Motherboard Form Factors

Without a doubt, the most important component in a PC system is the main board or motherboard. Some companies refer to the motherboard as a system board or planar. The terms motherboard, main board, system board, and planar are interchangeable, although I prefer the motherboard designation.

Several common form factors are used for PC motherboards. The form factor refers to the physical dimensions (size and shape) as well as certain connector, screw hole, and other positions that dictate into which type of case the board will fit.

Some are true standards (meaning that all boards with that form factor are interchangeable), whereas others are not standardized enough to allow for interchangeability. Unfortunately, these nonstandard form factors preclude any easy upgrade or inexpensive replacement, which generally means they should be avoided.

The more commonly known PC motherboard form factors include the following:

Obsolete Form Factors

Modern Form Factors

All Others

  • Baby-AT

  • Full-size AT

  • LPX (semiproprietary)

  • WTX (no longer in production)

  • ITX (flex-ATX variation, never produced)

  • ATX

  • micro-ATX

  • Flex-ATX

  • Mini-ITX (flex-ATX variation)

  • NLX

  • Fully proprietary designs (certain Compaq, Packard Bell, Hewlett-Packard, notebook/portable systems, and so on)

Motherboards have evolved over the years from the original Baby-AT form factor boards used in the original IBM PC and XT to the current ATX and NLX boards used in most full-size desktop and tower systems.

ATX has a number of variants, including micro-ATX (which is a smaller version of the ATX form factor used in the smaller systems) and flex-ATX (an even smaller version for the lowest-cost home PCs). A new form factor called mini-ITX is also available; it's really just a minimum-size version of flex-ATX designed for very small systems.

NLX is designed for corporate desktop–type systems; WTX was designed for workstations and medium-duty servers, but never became popular. Table below shows the modern industry-standard form factors and their recommended uses.

Form Factor



Standard desktop, mini-tower, and full-tower systems; most common form factor today; most flexible design for power users, enthusiasts, low-end servers/workstations, and higher-end home systems; ATX boards support up to seven expansion slots.


A slightly smaller version of ATX that fits into the same case as ATX. Many so-called ATX motherboards are actually mini-ATX motherboards; mini-ATX boards support up to six expansion slots.


A smaller version of ATX, used in Mid-range desktop or mini-tower systems. Fits micro-ATX or ATX chassis.


Smallest version of ATX, used in expensive or low-end small desktop or mini-tower systems; entertainment or appliance systems. Fits in flex-ATX, micro-ATX, or ATX chassis.


Minimum-size flex-ATX version, used in set-top boxes and compact/small form factor computers; highly integrated with one PCI expansion slot. Fits in mini-ITX, flex-ATX, micro-ATX, or ATX chassis.


Corporate desktop or mini-tower systems; fast and easy serviceability.

Although the Baby-AT, Full-size AT, and LPX boards were once popular, they have all but been replaced by more modern and interchangeable form factors. The modern form factors are true standards, which guarantees improved interchangeability within each type.

This means that ATX boards can interchange with other ATX boards, NLX with other NLX, and so on. The additional features found on these boards as compared to the obsolete form factors, combined with true interchangeability, has made the migration to these newer form factors quick and easy.

Today I recommend purchasing only systems with one of the modern industry-standard form factors. Each of these form factors, however, is discussed in more detail in the next articles. Anything that does not fit into one of the industry-standard form factors is considered proprietary.

Unless there are special circumstances, I do not recommend purchasing systems with proprietary board designs. They will be virtually impossible to upgrade and very expensive to repair later because the motherboard, case, and often power supply will not be interchangeable with other models. I call proprietary form factor systems "disposable" PCs because that's what you must normally do with them when they are too slow or need repair out of warranty.


"Disposable" PCs might be more common than ever. Some estimate that as much as 60% of all PCs sold today are disposable models, not so much because of the motherboards used, but because of the tiny power supplies and cramped micro-tower cases that are favored on most retail-market PCs today.

Although low-cost PCs using small chassis and power supplies are theoretically more upgradeable than past disposable type systems, you'll still hit the wall over time if you need more than three expansion slots or want to use more than two or three internal drives.

Because mini-tower systems are so cramped and limited, I consider them to be almost as disposable as the LPX systems they have largely replaced. You also need to watch out for systems that only appear to meet industry standards, such as certain Dell computer models built from 1996 to the present—especially the XPS line of systems.

These computers often use rewired versions of the ATX power supply (or even some that are completely nonstandard in size and shape) and modified motherboard power connectors, which makes both components completely incompatible with standard motherboards and power supplies.

In some of the systems, the power supply has a completely proprietary shape as well and the motherboards are not fully standard ATX either. If you want to upgrade the power supply, you must use a special Dell-compatible power supply. And if you want to upgrade the motherboard (assuming you can find one that fits), you must buy a standard power supply to match.

The best alternative is to replace the motherboard, power supply, and possibly the case with industry-standard components simultaneously. For more details about how to determine whether your Dell computer uses nonstandard power connectors. If you want to have a truly upgradeable system, insist on systems that use ATX motherboards in a mid-tower or larger case with at least five drive bays.