What is SFX?
In PC hardware, SFX is a power supply form factor, formally SFX12V, defined by Intel for small form factor systems. An SFX power supply does the same job as an ATX one, converting wall AC into the DC rails the PC needs, in a much smaller box. You'll find it in Mini-ITX and other compact cases that don't have room for an ATX unit.
SFX vs. ATX size
| Form factor | Size (W x H x D) | Typical use |
|---|---|---|
| ATX | 150 x 86 mm, usually 140 mm deep or more | Standard desktops and towers |
| SFX | 125 x 63.5 x 100 mm | Mini-ITX and other small cases |
| SFX-L | Same width and height as SFX, but deeper | Small cases with a bit more room; fits a larger, quieter fan |
The output connectors are the same as on an ATX unit (24-pin main, CPU power, PCIe, SATA), so an SFX PSU works with ordinary motherboards. Many ATX cases can mount an SFX unit with an adapter bracket, but an ATX unit won't fit a case built only for SFX.
Choosing and installing an SFX PSU
- Check the case. The case, not the motherboard, decides which PSU size fits. Mini-ITX cases may take SFX, SFX-L or full ATX.
- Wattage: size it for the CPU and graphics card like any PSU, and check the case's graphics card clearance at the same time.
- Cables: modular SFX units are common because spare cables have nowhere to go in a small case. Shorter cables help too.
- Noise and heat: a small fan has to spin faster to move the same air, so SFX units can be louder under heavy load. SFX-L's larger fan helps.
Other small PSU sizes
SFX isn't the only compact format. Slim office desktops often use TFX or Flex ATX power supplies, which have their own shapes and mounting. They aren't interchangeable with SFX, so match a replacement to the original form factor.
For the CompTIA A+ exam
SFX isn't named in the Core 1 (220-1201) objectives, which list ATX, microATX and ITX motherboard form factors under objective 3.5 and power supply installation under objective 3.6. In a small-build scenario, remember that a Mini-ITX case may need a smaller SFX power supply, and that wattage, modular cabling and connectors still have to match the build.