In the process of building a PC, the motherboard—acting as the "backbone" for core hardware—its form factor directly determines the system's compatibility, expandability, and application scenarios. Motherboards of different form factors vary significantly in key parameters such as size, number of expansion slots, and mounting holes. As the mainstream motherboard form factor system, the ATX series covers a wide range of sizes from mini to extra-large, catering to diverse needs from office and home use to professional workstations. Based on Sohoo's professional technical guide, this article provides a detailed analysis of the core parameters and application value of ATX series motherboard form factors.
Standard ATX (often simply referred to as "ATX") is the most widely used foundational form factor in the PC industry, and the top choice for most mid-to-high-end home computers and gaming rigs. Its core parameters are clear: the length is fixed at 12 inches (approximately 304.8mm), while the width ranges from 6.7 to 9.6 inches (about 169.9 to 243.8mm), providing ample space for hardware layout.
In terms of expandability, standard ATX motherboards feature a maximum of 7 expansion slots, supporting the simultaneous connection of multiple peripherals such as discrete graphics cards, sound cards, network cards, and storage expansion cards. This easily meets the multi-hardware expansion needs of gamers and content creators. For mounting holes, up to 10 are available, with 4 marked in red as optional. This design ensures installation stability while leaving room for manufacturers' design flexibility.
Thanks to its balanced size and expandability, standard ATX motherboards are compatible with most ATX cases, making them a "one-size-fits-all" choice that balances performance and compatibility—and a mainstream benchmark in the DIY PC building market.
For users seeking a compact chassis, compact motherboards smaller than standard ATX offer flexible solutions. All are compatible with standard ATX cases, eliminating the need to replace the hardware platform. Ordered by decreasing size, they include the following three types:
Micro-ATX is the second most popular form factor after standard ATX, ideal for office use, entry-level gaming, and other scenarios with moderate expandability requirements. Its length is fixed at 9.6 inches (approximately 243.8mm), and the width also ranges from 6.7 to 9.6 inches, featuring a compact size while retaining core functions.
In terms of expandability, Micro-ATX motherboards have a maximum of 4 expansion slots—3 fewer than standard ATX—sufficient for daily needs such as a single graphics card or one storage expansion. For mounting holes, up to 8 are available: 2 marked in green are optional for motherboard manufacturers, while those marked in red can be omitted if the board size is small enough to not require them. As a representative of balancing cost-effectiveness and compactness, Micro-ATX motherboards are widely used in mini PCs and office all-in-ones.
The DTX form factor is more compact than Micro-ATX, with a fixed length of 8 inches (approximately 203.2mm) and a width ranging from 6.7 to 9.6 inches. Mini-DTX is a sub-form factor of DTX, specifically referring to DTX motherboards with a width of 6.7 inches (about 169.9mm)—and the more common type in the DTX series.
In terms of expandability, DTX series motherboards offer a maximum of only 2 expansion slots, meeting only core hardware expansion needs. They are suitable for scenarios with strict size constraints, such as mini PCs and embedded devices. Up to 6 mounting holes are available, with red-marked holes omitted based on size requirements to further optimize compactness.
Mini-ITX is the smallest form factor in the ATX series, featuring a square design with both length and width fixed at 6.7 inches (approximately 169.9mm). It is the core choice for mini PC builds and HTPCs (Home Theater PCs).
Despite its extremely compact size, Mini-ITX retains full core functionality, with a maximum of 1 expansion slot supporting a single graphics card or other necessary expansion devices. The mounting hole design is highly standardized: all Mini-ITX motherboards use 4 mounting holes without exceptions, ensuring high compatibility with mini cases. Its small size and low power consumption make it widely used in living room PCs, portable workstations, and embedded systems.
