A parts list can be entirely reasonable component by component and still not assemble into a working computer. Compatibility is a property of combinations, not of parts, and the failures divide cleanly into two kinds. Electrical and logical mismatches — wrong socket, wrong memory type, insufficient power — prevent the machine from working. Physical clearance problems — a card too long, a cooler too tall — prevent it from going together at all.
The second kind is the more common source of returned parts, because clearance measurements are invisible in every product photograph and rarely mentioned in reviews. What follows is the complete set of checks, in the order in which they matter.
1. CPU socket and motherboard
The processor's socket must match the motherboard's socket exactly. There is no adapter, no partial compatibility, and no way to force it — the pin arrangement is physical.
A matching socket is necessary but not always sufficient. Sockets frequently span multiple processor generations, and a given motherboard chipset may support only some of them. A board can have the physically correct socket and still refuse to POST with a newer processor until its BIOS is updated. Manufacturers publish a CPU support list for every board; on a recent platform, checking it is worth the two minutes.
Some boards can update their BIOS without a working processor installed — a feature worth looking for if you are pairing a new processor with a board that may have been manufactured before it existed.
2. Memory type and capacity
Memory generation is dictated by the motherboard, not the processor. DDR4 and DDR5 modules are physically keyed so that neither will seat in the other's slot, and no board accepts both.
- Type: the kit must be the generation the board accepts.
- Slots: confirm the board has enough for the number of modules you are buying.
- Capacity: check the board's maximum total, and its maximum per module.
- Speed: a board supports memory up to a stated speed. Faster kits generally run, but at the board's ceiling rather than their rating.
Buy two modules rather than one wherever possible. Dual-channel operation is a free and substantial performance gain, and forfeiting it is the most common memory mistake in first builds.
3. Motherboard form factor and case
Cases accept their own board size and anything smaller, never larger. An ATX case takes ATX, MicroATX and Mini-ITX boards; a Mini-ITX case takes only Mini-ITX.
| Form factor | Approximate size | Typical use |
|---|---|---|
| ATX | 305 × 244 mm | Standard desktop; most expansion |
| MicroATX | 244 × 244 mm | Compact builds; good value |
| Mini-ITX | 170 × 170 mm | Small form factor; one expansion slot |
| E-ATX | 305 × 330 mm | Enthusiast; requires a case that states support |
Fitting a smaller board in a larger case is entirely fine and often sensible — it leaves more room to work and improves airflow.
4. Graphics card length and case clearance
This is the single most frequent physical incompatibility. Every case publishes a maximum graphics card length; every card publishes its own. The first must exceed the second, and by more than zero.
Two complications catch people out. Front-mounted radiators and drive cages reduce the available length below the case's headline figure, which is usually quoted with those absent. And a card's listed length sometimes excludes the power connector and the cable bend behind it, which can add fifteen to thirty millimetres of required space.
Leave at least twenty millimetres of margin over the published figures rather than assuming an exact fit will work.
5. CPU cooler height and case clearance
The same logic applies to cooler height. A tower cooler taller than the case's stated clearance will physically prevent the side panel from closing, and there is no workaround.
For liquid cooling the equivalent check is radiator support: confirm the case accepts your radiator size at the position you intend to mount it, and remember that case specifications sometimes quote radiator support without accounting for fan thickness. Add roughly 25 mm per fan layer.
6. Cooler socket compatibility
A cooler must ship with mounting hardware for your processor's socket. Most support a broad range, but recent sockets sometimes require a bracket the manufacturer supplies separately — occasionally free on request, occasionally not.
Also compare the cooler's rated dissipation against the processor's TDP, with margin rather than exactly. A cooler matched precisely to a processor will hold temperature, but only by running its fan at high speed permanently.
7. Power supply capacity
Add the graphics card's rated board power to the processor's maximum draw, add roughly 100 W for everything else, and multiply by about 1.4. Buy at or above the result.
The multiplier is not padding. It covers the efficiency band where supplies operate best, and — more importantly — the transient power spikes modern graphics cards produce. These last around a millisecond, never appear in average-draw figures, and are entirely capable of tripping the protection circuitry on a supply sized exactly to the calculated load. The symptom is an abrupt reboot under load with nothing actually faulty.
8. Power supply connectors
Capacity and connectors are separate checks, and a supply can pass the first while failing the second.
- Count the PCIe connectors your graphics card requires and confirm the supply provides that many, preferably on separate cables rather than daisy-chained.
- If the card uses a 12V-2x6 or 12VHPWR connector, confirm the supply has a native cable rather than relying on an adapter.
- Check the motherboard's CPU power requirement — most need one 8-pin EPS, some higher-end boards want an additional 4- or 8-pin.
- Count SATA connectors against the number of drives you are fitting.
Never use modular cables from one power supply with another. The pinout on the supply side is not standardised between manufacturers or even reliably between models from the same one, and mismatched cables can destroy connected hardware.
9. Storage interfaces
Storage is the least constrained part of a build, but there are two traps.
- An NVMe SSD needs an M.2 slot wired for PCIe. Some budget boards wire one or more M.2 slots for SATA only, and an NVMe drive in such a slot will not be detected at all.
- M.2 slots often share bandwidth with SATA ports or PCIe slots. Populating one can disable another, which the board manual documents and nothing else does.
For SATA drives, simply confirm a free port, a data cable and a spare power connector. M.2 length is almost always 2280, but the board specifies which lengths it accepts.
10. Front panel and connectivity
The least glamorous check, and the one that produces a working machine with a dead front panel.
- A front-panel USB-C port needs a corresponding internal header on the motherboard. Many budget boards do not have one.
- Count internal USB 2.0 and 3.0 headers if you are connecting a case front panel plus other internal devices — headers run out faster than expected.
- Check fan header count against the number of fans, and whether they are PWM or DC controlled.
- If you plan to use integrated graphics, confirm the processor actually has it. Several desktop processor lines do not, and a board's video outputs are inert without it.
The checks in build order
Choosing components in the sequence below means each decision constrains the next, and most of the checks above resolve themselves.
- Processor — decided by budget and workload.
- Motherboard — socket must match; verify the chipset supports that processor generation.
- Memory — type dictated by the board; two modules; within the board's capacity and speed limits.
- Graphics card — chosen for your resolution target; note its length and power draw.
- Storage — NVMe if the board has a PCIe-wired M.2 slot.
- Cooler — socket supported; capacity above the processor TDP with margin; note its height.
- Power supply — calculated capacity; connectors matching the graphics card and board.
- Case — accepts the board form factor, the graphics card length and the cooler height.
Our PC Builder runs ten compatibility checks against your parts list as you build it, covering socket matching, memory type, power adequacy, and both clearance measurements. It will tell you immediately when a combination does not work, rather than after delivery.
Failures that are not compatibility problems
Two things regularly get blamed on compatibility when they are something else. A machine that powers on but does not display anything is more often a memory seating problem or a BIOS version issue than a genuine incompatibility. And a machine that works but underperforms is usually a configuration problem — memory profile not enabled, single-channel operation, thermal throttling — rather than mismatched parts.
Before concluding that a component is wrong, reseat the memory, check that XMP or EXPO is enabled, and confirm nothing is running hot.