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In computing, the motherboard 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:

To serve as a central backbone to which all other modular parts such as CPU, RAM, and hard drives can be attached as required to create a computer
To be interchangeable (in most cases) with different components (in particular CPU and expansion cards) for the purposes of customization and upgrading
To distribute power to other circuit boards
To electronically co-ordinate and interface the operation of the components

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 IBM's long-standing standard, AT (Advanced Technology), was superseded in 1995 by the current industry standard ATX (Advanced Technology Extended),^[1] which still governs the size and design of the motherboard in most modern PCs.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 (also known as ITX, and not to be confused with EPIC) 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. The E-ATX form factor is not standarized and may vary according to the motherboard manufacturer.^[2]

Comparisons

Tabular information

Form factor	Originated	Date	Max. size^{[info 1]} width × depth	Notes (typical usage, Market adoption, etc.)
XT	IBM	1983	216 × 279 mm (8.5 × 11 in)	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 (Advanced Technology)	IBM	1984	305 × 279–330 mm (12 × 11–13 in)	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-AT	IBM	1985	216 × 254–330 mm (8.5 × 10–13 in)	IBM's 1985 successor to the AT motherboard. Functionally equivalent to the AT, it became popular due to its significantly smaller size.
ATX	Intel	1995	305 × 244 mm (12 × 9.6 in)	Created by Intel in 1995. As of 2017,^[update] 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 CEB	SSI	?	305 × 267 mm (12 × 10.5 in)	Created by the Server System Infrastructure (SSI) 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 EEB	SSI	?	305 × 330 mm (12 × 13 in)	Created by the Server System Infrastructure (SSI) 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 MEB	SSI	?	411 × 330 mm (16.2 × 13 in)	Created by the Server System Infrastructure (SSI) forum. Derived from the EEB and ATX specifications.
microATX	Intel	1996	244 × 244 mm (9.6 × 9.6 in)	A 20 % shorter 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 as of 2017.^[update]
Mini-ATX	AOpen	2005	150 × 150 mm (5.9 × 5.9 in)	Mini-ATX is considerably smaller than Micro-ATX. Mini-ATX motherboards were designed with MoDT (Mobile on Desktop Technology) which adapt mobile CPUs for lower power requirement, less heat generation and better application capability.
FlexATX	Intel	1999	228.6 × 190.5 mm max (9.0 × 7.5 in)	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-ITX	VIA	2001	170 × 170 mm max (6.7 × 6.7 in)	A small, highly integrated form factor, designed for small devices such as thin clients and set-top boxes.
Nano-ITX	VIA	2003	120 × 120 mm (4.7 × 4.7 in)	Targeted at smart digital entertainment devices such as PVRs, set-top boxes, media centers and Car PCs, and thin devices.
Pico-ITX	VIA	2007	100 × 72 mm max (3.9 × 2.8 in)
Mobile-ITX	VIA	2007	75 × 45 mm (2.953 × 1.772 in)
Neo-ITX	VIA	2012	170 × 85 × 35 mm (6.69 × 3.33 × 1.38 in)	Used in the VIA Android PC
Deep Mini-ITX	ASRock Rack	?	170 × 208 mm max (6.7 × 8.2 in)	Proprietary Mini-ITX variant i.a. for more than two memory slots, designed for chassis with support for Micro-ATX size and above
Mini-STX	Intel	2015	147 × 140 mm (5.79 × 5.51 in)	Smaller 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 (Balanced Technology Extended)	Intel	2004	325 × 267 mm max (12.8 × 10.5 in)	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 (or uBTX)	Intel	2004	264 × 267 mm max (10.4 × 10.5 in)	MicroBTX (also called uBTX) is a computer motherboard form factor. A microBTX is 10.4 × 10.5 in (264 × 267 mm) and can support up to four expansion slots.
