Motherboard form factors determine case compatibility, expansion capacity, and cooling clearance — worth understanding before buying either component, since a mismatch is a genuinely common and avoidable build mistake.
The Main Form Factors
- ATX (305×244mm) — the standard, most common size, offering the most PCIe slots and RAM slots for a typical build
- Micro-ATX (244×244mm) — a genuine size reduction with somewhat fewer expansion slots, fits smaller cases while still supporting a full-size GPU
- Mini-ITX (170×170mm) — genuinely compact, typically just one PCIe slot and two RAM slots, for small form factor builds
- E-ATX (larger than standard ATX) — for high-end workstations needing extra space for multiple GPUs or extensive VRM cooling
What You Actually Trade Off Going Smaller
- Fewer RAM slots (Mini-ITX is typically limited to 2, capping total memory capacity more than a full ATX board)
- Fewer PCIe slots for expansion cards beyond the GPU
- Often somewhat less robust VRM (power delivery) cooling, a genuine consideration for high-end CPUs under sustained load
Always verify your specific case’s supported form factors before purchasing a motherboard — cases explicitly list which form factors they physically fit, and a Micro-ATX board will fit in an ATX case, but not the reverse.