ATX Motherboard Dimensions: Full Size Guide

ATX motherboards measure 305 mm × 244 mm (12.0 × 9.6 inches), a standard set by Intel in 1995. This size defines the full-ATX form factor, and it determines which cases, power supplies, and expansion slots you can use. Below is everything you need to know about these dimensions, their variations, and how to work with them.

Formula

The dimensions define a rectangle. The area is width × height, and the diagonal is the square root of the sum of the squares (Pythagorean theorem).

• Width (w) – the shorter side, 244 mm (9.6 in)
• Height (h) – the longer side, 305 mm (12.0 in)
• Area – w × h in mm², divided by 100 to get cm²
• Diagonal – √(w² + h²) in mm

Worked Example

Using the default values (244 mm × 305 mm):

1. Area = 244 × 305 = 74,420 mm²
2. Convert to cm²: 74,420 / 100 = 744.2 cm²
3. Diagonal = √(244² + 305²) = √(59,536 + 93,025) = √152,561 ≈ 390.6 mm

The calculator shows 744 cm² and 391 mm, matching these results.

Why 305×244 mm?

Intel introduced ATX in 1995 as an evolution of the older AT form factor. The goal was to improve airflow, simplify cable routing, and allow the CPU to be placed away from expansion slots. The 305×244 mm rectangle became the standard, and nearly every full-ATX case, power supply, and mounting hole pattern follows it. The size also allows for 7 expansion slots (the metal brackets on the rear of the case), which is the most of any mainstream consumer form factor.

ATX vs. Other Form Factors

ATX is the largest common motherboard, but there are smaller variants that share the same mounting holes and connector layout. Here is how they compare:

• E-ATX – Extended ATX, typically 305×330 mm (12×13 in), but some are wider. Not all ATX cases fit E-ATX.
• Micro-ATX (mATX) – 244×244 mm (9.6×9.6 in). Fits in most ATX cases.
• Mini-ITX – 170×170 mm (6.7×6.7 in). Much smaller, usually 1 expansion slot.
• ATX (full) – 305×244 mm, the subject of this article.

All of these share the same front-panel connectors and power supply standards, so you can often swap between them in the same case (if the case supports the size).

How to Measure and Check Compatibility

Before buying a motherboard or case, you need to verify the dimensions physically. Here are the steps:

1. Measure the case’s interior width and height. Look for the specification sheet, or use a tape measure.
2. Check the mounting hole pattern. ATX boards have 9 screw holes, but the exact positions are standardized. Most cases have standoffs that you can move.
3. Verify the expansion slot cutouts. Full ATX needs 7 slots on the rear panel. If your case has only 4 or 5, a full ATX board won’t fit.
4. Check for clearance around the CPU cooler and RAM. The board dimensions are fixed, but the components on top can be tall.

Common Mistakes and Misconceptions

One common error is assuming that a case labeled “ATX� automatically fits every ATX board. Some cases are actually “ATX� but only support mATX or Mini-ITX. Another mistake is measuring the board from the edge of the I/O shield to the front edge – that is not the standard. The 305 mm dimension is the depth (front to back), and 244 mm is the width (side to side). Always check the official spec sheet from the manufacturer.

Another misconception is that E-ATX boards are the same width as ATX. They are often wider, so they may not fit in a standard ATX case even if the length is the same.

Costs and Practical Considerations

Full ATX boards tend to be more expensive than smaller ones because they offer more features: more PCIe slots, more M.2 slots, more USB ports, and better VRM cooling. However, the price difference is not just about size – it’s about the chipset and features. If you don’t need many expansion cards, a smaller board might save money and space. But if you plan to use multiple GPUs, capture cards, or NVMe drives, the extra space is worth it.

Frequently Asked Questions

Can I fit an ATX motherboard in a Micro-ATX case?

No. Micro-ATX cases are shorter (front to back) and have fewer expansion slots. A full ATX board will not fit. You need a case that explicitly supports ATX.

What is the difference between ATX and E-ATX?

ATX is 305×244 mm. E-ATX is usually 305×330 mm, but the width can vary. E-ATX boards have more room for extra PCIe lanes, memory slots, and VRM components, but they require a case with extra clearance.

Do all ATX cases have the same mounting holes?

Yes, the 9 mounting holes for ATX are standardized. However, some cases may have additional holes for E-ATX or other variants. Always check the case’s manual for the correct standoff positions.

How many expansion slots does an ATX motherboard have?

Standard ATX has 7 expansion slots (the metal brackets on the rear). This is the maximum for consumer boards. Some boards may have fewer physical slots, but the 7-slot layout is the standard.

Is a full ATX motherboard better than a Micro-ATX?

Not necessarily. Full ATX offers more room for expansion and often better power delivery, but for most users, a Micro-ATX board is sufficient. The choice depends on how many expansion cards you need and the size of your case.

Can I use an ATX power supply with a Micro-ATX motherboard?

Yes. ATX power supplies are compatible with Micro-ATX and Mini-ITX boards, as long as the case can physically fit the PSU. The 24-pin and 8-pin connectors are the same.

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in w | Width | mm | 244 | 0
in h | Height | mm | 305 | 0
out area | Board area | cm² | (w * h) / 100 | 0
out diag | Diagonal | mm | sqrt(w^2 + h^2) | 0