Micro-ATX, often abbreviated mATX, is a motherboard form factor that specifies a board of at most 244 x 244 mm (9.6 x 9.6 inches) with up to four expansion slots, while keeping the mounting holes, rear I/O opening and power connectors of full-size ATX. In the Motherboards topic of computer Hardware, it is the middle ground between the full ATX standard and the compact Mini-ITX standard.
Intel published the Micro-ATX specification in December 1997 as a shorter derivative of ATX. Because it is a strict subset of the ATX layout, a Micro-ATX board fits in nearly every ATX case, uses the same power supply, and accepts the same CPUs, memory and graphics cards. The practical difference lies in fewer slots, fewer connectors and a smaller footprint, which usually means a lower price.
How the Micro-ATX form factor is defined
The Micro-ATX specification defines physical dimensions, mounting points and connector positions, not the electronics on the board. The maximum size is 244 mm square, but the standard allows boards to be narrower than the maximum, and many budget models measure around 226 x 183 mm or similar. What must stay fixed is the position of the rear I/O panel, the mounting holes that align with the case standoffs and the slot positions relative to the back of the case.
The depth of 244 mm matches the depth of a standard ATX board, so the difference is only in length. An ATX board is 305 mm long and carries seven slot positions; Micro-ATX removes the lower part of the board and keeps the top four slot positions at the same 0.8-inch (20.32 mm) slot pitch. The case therefore needs only four rear brackets, though most cases provide seven anyway.
The following table lists the main parameters that the specification and common practice fix for Micro-ATX boards.
| Parameter | Typical value | What it affects |
|---|---|---|
| Maximum board size | 244 x 244 mm | Case compatibility and component density |
| Expansion slot positions | Up to 4 | Number of PCIe add-in cards |
| Slot pitch | 20.32 mm (0.8 in) | Clearance for multi-slot coolers and cards |
| Rear I/O opening | 158.75 x 44.45 mm | Same I/O shield size as ATX |
| Main power connector | 24-pin ATX plus 4- or 8-pin CPU | Uses any standard ATX power supply |
| Memory slots | Usually 2 or 4 DIMM | Maximum capacity and channel population |
Micro-ATX compared with ATX and Mini-ITX
Micro-ATX sits between the two dominant desktop form factors, and the choice among them is mainly a trade-off between expansion and size. All three share the same rear I/O panel position and power connectors, so a smaller board fits in a larger case but never the reverse.
| Form factor | Maximum size | Slot positions | Typical memory slots |
|---|---|---|---|
| ATX | 305 x 244 mm | 7 | 4 |
| Micro-ATX | 244 x 244 mm | 4 | 2 or 4 |
| Mini-ITX | 170 x 170 mm | 1 | 2 |
| FlexATX | 229 x 191 mm | Up to 3 | 2 |
In practice, a Micro-ATX board offers the same CPU socket and chipset options as an ATX board of the same generation. Manufacturers often mark Micro-ATX variants with an M suffix in the model name, for example a B-series board ending in M. The reduced area leaves less room for M.2 slots, fan headers and VRM phases, so a Micro-ATX board typically has fewer of each than its ATX sibling, although high-end exceptions exist.
Compared with Mini-ITX, Micro-ATX gives up compactness but gains four DIMM slots on many models, a second or third PCIe slot and more space between components. The larger area also eases power delivery cooling and makes building and cable routing easier than in a small-form-factor system.
Expansion slots and the graphics card problem
The four slot positions of Micro-ATX look adequate on paper, but modern graphics cards reduce the usable count. A typical gaming card occupies two to three slot widths, so it covers one or two of the slots below the primary PCIe x16 slot. The remaining slot is often limited to x1 or x4 electrical lanes from the chipset.
The table below shows how a common four-position layout is used once a large graphics card is installed.
| Slot position | Common slot type | Status with a three-slot graphics card |
|---|---|---|
| Position 1 | PCIe x16 (CPU lanes) | Occupied by the graphics card |
| Position 2 | PCIe x1 or empty | Covered by the card cooler |
| Position 3 | PCIe x1 or empty | Covered by the card cooler |
| Position 4 | PCIe x16 physical, x4 electrical | Usable for one add-in card |
Some boards place the first slot in the second position to leave room for a large CPU cooler or an M.2 heatsink, which shifts the graphics card downward and can make it overhang the bottom edge of the board. In a Micro-ATX case with only four rear brackets, a three-slot card in the lowest position may not fit at all, so checking bracket count and slot placement before purchase is essential.
Choosing a Micro-ATX board for a build
A Micro-ATX board is the right choice when a system needs one graphics card and few other cards, which describes most home, office and gaming computers today. Since storage moved to M.2 NVMe drives and networking, audio and Wi-Fi moved onto the board, the extra slots of ATX often remain empty.
| Situation | Recommended form factor | Reason |
|---|---|---|
| Gaming PC with one graphics card | Micro-ATX | Full performance at lower cost and size |
| Office or home desktop | Micro-ATX | Enough ports and slots, cheaper boards |
| Workstation with several add-in cards | ATX or larger | More slots and more PCIe lanes |
| Very small living-room system | Mini-ITX | Smallest standard footprint |
| Heavy overclocking | ATX | Stronger VRM and more cooling area |
When comparing boards, check the number of DIMM slots, the count and generation of M.2 slots, the VRM design relative to the planned processor and the rear I/O connectivity. A Micro-ATX board with only two DIMM slots limits future memory upgrades, while one with four slots allows a later increase in capacity without replacing the existing modules.
Limitations and common mistakes
The most frequent mistake is assuming that every feature of an ATX board is present on the Micro-ATX variant with the same chipset. Manufacturers save cost on smaller boards, so the following points deserve attention before purchase.
- Weaker power delivery on entry-level boards can throttle high-core-count processors under sustained load.
- Fewer fan headers may require a fan hub in a case with many fans.
- Shared PCIe lanes can disable a SATA port or an M.2 slot when another device is installed.
- Small Micro-ATX cases may restrict graphics card length, cooler height or radiator size.
- Front panel connectors such as USB-C headers are sometimes omitted on budget models.
None of these limits is inherent to the form factor itself. They follow from cost targets and board area, and premium Micro-ATX boards can match ATX boards in power delivery and connectivity.
A plain-language summary
Micro-ATX is a standard size for the main circuit board inside a desktop computer. It is shorter than the common full-size board but uses the same screws, the same power cables and the same back-panel opening.
Think of it as a compact car built on the same platform as a larger model. It runs the same engine and parts but has fewer seats. For most people, who use one graphics card and nothing else, those missing seats are never needed, and the smaller board costs less.
An example from practice
A small design studio needs three new desktop workstations for photo editing. Each machine will have an eight-core processor, 64 GB of memory, two NVMe drives and one mid-range graphics card. The person assembling them compares an ATX board and its Micro-ATX sibling with the same chipset.
The Micro-ATX model offers four DIMM slots, two M.2 slots and one free PCIe slot below the graphics card, which covers every requirement. It costs noticeably less and fits a compact mid-tower that takes less desk space. The finished workstations deliver the same performance as an ATX build, and the spare slot later receives a 10-gigabit network card for faster file transfers to the studio server.