ATX vs SFX Power Supplies
ATX is the full-size standard and SFX is the small-form-factor one. An SFX unit is physically smaller in every dimension, which is the entire point of it: it exists so a compact case can hold a real power supply. An SFX unit will mount in an ATX case with an adapter bracket, but an ATX unit will not fit an SFX bay at all, so the case decides this, not preference.
Buy the form factor your case takes. If the case accepts ATX, take ATX: it costs less per watt, the bigger fan has an easier time staying quiet, and nothing about it is worse. Choose SFX when the case requires it, and accept that you are paying for the size rather than for the power supply.
The physical difference
These are fixed standards, the same for every unit of that size.
Will it fit your case?
Two things the standard alone does not tell you.
The clearance figure is a second number
Your case names a power supply standard and, separately, a maximum depth. The standard says what can mount; the clearance figure says how deep a unit may actually be once the cable run and the drive cage are in the way. Both come from the case maker, and depth is the one people skip.
Cable length matters in small builds
Small-form-factor units ship with shorter cables because that is what a compact case needs. If you put one in a full-size case, check that the motherboard and CPU cables reach, since those two runs are the longest in any build.
The difference in practice
How the two actually differ across units you can buy today, rather than what the standard permits. Measured across 442 ATX and 29 SFX units with a live offer at the retailers we track, and updated daily.
Depth and noise are quoted against their own samples because neither is published for every unit, and assuming a figure for the ones that do not publish one would change the answer.
The picks on each side
The cheapest unit on each side, and the cheapest one that clears a bar worth clearing: at least 40 of 100 on the quality score. Recalculated daily from live prices.
ATX
SFX
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Common questions
Is an SFX power supply less powerful than an ATX one?
Not any more. SFX units are available well past 1000 W, which is more than a single-graphics-card build needs. The limits you actually run into are physical: fewer cable outlets and a smaller fan, rather than a wattage ceiling.
Are SFX power supplies louder?
They can be, because a smaller fan has to spin faster to move the same air. Plenty of SFX units still earn top noise grades in lab testing, so it is worth checking the measured grade for the specific unit rather than assuming.
Why do SFX power supplies cost more?
You pay for the engineering that fits the same output into a smaller box, and the volumes are lower than for ATX. Compare the price per watt of the two and the gap shows up clearly.
Keep going
Browse ATX units or SFX units with every filter, read up on how much wattage you need, or start from the power supply comparison hub.