Smaller Than microATX

There are several motherboard form factors smaller than microATX: FlexATX, DTX, Mini-DTX and Mini-ITX. They are all designed for "half-length" (175mm=6.9" long) or shorter PCI cards, but DTX and Mini-DTX retain microATX's large vertical clearance. FlexATX reduces the clearance to 58mm (2.3"). (Mini-ITX does not appear to specify vertical heights.) DTX is as deep as microATX.

Motherboard electronics have proven to fit onto Mini-ITX motherboards (170mm x 170mm, 6.7"x6.7"), although barely. Mini-ITX systems tend to use soldered CPUs, laptop RAM, single-channel RAM, and/or a smaller I/O panel. Most of them are intended for industrial use; they appear to use 6-layer PCBs rather than 4-layer PCBs common in consumer motherboards, which is acceptable in a relatively high-price niche market. Mini-DTX boards tend to add only a single-lane (x1) PCIe slot compared with Mini-ITX, because with an x16 slot they would hardly gain usable motherboard space, and do not seem to be available on the market (as of November 2007).

FlexATX

The FlexATX form factor seems to be the most attractive existing motherboard form factor for small PCs using desktop components. (Even smaller form factors are appropriate in combination with all-laptop components.) With 229mm x 191mm (9"x7.5") FlexATX is less wide and deep than microATX, it lowers the vertical clearance, and it provides more usable motherboard space than Mini-ITX and Mini-DTX. It allows for up to three expansion slots in a dedicated FlexATX case, or two expansion slots and even more motherboard space. (In a microATX case, an additional slot at the left edge is usable.)

Form Factor Comparison: Usable Motherboard Areas

Size of motherboard areas that allow 30.5mm (1.2") high parts or higher: Motherboard size minus PCI slot area ("area B"), I/O panel area (roughly 6.2"x0.75"), and other areas specified for very low parts height (FlexATX "area E"). Mounting holes are not counted. (Sizes are rounded to two digits.)

Usable Area; Height ≥1.2"	Slots	usable sq in	usable cm2
Mini-ITX	1	37	240
Mini-DTX	2	37	240
FlexATX	4*	43	280
FlexATX	3	48	310
DTX	2	60	390
microATX	4	55 [64]*	350 [410]

Note about FlexATX: One of the FlexATX rows assumes 4 expansion slots, the other one only 3 slots, adding 7"x0.8" of usable motherboard space. Note also that while a microATX case will accommodate up to 4 slots, a dedicated FlexATX case may not accommodate a card in the slot at the left edge. (That is, a FlexATX board with 3 slots may only provide 2 usable PCI slots when used in a FlexATX case.)

Note about microATX: There is a large part of "area B" (under expansion cards) which is usable for very low motherboard parts but not for a northbridge heat sink, memory slots or tall connectors. This area is about 9.4 sq in = 60cm² and included in the bracketed sizes.