Mini ITX vs Micro ATX vs ATX: Size and Compatibility Comparison

Mini-ITX measures 170 × 170 mm, Micro-ATX 244 × 244 mm, and ATX 305 × 244 mm — and a Micro-ATX board usually fits any ATX case, while Mini-ITX fits only compact cases built for it.

A board can measure perfectly and still refuse to seat, because a case needs the right standoff positions and a rear-I/O opening that lines up with your board’s back panel. The three mainstream sizes below cover the whole desktop market, and knowing the exact dimensions settles most “will this fit” arguments before you order anything.

Here is how the numbers differ, what each size costs you in expansion slots, and how to check a case before you buy.

The Exact Board Dimensions, Side By Side

ATX is the largest mainstream desktop format at 305 × 244 mm, Micro-ATX is a 244 × 244 mm square, and Mini-ITX shrinks that to a 170 × 170 mm square. VIA Technologies introduced Mini-ITX in 2001 specifically for compact systems, and it remains the smallest form factor that still uses standard ATX mounting holes.

The slot difference is the real story. Corsair’s form-factor explainer notes that Micro-ATX has three fewer expansion slots than standard ATX, and Mini-ITX typically gives you a single slot position. If you plan on a graphics card plus a capture card plus a sound card, the board size decision was already made for you.

What Actually Fits In Which Case

Micro-ATX boards almost always drop into ATX cases, and ATX cases frequently accept Mini-ITX as well, but Mini-ITX generally requires a case designed around it. A big case simply has more mounting holes and a larger tray, so a smaller board leaves empty space instead of failing.

The reverse never works. A 305 mm ATX board will not fit into a Mini-ITX chassis no matter how you angle it, so if you already own the board, start your case shopping from its dimensions.

Two fit problems survive even when the sizes match up. First, the case must have standoffs in the pattern your board uses. Second, the rear-I/O cutout has to align with your board’s back panel — a board can be exactly 170 × 170 mm and still not line up. Smaller boards also do not buy you internal clearance: a Mini-ITX build usually means tighter limits on graphics card length, cooler height, and cable routing than the same parts in an ATX tower. If you are weighing compact cases for a small build, this tested roundup of the best mini itx case options covers which ones actually clear a full-size GPU.

Which Form Factor Should You Buy?

Pick Micro-ATX when you want a compact machine without giving up expansion, and ATX when you want maximum room for cards and headers. Choose Mini-ITX only when small size is the priority itself, because you are trading slot count and clearance for footprint.

Micro-ATX is the quiet middle ground for most builders. It shares ATX’s 244 mm depth, fits a broad range of cases, and loses only three slots — a trade most single-GPU builds never notice. ATX remains the most common full-size desktop format for builds that need more room for cards and front-panel headers.

The latest published guidance sits in Intel’s ATX Version 3.0 design specification, which anchors the ATX baseline the other sizes are measured against.

One naming trap worth knowing: older ATX-family documents sometimes mention a “Mini-ATX,” which has nothing to do with modern Mini-ITX. If a listing says Mini-ATX, treat it as a historical label and check the actual millimeter dimensions instead.