Most build failures are not compatibility failures in the electrical sense. The parts would work together fine. They just do not fit in the box.
Four measurements decide it, every one of them is published by somebody, and almost none of them appear in the same place.
The Four Numbers
| Measurement | Part it constrains | Where the limit is published |
|---|---|---|
| GPU clearance | Graphics card length | Case spec sheet |
| CPU cooler clearance | Air cooler height | Case spec sheet |
| PSU maximum length | Power supply depth | Case spec sheet |
| Radiator support | AIO radiator plus fans | Case spec sheet, per mounting position |
The card, cooler and supply publish their own dimension. The case publishes the ceiling. Compatibility is one subtraction, and it still goes wrong constantly because the two numbers live on different manufacturers websites.
GPU Length
The number to find on the case is GPU clearance, sometimes called maximum graphics card length. The number on the card is its length in millimetres.
Two things make this less simple than it looks.
Front radiator or fans eat into it. Many cases publish two figures, one clear and one with a front-mounted radiator installed, and the second can be 60mm shorter. If you are running an AIO in the front, use that second number.
Cable clearance is not in the figure. A card that clears by 5mm may still not fit once the power connector is attached, because 12V-2x6 and 8-pin connectors stand proud of the end of the card and the cable needs a bend radius. Leave 20 to 30mm beyond the card length for anything with a top-mounted connector.
Slot width matters separately. A card rated 3.5 slots in a case with three usable slots physically mounts but blocks the slot below and may foul the side panel.
CPU Cooler Height
The case publishes CPU cooler clearance. The cooler publishes its height. Subtract.
Leave 5mm. A tower cooler that clears by 2mm will press against the side panel once the panel bows slightly, and on a tempered glass panel you will not be able to close it at all.
The trap here is that cooler height and radiator thickness are frequently stored in the same field on retail listings. A 360mm AIO has no meaningful height, it has a radiator thickness of perhaps 27mm and a fan of 25mm, and those stack. If a listing shows a suspiciously small height for a liquid cooler, it is quoting the radiator.
RAM clearance is the related figure and the one most often missing. A large air cooler overhangs the first memory slot, and the cooler maker publishes a RAM clearance height that your memory must sit under. Not every maker publishes it. Thermalright, notably, does not for the Peerless Assassin. Where it is not published, measure or choose low profile memory.
PSU Length
Case publishes PSU maximum length, supply publishes its depth. This one is usually comfortable in a mid tower and tight in anything small form factor.
The complication is the same as the GPU one: cables. A fully modular supply needs 40 to 50mm behind it for the connectors and the bend. In a case where the supply sits in a shroud with a hard drive cage behind it, the published maximum often assumes the cage is removed.
Form factor is a separate check. An SFX supply fits an ATX bracket with an adapter, an ATX supply does not fit an SFX bracket at all.
Radiator Support
The hardest of the four, because the case publishes it per position and the constraint is a stack rather than a single number.
A 360mm radiator needs 360mm of mounting length, but the real constraint is thickness: radiator plus fans. A 27mm radiator with 25mm fans is a 52mm stack, and a top mount with 55mm of clearance to the motherboard is tight.
Check the position specifically. Cases commonly support 360mm in the front and only 240mm in the top, and a top-mounted radiator is the one that collides with tall memory and VRM heatsinks.
Spec Block: The Checks, In Order
- GPU length against GPU clearance, with the front radiator figure if you are using one, plus 20 to 30mm for the power connector
- Cooler height against cooler clearance, plus 5mm
- Memory height against the cooler RAM clearance, where published
- PSU depth against PSU maximum length, plus 40 to 50mm for cables
- PSU form factor against case PSU form factor
- Radiator size against the specific mounting position, not the case headline
- Radiator plus fan thickness against clearance at that position
- Motherboard form factor against the case supported list
Winner
The measurement that catches most people is GPU clearance with a front radiator installed, because the headline figure on the case assumes an empty front. If you are building an AIO system with a long card, check that second figure first.
The measurement most often missing entirely is cooler RAM clearance, and the safe response to a missing figure is low profile memory rather than an assumption.
Pros and Cons of Buying to the Limit
Pros: Maximises what you can fit in a given chassis size, and small form factor builds are only possible by using the tolerances deliberately.
Cons: Removes any margin for a future upgrade, makes cable routing considerably harder, and a part that clears by 2mm on paper may not clear at all once connectors and panel flex are included.
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