For professional scenarios requiring multiple CPUs and expansion devices (such as servers, graphics workstations, and high-performance computing), extended motherboards larger than standard ATX provide sufficient hardware support. These motherboards are not compatible with standard ATX cases but support backward compatibility with standard ATX and smaller form factors. Ordered by increasing size, they include the following three types:
SSI-CEB (Server System Infrastructure Compact Electronics Bay) is a form factor for entry-level dual-CPU systems. It shares the same 12-inch length as standard ATX but has an expanded width of 9.6 to 10.5 inches (about 243.8 to 266.7mm)—wider than standard ATX—to accommodate dual-CPU layouts.
In terms of expandability, SSI-CEB retains 7 expansion slots (on par with standard ATX), supporting professional devices such as multi-graphics cards and multi-storage expansion cards. Up to 8 mounting holes ensure stability for the power supply and heat dissipation needs of dual-CPU systems. This form factor is common in entry-level servers and dual-socket workstation motherboards. For example, some ASUS ROG series motherboards (12×10.6 inches), although slightly larger than the SSI-CEB standard, are still compatible with CEB-compliant cases (e.g., RAVEN RV02, Fortress FT02).
SSI-EEB (Extended Electronics Bay) is the mainstream form factor for high-end dual-CPU systems. It also has a 12-inch length but a further expanded width of 10.5 to 13 inches (about 266.7 to 330.2mm), providing ample space for dual CPUs, multiple memory modules, and multiple expansion devices.
The number of expansion slots remains 7, meeting the needs of simultaneous connection of professional graphics cards, high-speed network cards, storage array cards, and other devices. Up to 11 mounting holes significantly enhance motherboard stability under high-load scenarios. This form factor is widely used in high-end servers and professional graphics workstations, serving as the core choice for high-performance dual-CPU platforms.
SSI-MEB (Maximum Electronics Bay) is the top-tier form factor in the ATX ecosystem designed for quad-CPU systems. It has an extended length of 16 inches (approximately 406.4mm) and a fixed width of 13 inches (about 330.2mm). Compared to SSI-EEB, it features additional length at the top to accommodate two extra CPU sockets.
The number of expansion slots remains 7, sufficient to support professional multi-device expansion. Up to 14 mounting holes ensure the stable operation of quad-CPU systems. This form factor is mainly used in ultra-high-performance servers, large computing nodes, and other professional scenarios to meet extreme computing power needs.
In addition to standard ATX series form factors, some manufacturers launch non-standard motherboards for special applications. Common types include:
The XL-ATX form factor has a length of 13.5 inches (approximately 342.9mm) and a width of 10.3 inches (about 261.6mm). It offers 9 expansion slots and up to 12 mounting holes. Slightly larger than standard ATX with enhanced expandability, it is common in high-end gaming rigs and enthusiast-grade workstation motherboards.
HPTX is one of the larger non-standard form factors, with a length of 13.6 inches (approximately 345.4mm) and a width of 15 inches (about 381mm). It also features 9 expansion slots and up to 17 mounting holes. Designed for extreme performance needs, this form factor is common in flagship servers and professional computing equipment.
Office/Home Use/Entry-Level Gaming: Prioritize Micro-ATX or standard ATX for a balance of cost-effectiveness and expandability, compatible with most cases and hardware.
Mini PC/HTPC: Mini-ITX is the first choice—extremely compact yet fully functional, suitable for space-constrained scenarios.
Professional Workstation/Dual-CPU System: Choose SSI-CEB for entry-level needs and SSI-EEB for high-end requirements to ensure stable operation of multiple CPUs and expansion devices.
Ultra-High-Performance Computing/Quad-CPU System: SSI-MEB is the only option to meet extreme computing power demands.
Enthusiast-Grade Custom Builds: Consider non-standard form factors such as XL-ATX and HPTX, paired with custom cases to achieve exclusive performance and aesthetics.
Selecting a motherboard form factor essentially involves balancing compatibility and expandability. Understanding the core differences between ATX series and non-standard form factors helps users accurately match their needs during PC building, avoid hardware compatibility issues, and reserve sufficient space for future upgrades. Whether for daily use or professional scenarios, there is always a motherboard form factor that perfectly fits your build requirements.
