What Is an SFX PSU? Small Form Factor Power Supplies Explained

An SFX PSU is a compact power supply measuring 125 mm wide, 63.5 mm tall, and 100 mm deep, built for mini-ITX and other small-form-factor cases that cannot fit a standard ATX unit.

An SFX PSU exists because mini-ITX cases have no room for a full-size power supply. Corsair’s power supply form factor guide draws the line clearly: SFX units are intended for mini-ITX builds and similar small enclosures, while ATX units are physically much larger. That size gap is the whole point of the standard — it is what lets a 10-liter case close its side panel.

The dimensions above come straight from Corsair’s published spec: 125 mm × 63.5 mm × 100 mm. SilverStone, which helped define the category, lists the same numbers. Memorize those three figures and most SFX compatibility questions answer themselves.

What Do The SFX and SFX-L Labels Actually Mean?

SFX is the standard size; SFX-L is the same width and height but 30 mm longer, at 125 mm × 63.5 mm × 130 mm.

SilverStone created SFX-L in 2014 alongside the SX500-LG announcement. The extra 30 mm buys room for a larger fan and more internal components, which is why higher-wattage SFX-L units exist. It costs you 30 mm of case depth, and in a tightly packed build that can be the difference between a clean install and a cable jammed against a drive cage.

Treat the two as separate form factors. A case listed for SFX may not clear an SFX-L unit, even though the mounting holes line up.

What Specs Matter On A Real SFX Unit?

Wattage, ATX version support, fan size, and modular cabling are the specs that decide whether a unit works in your build — not the form factor label alone.

Current models show how wide the range runs.

Two things to notice. First, fan size varies — 80mm and 92mm both appear in current SFX designs, so never assume. Second, version labels are not interchangeable: sources list SFX12V 3.3, SFX12V 4.0, ATX v3.0, and ATX12V 3.1 across different products. Quote the exact version printed on the manufacturer’s page for the model you are buying.

Corsair’s product pages list AC input of 100–240V for the SF750 and SF850, so those units handle both North American and European wall voltage without a switch. If you are shopping for a 750W unit specifically, our roundup of the best 750W SFX PSUs puts individual models side by side.

Will An SFX PSU Fit Your Case?

Wattage alone never guarantees a fit — case clearance depends on the unit’s length or depth and the space left for cables.

Work through this order before buying:

1. Check the case manual for supported PSU form factors. If it says SFX only, skip SFX-L.
2. Measure the PSU bay depth. Standard SFX needs 100 mm; SFX-L needs 130 mm.
3. Budget cable space. Fully modular units let you skip unused cables, which matters more in small cases than in towers.
4. Match the wattage to your GPU and CPU, then confirm the ATX version your motherboard expects.

Two safety notes worth knowing. be quiet! rates the SFX L Power 600W for operation up to 40°C, so a cramped case with poor airflow can push a PSU past its comfortable range. And protections like OCP, OVP, SCP, OPP, and OTP are not marketing filler — they are what stops a short circuit from taking your motherboard with it. Corsair’s form factor guide is the source that explains the ATX-versus-SFX size gap directly: Corsair’s power supply form factor guide.