DTX	AMD	2007	200 × 244 mm max (8.0 × 9.6 in)	DTX backward compatible with ATX cases. Shorter variant, Mini-DTX (mDTX) of 8 × 6.7 inches (203 × 170 mm)
smartModule	Digital-Logic	?	66 × 85 mm (2.60 × 3.35 in)	Used in embedded systems and single-board computers. Requires a baseboard.
ETX	Kontron	1999	95 × 114 mm (3.74 × 4.49 in)	Used in embedded systems and single-board computers. Requires a baseboard.
COM Express Basic	PICMG	2005	95 × 125 mm (3.74 × 4.9 in)	Used in embedded systems and single-board computers. Requires a carrier board.
COM Express Compact	PICMG	2005	95 × 95 mm (3.74 × 3.74 in)	Used in embedded systems and single-board computers. Requires a carrier board.
COM Express Mini	PICMG	2005	55 × 84 mm (2.17 × 3.31 in)	Used in embedded systems and single-board computers. Requires a carrier board. Adheres to pin-out Type 10^[3]
COM-HPC Size A	PICMG	2020	95 × 120 mm (3.7 × 4.7 in)	Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Client Type modules.
COM-HPC Size B	PICMG	2020	120 × 120 mm (4.7 × 4.7 in)	Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Client Type modules.
COM-HPC Size C	PICMG	2020	160 × 120 mm (6.3 × 4.7 in)	Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Client Type modules with multiple SODIMM memory sockets.
COM-HPC Size D	PICMG	2020	160 × 160 mm (6.3 × 6.3 in)	Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Server Type modules with 4x full size DIMM memory sockets.
COM-HPC Size E	PICMG	2020	200 × 160 mm (7.9 × 6.3 in)	Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Server Type modules with 8x full size DIMM memory sockets.
CoreExpress	SFF-SIG	?	58 × 65 mm (2.28 × 2.56 in)	Used in embedded systems and single-board computers. Requires a carrier board.
Extended ATX (EATX)	Unknown	?	305 × 330 mm (12 × 13 in)	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 (EEATX)	Supermicro	?	347 × 330 mm (13.68 × 13 in)	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.
LPX	Western Digital	?	229 × 279–330 mm (9 × 11–13 in)	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-LPX	Western Digital	?	203–229 × 254–279 mm (8–9 × 10–11 in)	Used in slimline retail PCs.
PC/104	PC/104 Consortium	1992	97 × 91 mm (3.8 × 3.6 in)	Used in embedded systems. AT Bus (ISA) architecture adapted to vibration-tolerant header connectors.
PC/104-Plus	PC/104 Consortium	1997	97 × 91 mm (3.8 × 3.6 in)	Used in embedded systems. PCI Bus architecture adapted to vibration-tolerant header connectors.
PCI/104-Express	PC/104 Consortium	2008	97 × 91 mm (3.8 × 3.6 in)	Used in embedded systems. PCI Express architecture adapted to vibration-tolerant header connectors.
PCIe/104	PC/104 Consortium	2008	97 × 91 mm (3.8 × 3.6 in)	Used in embedded systems. PCI/104-Express without the legacy PCI bus.
NLX	Intel	1999	203–229 × 254–345 mm (8–9 × 10–13.6 in)	A low-profile design released in 1997. It also incorporated a riser for expansion cards,^[4] and never became popular.
UTX	TQ-Components	2001	88 × 108 mm (3.46 × 4.25 in)	Used in embedded systems and IPCs. Requires a baseboard.
WTX	Intel	1998	355.6 × 425.4 mm (14 × 16.75 in)	A large design for servers and high-end workstations featuring multiple CPUs and hard drives.
SWTX	Supermicro	?	418 × 330 mm (16.48 × 13 in)	A proprietary design for servers and high-end workstations featuring multiple CPUs.
HPTX	EVGA	2008	345 × 381 mm (13.6 × 15 in)	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.
XTX	Ampro / Congatec	2005	95 × 114 mm (3.74 × 4.49 in)	Used in embedded systems. Requires a base.
YTX	Asus, MSI & Maxsun	2023	175 × 245 mm (6.89 × 9.65 in)	New form factor supporting the DIY-APE initiative

^ For boards which take expansion slots, the length of the expansion card aligns with the depth of the system board. The case may support cards longer than the depth of the mainboard.

