Motherboard Form Factors: ATX vs microATX vs Mini-ITX Sizes
A motherboard's form factor sets its size, where its mounting holes sit and which cases it fits. The three you'll meet most are ATX (305 × 244 mm), microATX (244 × 244 mm) and Mini-ITX (170 × 170 mm). CompTIA A+ Core 1 (220-1201) objective 3.5 lists them as ATX, microATX and ITX.
The sizes, drawn to scale
| Board | Size (mm) | Size (in) | Expansion slots | Typical case | In the V15 objectives? |
|---|---|---|---|---|---|
| ATX | 305 × 244 mm | 12 × 9.6 in | Up to 7 | Mid tower and full tower | Yes |
| microATX | 244 × 244 mm | 9.6 × 9.6 in | Up to 4 | Mini tower and many ATX cases | Yes |
| Mini-ITX | 170 × 170 mm | 6.7 × 6.7 in | Usually 1 | Small form factor cases, plus ATX and microATX cases | Yes |
| E-ATX | 305 × 330 mm | 12 × 13 in | Varies | Full towers and larger mid towers that list E-ATX support | No |
The specifications set maximum sizes, so a board can be smaller than its form factor allows. ATX and microATX are both up to 244 mm (9.6 in) deep; the width, 12 in against 9.6 in, is what tells them apart. E-ATX has no single size; the table shows the SSI EEB maximum.
What fits in which case
- Mini-ITX in an ATX or microATX case: Mini-ITX uses a subset of the ATX mounting holes, and Intel's Mini-ITX addendum says it can be installed in any ATX case.
- microATX in an ATX case: Intel's ATX specification says many ATX cases accept microATX boards. The microATX spec also adds two mounting points (R and S) that ATX doesn't use, so check that the case lists microATX support.
- Standoffs: remove or move any standoff that doesn't sit under a mounting hole in the board, so it can't short or scratch the back of the board.
- Rear I/O: all three use the same rear I/O panel opening, and each board has its own I/O shield, either in the box or built onto the board.
- Bigger boards don't fit smaller cases: an ATX board won't go in a case built only for microATX or Mini-ITX. The case's spec page lists the form factors it supports.
How to tell which form factor a board is
- Check the model. Find the model printed on the board, or in System Information (msinfo32, the BaseBoard Product line). In PowerShell, Get-CimInstance Win32_BaseBoard | Select-Object Manufacturer, Product shows the same thing. Look that model or part number up on the maker's site: Windows reports the model, not the form factor.
- Measure it. Measure the long side: about 12 in (305 mm) means ATX, 9.6 in (244 mm) or less means microATX, and a board 170 mm square is Mini-ITX.
- Count the expansion slots. Five or more means ATX, since microATX allows at most four and Mini-ITX usually has one. Boards often have fewer slots than their form factor allows, so treat the count as a clue, not proof.
- The older wmic baseboard command no longer works on current Windows 11 releases, because Microsoft has removed the wmic tool. Use PowerShell instead.
For the A+ exam (objective 3.5)
Objective 3.5 covers installing and configuring motherboards, CPUs and add-on cards, and its form factor list is ATX, microATX and ITX. In practice, ITX means Mini-ITX. The objective starts with "Given a scenario", so expect questions that put you in a build or upgrade, such as picking a board for a small case.
The same objective covers connector types, CPU sockets and BIOS/UEFI settings. The full lesson goes deeper on form factors, and the free Motherboard Identification PBQ has you name the parts of a board.
E-ATX and other sizes
- E-ATX (Extended ATX) has no Intel specification. The closest standard, SSI EEB, sets 305 × 330 mm (12 × 13 in), but makers label different sizes as E-ATX, and many are 12 by about 10.9 inches. Check the board's exact size against the case. E-ATX isn't in the A+ objectives.
- Nano-ITX and Pico-ITX are smaller boards from VIA, which introduced Mini-ITX in 2001: 120 mm square and 100 by 72 mm, mostly used in embedded systems.
- BTX, an Intel design from 2003, rearranged the board for better cooling but never replaced ATX. It isn't in the current A+ objectives.