Definition: Form factor defines the physical dimensions and mounting standard of a desktop motherboard and the case that houses it. The three main consumer form factors are ATX (full-size), Micro-ATX (mid-size), and Mini-ITX (compact). Form factor determines the number of expansion slots, case compatibility, and how much upgrade headroom exists in a build.
Why it matters for desktop upgrades
Before upgrading a PSU, adding a GPU, or fitting a new CPU cooler, you need to know your case’s form factor. A large air cooler may not fit in a Micro-ATX case. A full-length GPU may not clear the PSU in a compact case. PSU sizes also vary — standard ATX PSUs fit most mid and full towers; SFX PSUs are required for small form factor cases.
Form factor comparison
- ATX (305×244mm): Full-size. Up to 7 PCIe slots, 4 RAM slots, multiple M.2 slots. Best upgrade headroom. Fits mid-tower and full-tower cases.
- Micro-ATX (244×244mm): Mid-size. Up to 4 PCIe slots, 4 RAM slots. Good balance. Fits mid-tower cases and dedicated mATX cases.
- Mini-ITX (170×170mm): Compact. 1 PCIe x16 slot, 2 RAM slots. Very limited expansion. Requires SFF case. Popular for living room or portable builds.
PSU form factors
- ATX PSU: Standard size. Fits all mid and full-tower cases. Widest range of wattage and quality options.
- SFX PSU: Small form factor. Required for compact cases like the NZXT H1 or Fractal Terra. Fewer options, generally more expensive per watt.
- SFX-L PSU: Slightly longer than SFX, fits more efficient fan. Better cooling than standard SFX.
Common mistakes
- Buying a Micro-ATX board for a Mini-ITX case. It will not fit — mounting hole patterns differ between form factors.
- Installing an ATX PSU in an SFX case. Physically impossible — check case PSU clearance specifications before purchasing.
- Underestimating GPU length. High-end GPUs like the RTX 4090 are 336mm long. Many mid-tower cases max out at 340mm — measure before buying.
Related terms
- Motherboard — form factor defines motherboard size
- PSU — PSU form factor must match the case
- PCIe — number of PCIe slots varies by form factor
Not sure what fits in your case?
BreakFixNow checks form factor and clearance compatibility before installing any component. Walk in — no appointment needed.
BreakFixNow checks form factor and clearance compatibility before installing any component. Walk in — no appointment needed.