AIO Radiator Size Guide: 240mm vs 280mm vs 360mm, Matched to Real CPU Heat Output
All-in-one liquid coolers are sold by radiator size — 240mm, 280mm, and 360mm being the common desktop options — but the size number alone doesn't tell you whether a given radiator can actually keep a specific CPU cool under sustained load. This guide matches radiator surface area to real CPU heat output so you can size a cooler correctly instead of guessing from marketing copy.
What radiator size is actually measuring
The millimeter figure refers to the total length available for mounting fans, which roughly determines total radiator surface area: a 240mm radiator mounts two 120mm fans, a 280mm radiator mounts two 140mm fans, and a 360mm radiator mounts three 120mm fans. More surface area means more room for the radiator's fins to reject heat from the coolant into the air passing through them, and more fan area means more air volume can be moved through those fins at a given noise level. Going from 240mm to 360mm roughly increases usable radiator surface area by about 50%, which translates fairly directly into either lower sustained temperatures at the same fan speed, or the same temperatures at meaningfully lower (quieter) fan speed.
Matching radiator size to CPU heat output
The relevant number to size against isn't the CPU's rated TDP — it's the actual sustained power draw the chip pulls under full load, which is frequently far higher than the box TDP once motherboard firmware power-limit defaults are factored in (most enthusiast boards quietly raise or remove the CPU's stock power ceiling out of the box). A chip like the Intel Core i9-14900K can sustain well over 250W under all-core load on a board that removes Intel's default power ceiling, and that kind of sustained heat output is genuinely at the edge of what a 240mm AIO can dissipate quietly — a 280mm or 360mm radiator gives meaningfully more thermal headroom and lets fan curves stay quieter at the same chip temperature.
As a practical rule of thumb: a 240mm AIO comfortably handles most mid-range CPUs up to roughly 125-150W sustained, a 280mm AIO extends comfortably into the 150-200W range, and a 360mm AIO is the appropriate choice for anything sustaining 200W or more, including unrestrained high-core-count Intel chips and heavily PBO-tuned AMD flagships like the Ryzen 9 7950X running its full 230W PPT ceiling. These aren't hard cutoffs — a well-built 240mm unit with premium fans can still handle a 200W chip, just at higher noise levels and closer to its thermal ceiling — but they're a sound starting point for sizing a cooler to a specific CPU rather than picking a radiator size arbitrarily.
Case compatibility is the real limiting factor for most builds
Radiator size selection is frequently constrained by the case rather than by cooling preference. A compact mATX or ITX case may only physically support a 240mm or 280mm radiator regardless of what the CPU could ideally use, which is exactly the kind of trade-off covered in our case form factor guide. Always check a specific case's radiator clearance specifications (including thickness clearance if you're running push-pull fans, which roughly doubles the depth needed) before assuming a 360mm unit will fit — top-mount radiator clearance is one of the most commonly overlooked compatibility issues in ITX and compact mATX builds specifically.
Airflow direction and case interaction
Radiator size interacts directly with overall case airflow strategy, not just with the CPU it's cooling. A top-mounted radiator pulling warm air up and out of the case works well with front-to-back or front-to-top airflow patterns but can fight against a case that's primarily set up for bottom-to-top exhaust. Our case airflow and fan curve deep dive covers how radiator placement should be planned alongside the rest of a case's intake and exhaust fans rather than treated as an isolated component decision — a mismatched radiator orientation can genuinely undercut the thermal advantage of stepping up to a larger radiator size in the first place.
Push, pull, and push-pull fan configurations
Radiator fans can be mounted pulling air through the radiator (pull), pushing air into it (push), or both simultaneously (push-pull) with a second set of fans on the opposite face. Push-pull configurations do improve static-pressure-limited airflow through a dense radiator core, but the gains are modest on modern radiators with efficient fin designs — typically a few degrees under sustained load — while the cost is real: double the fan count, double the potential fan noise, and a meaningful increase in the physical depth the radiator assembly needs inside the case, which is exactly where a 360mm push-pull setup can suddenly stop fitting in a case that comfortably cleared a single-fan 360mm radiator. For most builds, a single well-chosen set of static-pressure-optimized fans on one side of the radiator, sized correctly for the CPU's actual heat output in the first place, delivers better value than adding a second set of fans to compensate for an undersized radiator.
Diminishing returns and noise
Bigger radiators don't scale their cooling benefit linearly forever — the difference between a 280mm and 360mm unit on a genuinely low-power CPU (under 100W sustained) is often only a few degrees, well within the noise floor of normal ambient temperature variation. Where a larger radiator earns its keep on a lower-power chip isn't lower peak temperatures so much as the ability to run fans at noticeably lower, quieter RPM for the same temperature target — directly relevant if you're building toward a silent PC build rather than chasing the lowest possible number on a temperature readout.
Bottom line
Size an AIO radiator to your CPU's actual sustained power draw under your motherboard's real-world power limit settings, not the box TDP number, and confirm your case has genuine clearance for the radiator thickness plus fans before buying. A 360mm radiator is the right call for unrestrained high-core-count flagships; a 240mm or 280mm unit is entirely sufficient, and often quieter for the money, on most mid-range CPUs running realistic power limits. When in doubt between two sizes that both physically fit your case, the larger radiator is almost always the safer long-term choice — it gives you quieter headroom today and keeps pace if a future CPU upgrade or a more aggressive power-limit setting raises your sustained heat output later.