Motherboard Form Factors
The form factor determines the general layout, size, and feature placement on a motherboard. Different form factors usually require different style cases. Differences between form factors can include; physical size and shape, mounting hole location, feature placement, power supply connectors, and others.
Form Factor Specifications

12"

11-13"

Very Old PCs

Full AT, Full Tower

8.5"

10-13"

Older PCs

All but Slimline, ATX

12"

9.6"

Newer PCs

ATX

11.2"

8.2"

Newer PCs

ATX

9.6" 9.6" Specialty PCs Slimline
9.0" 7.5" Specialty PCs Custom Design
9"

11-13"

Older Retail PCs

Slimline

8-9"

10-11"

Older Retail PCs

Slimline

8-9"

10-13.6"

Newer Retail PCs

Slimline

AT Form Factor
The AT form factor is the oldest and the biggest form factor. It was popular until the Baby AT  was released, which was around the time of the 386 processor (1992-93). The reason that prompted the Baby AT was the width of the AT (12") and the fact that the board was difficult to install, service, and upgrade.
BABY AT
The Baby AT was the standard in the PC industry from roughly 1993-1997. It is still being used today, usually in Pentium class products.

Some issues with the AT and Baby AT design is the location of the features on the board.  The CPU socket is placed so that it may interfere with longer bus cards.  In some designs the memory sockets are similarly placed.  This can limit the amount and selection of peripheral cards you can install.  Also the IO ports are separate and mounted on the case and connected to pin-outs on the motherboard. These are usually located near the floppy and IDE pin-outs and can result in quite a jumble of ribbon cables.

ATX
ATX was developed as an evolution of the Baby AT form factor and was defined to address four areas of improvement: enhanced ease of use, better support for current and future I/O, better support for current and future processor technology, and reduced total system cost.

The ATX is basically a Baby AT rotated 90 degrees and providing a new mounting configuration for the power supply. The processor is relocated away from the expansion slots, allowing them to hold full length add-in cards. The longer side of the board is used to host more on-board I/O. The ATX power supply, rather than blowing air out of the chassis, as in most Baby AT platforms, provides air-flow through the chassis and across the processor.

Mini-ATX

This form factor is basically the same as ATX with a smaller allowable board size.

  • ATX = 12" x 9.6"
  • Mini-ATX = 11.2" x 8.2"
microATX

This form factor was developed as a natural evolution of the ATX form factor to address new market trends and PC technologies. microATX supports:

  • Current processor technologies
  • The transition to newer processor technologies
  • AGP high performance graphics solutions
  • Smaller motherboard size
  • Smaller power supply form factor

FlexATX

A subset of the microATX design. FlexATX offers the opportunity for system developers to create many new personal computer designs. FlexATX allows enhanced flexibility where conforming motherboards may be enclosed; that is, all-in-one computing devices, LCD-personal computers, or standard desktop systems.

This form factor is designed to allow very custom case and board designs to be manufactured.  For example; The NBA could commission computers that looked like basketballs.  There is not too much limit on the shape of the board and case.  We should see some very interesting system designs emerging from this form factor.

  • Supports current socketed processor technologies
  • Smaller motherboard size
  • ATX 2.03 I/O panel
  • Same mounting holes as microATX
  • Socket only processors to keep the size small
LPX & Mini LPX

This is based on a design by Western Digital. The expansion slots are on a single riser card which is mounted onto the planar board. Mainly OEM manufacturers (i.e. Packard Bell/NEC, Dell, etc) use these boards.

LPX is an older form factor (8.67" x 9.25") that has been replaced by NLX. The LPX form factor is usually found in desktop model PCs. The LPX case is a slim-line, low-profile case with a riser card arrangement for expansion cards. This means that expansion boards are parallel to the motherboard, rather than perpendicular to it as in other common form factors, such as AT and ATX. This allows for smaller cases, but limits the number of expansion slots, usually to two or three.

LPX motherboards often have the video adapters integrated onto the motherboard, and they may have integrated sound as well. This can provide a high-quality product at low cost, but can make upgrading or repair difficult. It is not always possible to disable the built-in video adapter cards to allow for an upgrade. LPX motherboards also usually come with serial, parallel, and mouse connectors attached to them, like ATX.

The LPX case and motherboard design are not designed for a home PC builder, as they can be cramped and difficult to work in, as well as being non-standard. They also offer poor expandability, poor upgradability, poor cooling, and difficulty of use for the home PC builder.

NLX

NLX is a new low profile motherboard form factor designed to improve upon today’s low profile form factors and to adapt to new market trends and PC technologies. NLX does the following:

  • Supports current and future processor technologies
  • Supports new Accelerated Graphics Port (A.G.P.) high performance graphics solutions
  • Supports tall memory technology
  • Provides more system level design and integration flexibility; for example, the new design flexibility allows system designers to implement a motherboard that can be removed quickly, in most cases without removing screws, thus lowering the PC’s total cost of ownership.

The picture above shows an example of an NLX board and riser.

  • The add-in card riser is located at the right edge of the motherboard (as viewed from the front).
  • The processor is located at the front, left section of the motherboard, improving thermal and clearance issues.
  • Taller components such as the processor and tall memory are preferred to be located on the left side of the motherboard, allowing the I/O slots to hold full length add-in cards in many system configurations.
  • At the back of the motherboard (as viewed from the front), the I/O connectors are stacked single and double high to support more connectors.

Several major PC vendors world-wide worked jointly to define the NLX form factor and to incorporate flexibility to accommodate the best designs for current and future PCs. NLX is a public specification intended for widespread use in many types of systems.

Also see Motherboard FAQ.