Computer form factor


In computing, the form factor is the specification of a motherboard – the dimensions, power supply type, location of mounting holes, number of ports on the back panel, etc. Specifically, in the IBM PC compatible industry, standard form factors ensure that parts are interchangeable across competing vendors and generations of technology, while in enterprise computing, form factors ensure that server modules fit into existing rackmount systems. Traditionally, the most significant specification is for that of the motherboard, which generally dictates the overall size of the case. Small form factors have been developed and implemented.

Overview of form factors

A PC motherboard is the main circuit board within a typical desktop computer, laptop or server. Its main functions are as follows:
As new generations of components have been developed, the standards of motherboards have changed too. For example, the introduction of AGP and, more recently, PCI Express have influenced motherboard design. However, the standardized size and layout of motherboards have changed much more slowly and are controlled by their own standards. The list of components required on a motherboard changes far more slowly than the components themselves. For example, north bridge microchips have changed many times since their introduction with many manufacturers bringing out their own versions, but in terms of form factor standards, provisions for north bridges have remained fairly static for many years.
Although it is a slower process, form factors do evolve regularly in response to changing demands. IBM's long-standing standard, AT, was superseded in 1995 by the current industry standard ATX, which still governs the size and design of the motherboard in most modern PCs. The latest update to the ATX standard was released in 2007. A divergent standard by chipset manufacturer VIA called EPIA is based upon smaller form factors and its own standards.
Differences between form factors are most apparent in terms of their intended market sector, and involve variations in size, design compromises and typical features. Most modern computers have very similar requirements, so form factor differences tend to be based upon subsets and supersets of these. For example, a desktop computer may require more sockets for maximum flexibility and many optional connectors and other features on board, whereas a computer to be used in a multimedia system may need to be optimized for heat and size, with additional plug-in cards being less common. The smallest motherboards may sacrifice CPU flexibility in favor of a fixed manufacturer's choice.

