SFX and ATX are two different physical sizes of power supply, not different technical standards. ATX is the standard size fitting most mid-tower and full-tower cases, while SFX is meaningfully smaller, designed specifically for compact and small-form-factor builds where space is limited.
- SFX and ATX differ in physical size, not electrical standards or quality.
- ATX fits standard mid-tower and full-tower cases; SFX fits compact builds.
- SFX PSUs often cost more than similarly specced ATX units.
- An SFX-to-ATX bracket adapter lets an SFX PSU fit in a standard case.
The Physical Size Difference
A standard ATX power supply measures roughly 150mm x 86mm x 140mm, the size assumed by most mid-tower and full-tower cases. SFX power supplies are considerably smaller, roughly 125mm x 63.5mm x 100mm, designed to fit small-form-factor cases where every millimeter of internal space matters.
Does Size Affect Performance?
Not inherently. SFX power supplies can offer the same wattage, efficiency ratings, and build quality as their ATX counterparts, according to component reviews from TechPowerUp. The smaller size means tighter internal component packing and often a smaller fan, which can result in slightly higher fan noise at a given load compared to a similarly rated ATX unit with more room to breathe.
Why SFX Costs More
SFX power supplies typically cost more than a similarly specced ATX unit, since the smaller size demands more precise manufacturing and tighter component packing. This premium is generally accepted as a necessary tradeoff for small-form-factor builders who need the compact footprint.
Can You Use an SFX PSU in an ATX Case?
Yes, with an inexpensive bracket adapter that lets the smaller SFX unit mount securely in a standard ATX-sized bay. This is occasionally done deliberately to free up extra case space for cooling or cable management, even in a standard-sized case.
Can an ATX PSU Fit an SFX Case?
No, an ATX power supply is too large to fit a case specifically designed for the smaller SFX form factor. If you’re building in a small-form-factor case, check its specifications carefully, since some support only SFX, some support SFX-L (a slightly longer SFX variant), and some support full ATX depending on the case’s internal design.
Which Should You Choose?
If you’re building in a standard mid-tower or full-tower case, ATX is the straightforward, more affordable, and widely available choice. Only choose SFX if your specific case requires it, following the general compatibility approach in how to choose PC parts that work together.
Tip: always check your case’s specifications for exactly which PSU form factors it supports before purchasing, since guessing based on case size alone can lead to a PSU that simply doesn’t fit.
See also what power supply do I need and how to install a power supply once you’ve confirmed your case’s form factor. Reviews from TechPowerUp and Tom’s Hardware cover specific SFX models in more depth.
Frequently Asked Questions
Is an SFX power supply worse than ATX?
No, SFX can match ATX in wattage, efficiency, and quality; it’s purely a smaller physical size for compact builds.
Can I use an SFX PSU in a standard case?
Yes, with an inexpensive bracket adapter that mounts the smaller unit securely in a standard ATX-sized bay.
Why do SFX power supplies cost more?
The smaller size requires more precise manufacturing and tighter component packing, which typically raises the price compared to similar ATX units.
Can I fit an ATX PSU in a small-form-factor case?
No, ATX is too large for cases specifically designed around the smaller SFX or SFX-L form factor.
