Short answer: check two things in your case manual: which form factor it takes (ATX, SFX or SFX-L) and the maximum power supply length. Then leave room for the cables to bend out of the back of the unit.
Step 1: the form factor
| Size | Width × height × depth |
|---|---|
| ATX | 150 × 86 × 140 mm or longer |
| SFX | 125 × 63.5 × 100 mm |
| SFX-L | 125 × 63.5 × 130 mm |
Most mid-tower cases take ATX. Small form factor cases usually take SFX or SFX-L. See the best SFX power supplies.
Step 2: the length
ATX units all share the same width and height, but length varies a lot. Your case manual will list a maximum. A few examples from my picks:
| Power supply | Length to note |
|---|---|
| Seasonic Core GX ATX 3 | 140mm, compact |
| Corsair HXi (2025) | 200mm, Tom’s Hardware warns it may not fit every case |
| Corsair RMx Shift | Side sockets and extra depth need side clearance |
High-wattage units tend to be longer. If your case is tight, compare lengths before you buy.
Step 3: cable space
- Modular cables plug into the back of the unit, so they add length. Leave a few centimetres for them to bend.
- The drive cage or a front-mounted radiator can steal space in the basement area.
- With side-socket units like the RMx Shift, check the gap between the unit and the side panel.
Step 4: fan direction
If your case has a filtered vent under the power supply, you’ll usually mount it fan down. See PSU fan up or down.
Compact picks
- Compact ATX · Tier ASeasonic Core GX ATX 380 Plus Gold · Fully modular · ATX · 650/750/850WOnly 140mm deep.Check 750W price
- SFX, with an ATX bracket · Tier ACorsair SF Platinum (2024)80 Plus Platinum · Fully modular · SFX · 750/850/1000WFits SFX cases, and ATX cases with the included bracket.Check 850W price
Questions
Are all ATX power supplies the same size?
Same width and height, but length varies. Check your case’s maximum.
Can I fit an SFX power supply in an ATX case?
Yes, with an SFX-to-ATX bracket. Some SFX units include one.
Where do I find my case’s maximum power supply length?
In the case manual or on the maker’s spec page.
Is a longer power supply better?
Not in itself. Longer units can fit bigger fans or more parts, but compact units can be just as good.
What if my power supply is slightly too long?
It may clash with the drive cage. Some cases let you remove or move it.
Sources
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