Comparisons

Tabular information

Form factorOriginatedDateMax. size
width × depth		Notes
XTIBM19838.5 × 11 in
216 × 279 mm		Obsolete, see Industry Standard Architecture. The IBM Personal Computer XT was the successor to the original IBM PC, its first home computer. As the specifications were open, many clone motherboards were produced and it became a de facto standard.
AT IBM198412 × 11–13 in
305 × 279–330 mm		Obsolete, see Industry Standard Architecture. Created by IBM for the IBM Personal Computer/AT, an Intel 80286 machine. Also known as Full AT, it was popular during the era of the Intel 80386 microprocessor. Superseded by ATX.
Baby-ATIBM19858.5 × 10–13 in
216 × 254–330 mm		IBM's 1985 successor to the AT motherboard. Functionally equivalent to the AT, it became popular due to its significantly smaller size.
ATXIntel199512 × 9.6 in
305 × 244 mm		Created by Intel in 1995. , it is the most popular form factor for commodity motherboards. Typical size is 9.6 × 12 in although some companies extend that to 10 × 12 in.
SSI CEBSSI12 × 10.5 in
305 × 267 mm		Created by the Server System Infrastructure forum. Derived from the EEB and ATX specifications. This means that SSI CEB motherboards have the same mounting holes and the same IO connector area as ATX motherboards.
SSI EEBSSI12 × 13 in
305 × 330 mm		Created by the Server System Infrastructure forum. Derived from the EEB and ATX specifications. This means that SSI CEB motherboards have the same mounting holes and the same IO connector area as ATX motherboards, but SSI EEB motherboards do not.
SSI MEBSSI16.2 × 13 in
411 × 330 mm		Created by the Server System Infrastructure forum. Derived from the EEB and ATX specifications.
microATX19969.6 × 9.6 in
244 × 244 mm		A smaller variant of the ATX form factor. Compatible with most ATX cases, but has fewer slots than ATX, for a smaller power supply unit. Very popular for desktop and small form factor computers.
Mini-ATXAOpen20055.9 × 5.9 in
150 × 150 mm		Mini-ATX is considerably smaller than Micro-ATX. Mini-ATX motherboards were designed with MoDT which adapt mobile CPUs for lower power requirement, less heat generation and better application capability.
FlexATXIntel19999.0 × 7.5 in
228.6 × 190.5 mm max.		A subset of microATX developed by Intel in 1999. Allows more flexible motherboard design, component positioning and shape. Can be smaller than regular microATX.
Mini-ITXVIA20016.7 × 6.7 in
170 × 170 mm max.		A small, highly integrated form factor, designed for small devices such as thin clients and set-top boxes.
Nano-ITXVIA20034.7 × 4.7 in
120 × 120 mm		Targeted at smart digital entertainment devices such as PVRs, set-top boxes, media centers and Car PCs, and thin devices.
Pico-ITXVIA20073.9 × 2.8 in
100 × 72 mm max.
Mobile-ITXVIA20072.953 × 1.772 in
75 × 45 mm
Neo-ITXVIA2012170 × 85 × 35 mmUsed in the VIA Android PC
Mini-STXIntel2015147 x 140 mmSmaller than Mini-ITX, but bigger than the NUC, this board is used in small form factor computers, using a socketed intel core processor and SO-DIMMS.
BTX Intel200412.8 × 10.5 in
325 × 267 mm max.		A standard proposed by Intel as a successor to ATX in the early 2000s, according to Intel the layout has better cooling. BTX Boards are flipped in comparison to ATX Boards, so a BTX or MicroBTX Board needs a BTX case, while an ATX style board fits in an ATX case. The RAM slots and the PCI slots are parallel to each other.
Processor is placed closest to the fan. May contain a CNR board.
MicroBTX Intel200410.4 × 10.5 in
264 × 267 mm max.		MicroBTX is a computer motherboard form factor. A microBTX is 10.4 × 10.5 in and can support up to four expansion slots.
PicoBTXIntel20048.0 × 10.5 in
203 × 267 mm max.
DTXAMD20078.0 × 9.6 in
200 × 244 mm max.
smartModuleDigital-Logic66 × 85 mmUsed in embedded systems and single board computers. Requires a baseboard.
ETXKontron95 × 114 mmUsed in embedded systems and single board computers. Requires a baseboard.
COM Express BasicPICMG95 × 125 mmUsed in embedded systems and single board computers. Requires a carrier board. Formerly referred to as ETXexpress by Kontron.
COM Express CompactPICMG95 × 95 mmUsed in embedded systems and single board computers. Requires a carrier board. Formerly referred to as microETXexpress by Kontron.
COM Express MiniPICMG55 × 84 mmUsed in embedded systems and single board computers. Requires a carrier board. Formerly referred to as nanoETXexpress by Kontron. Also known as COM Express Ultra and adheres to pin-outs Type 1 or Type 10
CoreExpressSFF-SIG58 × 65 mmUsed in embedded systems and single board computers. Requires a carrier board.
Extended ATX 12 × 13 in
305 × 330 mm		Used in rackmount server systems. Typically used for server-class type motherboards with dual processors and too much circuitry for a standard ATX motherboard. The mounting hole pattern for the upper portion of the board matches ATX.
Enhanced Extended ATX Supermicro13.68 × 13 in
347 × 330 mm		Used in rackmount server systems. Typically used for server-class type motherboards with dual processors and too much circuitry for a standard E.ATX motherboard.
LPXWestern Digital9 × 11–13 in
229 × 279–330 mm		Based on a design by Western Digital, it allowed smaller cases than the AT standard, by putting the expansion card slots on a Riser card. Used in slimline retail PCs. LPX was never standardized and generally only used by large OEMs.
Mini-LPXWestern Digital8–9 × 10–11 in
203–229 × 254–279 mm		Used in slimline retail PCs.
PC/104PC/104 Consortium19923.8 × 3.6 inUsed in embedded systems. AT Bus architecture adapted to vibration-tolerant header connectors.
PC/104-PlusPC/104 Consortium19973.8 × 3.6 inUsed in embedded systems. PCI Bus architecture adapted to vibration-tolerant header connectors.
PCI/104-ExpressPC/104 Consortium20083.8 × 3.6 inUsed in embedded systems.
PCI Express architecture adapted to vibration-tolerant header connectors.
PCIe/104PC/104 Consortium20083.8 × 3.6 inUsed in embedded systems.
PCI/104-Express without the legacy PCI bus.
NLXIntel19998–9 × 10–13.6 in
203–229 × 254–345 mm		A low-profile design released in 1997. It also incorporated a riser for expansion cards, and never became popular.
UTXTQ-Components200188 × 108 mmUsed in embedded systems and IPCs. Requires a baseboard.
WTXIntel199814 × 16.75 in
355.6 × 425.4 mm		A large design for servers and high-end workstations featuring multiple CPUs and hard drives.
SWTXSupermicro16.48 × 13 in
418 × 330 mm		A proprietary design for servers and high-end workstations featuring multiple CPUs.
HPTXEVGA200813.6 × 15 in
345.44 × 381 mm		A large design by EVGA currently featured on two motherboards; the eVGA SR2 and SRX. Intended for use with multiple CPUs. Cases require 9 expansion slots to contain this form-factor.
XTXAmpro / Congatec200595 × 114 mmUsed in embedded systems. Requires a base.

Maximum number of expansion card slots

ATX case compatible:
SpecificationNumber
HPTX9
ATX/EATX/SSI EEB/SSI CEB7
MicroATX4
FlexATX3
DTX/Mini-DTX2
Mini-ITX1

Visual examples of different form factors

PC/104 and EBX

is an embedded computer standard which defines both a form factor and computer bus. PC/104 is intended for embedded computing environments. Single board computers built to this form factor are often sold by COTS vendors, which benefits users who want a customized rugged system, without months of design and paper work.
The PC/104 form factor was standardized by the PC/104 Consortium in 1992. An IEEE standard corresponding to PC/104 was drafted as IEEE P996.1, but never ratified.
The 5.75 × 8.0 in Embedded Board eXpandable specification, which was derived from Ampro's proprietary Little Board form-factor, resulted from a collaboration between Ampro and Motorola Computer Group.
As compared with PC/104 modules, these larger SBCs tend to have everything of a full PC on them, including application oriented interfaces like audio, analog, or digital I/O in many cases. Also it's much easier to fit Pentium CPUs, whereas it's a tight squeeze to do so on a PC/104 SBC. Typically, EBX SBCs contain: the CPU; upgradeable RAM subassemblies ; Flash memory for solid state drive; multiple USB, serial, and parallel ports; onboard expansion via a PC/104 module stack; off-board expansion via ISA and/or PCI buses ; networking interface ; and video.

Mini PC

is a PC small form factor very close in size to an external CD or DVD drive. Mini PCs have proven popular for use as HTPCs.

Examples