Motherboard form factors: ATX, micro-ATX and mini-ITX

In short: The form factor defines the size and mounting points of a motherboard and determines which case it fits and how many slots it offers.

In more detail: Motherboards come in standardised sizes. If you buy case, motherboard and power supply to match, parts from different manufacturers can be combined. The size affects expandability, cooling and price.

In Depth

The common formats

Format	Size (approx.)	Typical slots
E-ATX	305 × 330 mm	many, for workstations
ATX	305 × 244 mm	usually 4 RAM, 5–7 expansion slots
Micro-ATX (mATX)	244 × 244 mm	usually 4 RAM, up to 4 slots
Mini-ITX	170 × 170 mm	usually 2 RAM, 1 slot (PCIe x16)
Mini-ITX is very compact, often with only two RAM slots and one graphics-card slot; in return it allows a small case.

Compatibility

A larger case accepts smaller boards (ATX cases also take mATX and mini-ITX), not the other way round. The mounting holes (standoffs) must match the format. The power supply has its own format (ATX, SFX for small cases). Also check the height of the CPU cooler and the length of the graphics card in the case.

Selection criteria

How many slots and ports do you need? M.2 slots for SSDs, SATA ports, USB ports, Wi-Fi and network card, number of RAM slots. Smaller boards have fewer of these and are often more expensive per function. An ATX board offers the best expandability and easier cooling through more space; mini-ITX fits living-room PCs and LAN-party computers.

Other formats

Existed earlier: BTX (Intel, did not catch on). For servers and industry there are formats of their own (SSI-EEB, mini-DTX). Laptops and mini PCs often use their own, permanently installed boards. After choosing processor and socket (for example AM5, LGA1700), the chipset (B650, Z790 …) determines the features and the form factor the size.

See also: Interplay of CPU, memory and motherboard, Power supply and 80 Plus, PCIe