multimedia Card) and SD (Secure Digital) Card; the Mini-SD; and the newest, XD-Picture Card. (To keep the record straight, most of these form factors are also used for I/O or other nonmemory devices.)
None of those form factors, of course, includes the more recent wave of "keychain" flash memory cards that plug directly into a USB port. Curiously, no single name or form factor descriptor has emerged for these USB devices.
Desktop PC Enclosures
How big (and what shape?) is the box that holds your PC? Besides referring to the type of motherboard(s) a case is designed to accept, we also describe the case as a tower (sometimes full tower, with the biggest capacity for multiple disk drives), midtower or minitower (same basic shape, but shorter); microtower (smaller still, usually capable of handling only a single floppy drive and optical drive); the superflat pizza box; and finally the all-in-one (combining monitor, storage and electronics in a single package).
A somewhat confusing term these days is small form factor (sometimes abbreviated SFF), which usually refers to a small, near-cube-shaped box that may require proprietary components, best exemplified by Shuttle Computer Inc. computers or the Aria case from Antec Inc. Small form factor is also used to describe the smallest enclosures in a given maker's line, regardless of shape.