ATX (Advanced Technology eXtended) is the form factor specification for desktop motherboards that defines the board dimensions of 305 × 244 mm, the position of the mounting holes, the rear input/output panel and the power supply connectors. Among motherboards in desktop hardware, ATX is the reference size against which every other board is measured, and its rules let a board, a case and a power supply from different manufacturers fit and work together.
Intel introduced ATX in 1995 as a replacement for the older AT and Baby AT layouts. Its core geometry has remained compatible for three decades, while the power connectors and electrical rules have been revised many times, most recently in the ATX 3.x power supply guidelines.
What the ATX specification defines
The ATX specification is a mechanical and electrical contract between three parts of a desktop computer: the motherboard, the case and the power supply unit (PSU). It does not describe the chipset, the CPU socket or the memory type, which change with every platform generation. It describes only what must stay constant so that parts remain interchangeable.
The following table lists the main elements the standard fixes and why each one matters to builders.
| Element | What ATX specifies | Practical effect |
|---|---|---|
| Board size | Up to 305 × 244 mm (12 × 9.6 in) | Determines case compatibility |
| Mounting holes | Fixed standoff positions, up to nine | Board screws into any ATX case |
| Rear I/O shield | Cutout of about 159 × 44 mm | Ports align with the case opening |
| Expansion slots | Up to seven slots at 20.32 mm pitch | Cards align with case brackets |
| Main power connector | 24-pin connector (20-pin in early versions) | Delivers +12 V, +5 V, +3.3 V and standby |
| Standby power | +5 VSB rail and soft power-on signal | Enables software shutdown and wake events |
A key innovation of ATX was soft power control. Instead of a mechanical mains switch, the front-panel button sends a signal to the motherboard, which tells the PSU to turn on through the PS_ON line. The +5 V standby rail keeps a small part of the board powered at all times, which makes wake-on-LAN, keyboard power-on and operating-system shutdown possible.
ATX power connectors and their evolution
The power side of ATX has changed far more than the board geometry. As processors and graphics cards moved most of their consumption to the +12 V rail, the specification added dedicated auxiliary connectors and tightened voltage tolerances. Modern boards draw almost all CPU power through 8-pin EPS12V connectors, which feed the VRM that converts 12 V to the roughly 1 V that the processor core needs.
The table below summarizes the most important revisions and what each one changed.
| Revision | Main change | Why it mattered |
|---|---|---|
| ATX 1.x (1995) | 20-pin main connector, soft power | Replaced AT mains switch design |
| ATX12V 1.x (2000) | Added 4-pin +12 V CPU connector | Supported higher CPU power draw |
| ATX12V 2.x (2003) | 24-pin main connector, PCIe power | Added current for PCIe slots |
| ATX 3.0 (2022) | 12VHPWR connector, power excursion rules | Handled transient GPU spikes |
| ATX 3.1 (2023) | 12V-2×6 connector with shorter sense pins | Reduced risk of partial insertion |
ATX 3.0 and 3.1 matter mainly for graphics cards. They require a PSU to tolerate short power excursions of up to 200 percent of rated output, because modern GPUs draw sharp transient peaks lasting microseconds to milliseconds. The 12V-2×6 connector can deliver up to 600 W through a single cable, but it must be fully seated to work safely.
ATX compared with other motherboard form factors
ATX sits in the middle of a family of related form factors. Micro-ATX and Mini-ITX reuse a subset of the ATX mounting holes and the same I/O shield position, so a smaller board fits inside a larger ATX case. The reverse is not true: a full ATX board does not fit into a case built only for smaller boards.
| Form factor | Maximum size | Typical expansion slots | Typical use |
|---|---|---|---|
| E-ATX | 305 × 330 mm (not strictly standardized) | 7 or more | Workstations, dual-socket boards |
| ATX | 305 × 244 mm | Up to 7 | Mainstream and enthusiast desktops |
| Micro-ATX | 244 × 244 mm | Up to 4 | Budget and compact desktops |
| Mini-ITX | 170 × 170 mm | 1 | Small form factor builds |
E-ATX is a loose label, not a single published standard. Different vendors use widths between roughly 264 and 280 mm or more, so a board marketed as E-ATX may or may not fit a given case. Builders should compare the exact board dimensions with the case support list rather than rely on the name.
Choosing an ATX board and case
Choosing ATX is mainly a question of expansion needs versus physical size. The extra board area gives room for more PCIe slots, more M.2 slots, larger heatsinks on the power stages and more fan and USB headers. In return, it requires a mid-tower or full-tower case and usually costs more than an equivalent smaller board.
| Situation | Recommended form factor | Reason |
|---|---|---|
| Multiple GPUs or add-in cards | ATX or E-ATX | Needs several full-length slots |
| High-core-count CPU with overclocking | ATX | Room for larger VRM and cooling |
| Single GPU, standard workloads | Micro-ATX or ATX | Both provide enough slots |
| Compact living-room PC | Mini-ITX | ATX would require a much larger case |
Before buying, check three compatibility points: the case must list ATX support, the PSU must provide the correct number of EPS12V connectors for the board, and the CPU cooler and graphics card must fit the clearance limits of the case. The ATX label guarantees that the board mounts correctly, not that every component fits around it.
Common ATX mistakes and misconceptions
Most problems with ATX builds come from assuming that the form factor guarantees more than it actually does. The following points cover the frequent errors.
- Missing standoffs under the board, or an extra standoff in the wrong place, can short the board against the case.
- Connecting only one 4-pin CPU connector on a board that expects 8+8 pins can limit power delivery or prevent booting.
- ATX does not mean the same chipset or socket; two ATX boards can support entirely different processors.
- A partially seated 12V-2×6 or 12VHPWR plug can overheat; the connector must click fully into place.
- Back-connector designs that move cable headers to the rear of the board need a case designed for them, even though they keep ATX dimensions.
Put simply
ATX is a shared set of measurements for the main circuit board inside a desktop computer. It sets the size of the board. It fixes where the screws go and where the ports appear at the back. It also defines how the board plugs into the power supply.
Every maker follows the same measurements. As a result, a board, a case and a power supply from different companies fit together. Standard paper sizes work the same way: any printer can handle a sheet of letter paper. The key point is simple. ATX controls the fit and the power plugs, not the speed of the computer.
In practice
A video editor builds a workstation around a 16-core desktop processor, a large graphics card, a capture card and a 10-gigabit network card. A Mini-ITX board offers only one slot, and a Micro-ATX board leaves too little space between cards for airflow, so the editor chooses an ATX board with three full-length PCIe slots and four M.2 sockets.
The board mounts on the standard nine standoffs of a mid-tower ATX case. An ATX 3.1 power supply rated at 1,000 W supplies the 24-pin main connector, two 8-pin EPS12V cables to the processor power stages and a single 12V-2×6 cable to the graphics card. The result is a system that assembles without adapters, keeps cards spaced for cooling and can absorb the short power spikes of the graphics card without unexpected shutdowns.