Form factor

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For components and devices in engineering, principally electronic, a form factor defines the linear dimensions, mounting and connector standards, and other parameters that categorize the item as belonging to a mechanically compatible family. For example, personal computer motherboards come in form factors defined by such things as ATX, and PC towers are in micro, mini, and full size form factors. Peripheral component cards are in one of several PCI[1] form factors, with the "half-length" having become the most common.

To simplify logistics (military)|logistics by using Line Replaceable Units, as few form factors as possible should be used by the LRUs.

Rack mounting

In the general electronics industry, "rack mountable" devices are 19 inches/482.6 mm wide and some multiple of 1.75 inches/44.45 mm tall, with mounting flanges protruding from the front.

The flanges have holes through which standard mounting screws go into pre-tapped holes in a mounting rail on both sides of the device.[2]The mounting rails have a width 0.625 inches/15.875 mm wide, separated by 17.75 inches/450.85 mm, giving the total 19 inch width. Mounting rails are paired, such that each hole is separated, center-to-center, by 18.3 inches/464.82 mm.

Joint Tactical Radio System: form factor reduction

The Joint Tactical Radio System (JTRS) is a radical simplification of different form factors used by U.S. military electronics, primarily radios and encryption devices. JTRS is a software defined radio, in which many of the discrete electronic components, and even fundamental techniques such as superheterodyne operation, are replaced by computer-controlled digital signal processors.[3] 26 different form factors were identified as being in military use, and, with the smaller size and programmability of the JTRS architecture, reduced to 13 form factors:

  1. Manpack
  2. Handheld
  3. Airborne, Maritime and Fixed Site Small Airborne (AMF-SA)
  4. AMF-MF (Maritime/fixed site)
  5. Small form factor (SFF) A&H (for Intelligent Munitions Systems and Unattended Ground Sensors (UGS) in the Future Combat Systems(FCS))
  6. SFF B, C and I (for Ground Soldier Systems)
  7. SFF D (for aerial systems)
  8. SFF J (for Networked Missile Launcher System in FCS)

Mobile telephones

Cases for cellular telephones and physically compatible electronics come in several form factors:[4]

  • Bar
  • Clamshell or folder
  • Flip
  • Slide
  • Swivel

Component packaging

Vacuum tubes, while obsolete for virtually all applications, became quite standardized in a series of sizes and socket types. A designation such as "5U4" meant a five-volt power supply and a 4-pin socket, while a 12AT7 had a 12-volt power supply and a 7-pin socket.

Integrated circuits went into widespread production as dual in-line packages.


Standardizing electronic and optical connectors is part of effectively designing to common form factors. Many military electronics devices, even though they may have differently sized chassis, use the MIL-STD-1553 bus interface. The 8-pin RJ45 connector is widely used in structured cabling systems; it is often called the "Ethernet" or 10BaseT connector but is far more versatile than that application; it was actually developed for telephony interconnection.

In general, with higher and higher speeds available on individual conductors, the trend is to have fewer and fewer pins on connectors. In the past, when it was impractical to put multiple control signals on a single conductor, connectors often had tens of connectors.


  1. Peripheral Component Interconnect Special Interest Group
  2. Electronics Industries Association standard 310-D, International Electrotechnical Commission standard 60297
  3. Joint Program Executive Office, Joint Tactical Radio System
  4. "Form Factor", Phone Scoop