microATX (sometimes referred to as µATX, mATX or uATX) is a standard for motherboards that was introduced in December 1997. The maximum size of a microATX motherboard is 244 mm × 244 mm (9.6 in × 9.6 in), but some microATX boards can be as small as 171.45 mm × 171.45 mm (6.75 in × 6.75 in). The standard ATX size is 25% longer, at 305 mm × 244 mm (12 in × 9.6 in).
Backward compatibility 
microATX was explicitly designed to be backward-compatible with ATX. The mounting points of microATX motherboards are a subset of those used on full-size ATX boards, and the I/O panel is identical. Thus, microATX motherboards can be used in full-size ATX cases. Furthermore, most microATX motherboards generally use the same power connectors as ATX motherboards, thus permitting the use of full-size ATX power supplies with microATX boards.
microATX boards often use the same chipsets (northbridges and southbridges) as full-size ATX boards, allowing them to use many of the same components. However, since microATX cases are typically much smaller than ATX cases, they usually have fewer expansion slots.
Most modern ATX motherboards have five or more PCI or PCI-Express expansion slots, while microATX boards typically have only four (four being the maximum permitted by the specification). In order to conserve expansion slots and case space, many manufacturers produce microATX motherboard with a full-range of integrated peripherals (especially integrated graphics), which may serve as the basis for small form factor and media center PCs. For example, the Asus A8N-VM CSM motherboard (pictured right) features onboard GeForce 6 graphics, AC'97 audio, and gigabit Ethernet (among others), thus freeing up the expansion slots that would have been used for a graphics card, sound card, and Ethernet card. In recent years, however, it is common even for ATX boards to integrate all these components, as much of this functionality is contained in the typical northbridge/southbridge pair. With the "must-have" functions already present on the motherboard, the need for having many expansion slots has faded, and adoption of microATX has increased even to be used in ATX cases.
A more modern limitation of a microATX case is due to its reduction in drive bays. Current southbridges support up to six SATA devices, in addition to up to four legacy IDE devices. The full range of connectors are commonly found on microATX boards, and can be fully exploited if the board is mounted in an ATX case.
In addition, some microATX cases require the use of Low-Profile PCI cards and use power supplies with non-standard dimensions.
- See this thread on HardForum.com for an example of "mATX."
- Intel Developer Forum
- See this thread for an example of "uATX."
- Mueller, Scott (2003). Upgrading and Repairing PCs. Pearson Education. p. 211. ISBN 978-0-7897-2974-3.
- Intel Desktop Board D201GLY Overview (In Intel Technical Specifications this board (and others with dimensions 171.45 mm × 171.45 mm) named as "Mini-ITX, compatible with microATX", although standard size for mini-ITX boards is 170 mm × 170 mm)
- As of 2007[update], most motherboards follow the ATX12V 2.2 specification, which provides for a 24-pin main power connector, and a 4-pin auxiliary connector.