Size variants

List is incomplete

Form factor	Originated	Date	Max. size^[a] width × depth	Slots	Notes (typical usage, Market adoption, etc.)
ATX	Intel	1995	12 × 9.6 in (305 × 244 mm)	7 ^[1]	Original, successor to AT motherboard
Proprietary, specific to crypto-mining specific motherboards	Unknown	2011	12 × 8 in (305 × 203 mm)	3	3 double-slot add-in cards with 1 slots of free space in between
SSI CEB	SSI	?	12 × 10.5 in (305 × 267 mm)	7	Compact Electronics Bay
SSI MEB	SSI	2011	16.2 × 13 in (411 × 330 mm)	12	Midrange Electronics Bay
SSI EEB	SSI	?	12 × 13 in (305 × 330 mm)	7	Enterprise Electronics Bay
SSI TEB	SSI	?	12 × 10.5 in (305 × 267 mm)	7	Thin Electronics Bay, for rack-mount, has board component height specification
microATX	Intel	1997	9.6 × 9.6 in (244 × 244 mm)	4	Fits in ATX, and EATX cases.
FlexATX	Intel	1997	9 × 7.5 in (229 × 191 mm)	3
Extended ATX (standard)	Supermicro / Asus	?	12 × 13 in (305 × 330 mm)	7	Screw holes not completely compatible with some ATX cases. Designed for dual CPUs, and quad double slot video cards.
Extended ATX (commonly)	Unknown	?	12 × 10.1 in (305 × 257 mm) 12 × 10.4 in (305 × 264 mm) 12 × 10.5 in (305 × 267 mm) 12 × 10.7 in (305 × 272 mm)	7	ATX pattern screw holes
EE-ATX	Supermicro	?	13.68 × 13 in (347 × 330 mm)	7	Enhanced Extended ATX
Ultra ATX	Foxconn	2008	14.4 × 9.6 in (366 × 244 mm)	10	Intended for multiple double-slot video cards, and dual CPUs.
XL-ATX	EVGA	2009	13.5 × 10.3 in (343 × 262 mm)	9
XL-ATX	Gigabyte	2010	13.58 x 10.31 in (345 x 262 mm)	7
XL-ATX	MSI	2010	13.6 × 10.4 in (345 × 264 mm)	7
WTX	Intel	1998	14 × 16.75 in (356 × 425 mm).	9	Discontinued 2008
Mini-ITX	VIA	2001	6.7 x 6.7in (170 × 170 mm).	1	Originally designed for home theatre or other fanless applications
Mini-DTX	AMD	2007	8 × 6.7 in (203 × 170 mm)	2	Derived from Mini-ITX and DTX
BTX	Intel	2004	12.8 × 10.5 in (325 × 267 mm)	7	Canceled 2006. Also micro, nano, and pico variants. Not generally compatible with ATX mounting.
HPTX	EVGA	2010	13.6 × 15 in (345 × 381 mm)	6	Dual processors, 12 RAM slots
SWTX	Supermicro	2006	16.48 × 13 in (419 × 330 mm) and others	5	Quad processors, not compatible with ATX mounting

Maximum number of expansion card slots

ATX case compatible:


Specification	Number
HPTX	9
ATX/EATX^{[broken anchor]}/SSI EEB^{[broken anchor]}/SSI CEB	7
MicroATX	4
FlexATX	3
DTX/Mini-DTX	2
Mini-ITX	1

Visual examples of different form factors

Different form factors
ATX
(Abit KT7)
mini-ITX
(VIA EPIA 5000AG)
Pico-ITX
(VIA EPIA PX10000G)

PC/104 and EBX

PC/104 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.^[5] An IEEE standard corresponding to PC/104 was drafted as IEEE P996.1, but never ratified.^[6]

The 5.75 × 8.0 in Embedded Board eXpandable (EBX) 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 (but still reasonably embeddable) 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 (or expensive) to do so on a PC/104 SBC. Typically, EBX SBCs contain: the CPU; upgradeable RAM subassemblies (e.g., DIMM); 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 (from the PC/104 connectors); networking interface (typically Ethernet); and video (typically CRT, LCD, and TV).

Mini PC

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

AOpen XC mini
Apple Mac mini
Intel NUC
Gigabyte Brix
Zotac ZBOX
Asus Vivopc
Lenovo ThinkCentre Tiny
Dell Optiplex Mini/Micro
Acer Veriton

Intel NUC
Gigabyte BRIX

Notes

^ For boards which take expansion slots, the length of the expansion card aligns with the depth of the system board. The case may support cards longer than the depth of the mainboard.