The Short Answer
For a 300W GPU overclock on a custom loop, plan on 360mm of radiator surface as your baseline, with 480mm preferred if you want quiet fans and headroom for summer ambient temperatures. That translates to either one 360mm radiator (3x120mm) or one 280mm radiator (2x140mm) as an absolute minimum when the GPU is the only block on the loop, and 480mm to 600mm of total surface (for example, a 360mm + 240mm, or a 420mm + 240mm) when you are cooling both a 300W GPU and a 100W to 175W CPU together.
The math is straightforward. Most modern copper radiators dissipate roughly 80W to 125W per 120mm of length at a 10C water-to-air delta with fans at 1200 to 1500 RPM, depending on thickness and fan speed. A 300W overclocked GPU alone needs about three 120mm sections to keep water temperature under 10C to 12C above room temperature without running fans at full speed. If you want a near-silent build under 1000 RPM, add another 120mm to 240mm of surface. If you are chasing maximum overclock stability and want your coolant under 40C with a 22C room, the extra surface pays for itself in lower fan noise and more thermal stability.
For thickness, a standard 30mm thick radiator is ideal for most cases because it performs well with moderate fan speeds and fits easily. A 45mm to 60mm thick radiator can add 15 to 25 percent more cooling per 120mm section, but only if you use higher static pressure fans above 1300 RPM and have at least 65mm of total clearance including fans.
Standard Sizes
PC radiators are named by fan size and fan count, not by exact external dimensions. A 360mm radiator holds three 120mm fans, a 420mm holds three 140mm fans, and so on. External dimensions vary by 5mm to 15mm between brands due to end tanks and mounting rails, but the fan mount spacing is standardized. Use the table below to plan case fitment before you buy.
| Common Name | Fan Configuration | Approx. Length (in / cm) | Approx. Width (in / cm) | Common Thickness Options (in / cm) | Typical Weight Empty (lb / kg) |
|---|---|---|---|---|---|
| 120mm | 1 x 120mm | 6.1 / 15.5 | 4.7 / 12.0 | 1.06 – 2.36 / 2.7 – 6.0 | 0.9 – 1.5 / 0.4 – 0.7 |
| 140mm | 1 x 140mm | 6.9 / 17.5 | 5.5 / 14.0 | 1.06 – 2.36 / 2.7 – 6.0 | 1.1 – 1.7 / 0.5 – 0.8 |
| 240mm | 2 x 120mm | 10.9 / 27.7 | 4.7 / 12.0 | 1.06 – 2.36 / 2.7 – 6.0 | 1.5 – 2.6 / 0.7 – 1.2 |
| 280mm | 2 x 140mm | 12.4 / 31.5 | 5.5 / 14.0 | 1.10 – 2.36 / 2.8 – 6.0 | 1.8 – 3.1 / 0.8 – 1.4 |
| 360mm | 3 x 120mm | 15.7 / 39.8 | 4.7 / 12.0 | 1.06 – 2.36 / 2.7 – 6.0 | 2.2 – 3.7 / 1.0 – 1.7 |
| 420mm | 3 x 140mm | 17.9 / 45.5 | 5.5 / 14.0 | 1.10 – 2.36 / 2.8 – 6.0 | 2.6 – 4.4 / 1.2 – 2.0 |
| 480mm | 4 x 120mm | 20.4 / 51.8 | 4.7 / 12.0 | 1.18 – 2.36 / 3.0 – 6.0 | 3.0 – 5.0 / 1.4 – 2.3 |
Fan thickness is separate. Standard fans are 1.0 inch / 25mm thick, slim fans are 0.59 inch / 15mm. Add fan thickness to radiator thickness plus 5mm for screws and vibration pads to get total stack height. For example, a 30mm radiator with 25mm fans needs 55mm to 60mm of clearance. A 60mm thick radiator with 25mm fans needs 85mm to 90mm.
For a 300W GPU, use this quick surface guide. GPU-only loop, noise tolerant: 360mm x 30mm minimum. GPU-only loop, quiet target under 35 dBA: 360mm x 45mm or 420mm x 30mm. GPU plus CPU loop (400W to 475W combined): 480mm to 600mm total surface, such as 360mm + 240mm or 280mm + 360mm. GPU plus CPU plus overclocking headroom for 350W on the GPU: 600mm to 720mm total if your case allows it.
How to Measure Your Space
Most fitment issues come from assuming a case that supports a 360mm radiator will support any 360mm radiator. Thickness, port position, and fan overhang matter more than length. Measure before ordering.
- Identify your mount points. Check your case manual for supported radiator sizes per location (front, top, bottom, side). Note whether it lists 120mm or 140mm fan spacing. A case that fits a 360mm in front may only fit a 240mm on top due to RAM and motherboard heatsink clearance.
- Measure usable length, width, and depth. With a tape measure, measure the inside rail length where the radiator would sit. For length, you need at least the radiator length plus 0.5 inch / 1.3 cm for fittings and tubing bend. For width, confirm you have 4.9 inches / 12.5 cm for 120mm radiators or 5.7 inches / 14.5 cm for 140mm radiators.
- Measure stack height. Measure from the mounting surface to the nearest obstruction (RAM, motherboard VRM heatsink, power supply shroud, or side panel). Subtract 5mm for safety. If you measure 60mm total, you can fit a 30mm radiator with 25mm fans but not a 45mm radiator with 25mm fans.
- Check port clearance and offset. Measure distance from the radiator end tank to the case panel or fan hub. Many radiators have ports on one end that add 15mm to 20mm. Offset ports or side ports can save 10mm to 15mm in tight top mounts. Verify tubing route will not kink against the top panel.
- Confirm fan fit and airflow direction. Lay your fans on the mount and check screw alignment. The standard spacing is 15mm between 120mm fans and 20mm between 140mm fans. Decide on intake or exhaust. In a front mount, intake gives 2C to 4C lower water temperature but raises internal case temperature. Top mount as exhaust is common for GPU heat to exit directly.
- Account for dust filters and panels. If your case has a front filter or glass panel 10mm to 15mm in front of the fans, you lose some airflow. Add that restriction to your plan by choosing a slimmer radiator with higher surface area (for example, 420mm x 30mm instead of 360mm x 45mm) if front airflow is limited.
- Weigh your filled loop for vertical mounts. A filled 360mm x 30mm radiator weighs about 3.5 lb to 4.5 lb / 1.6 kg to 2.0 kg with fans and coolant. A 420mm x 45mm can exceed 6 lb / 2.7 kg. Ensure side or bottom mounts have full screw support and do not rely on two screws only.
By Room or Use Case
Compact Mid-Tower (Up to 360mm Total Support)
If your case only supports one 360mm or one 280mm mount, you can still cool a 300W GPU overclock, but you will trade noise. A single 360mm x 30mm with three 120mm fans at 1300 to 1600 RPM will handle 300W with a coolant delta of about 10C to 14C above ambient. Expect fan noise around 34 to 40 dBA at that speed. A 280mm x 30mm is roughly equal to a 360mm x 30mm in surface area (about 784 sq cm vs 864 sq cm of fan area) and is viable if your case is 140mm-optimized, but you will need slightly higher RPM to match a 360mm. For this category, choose a full copper 30mm to 45mm thick model and use high static pressure fans (2.0 to 3.5 mmH2O). Avoid thick 60mm radiators here unless your case explicitly lists 60mm top clearance, as you will choke airflow.
Standard Mid-Tower and Full Tower (480mm to 720mm Total Support)
This is the sweet spot for a 300W GPU overclock, especially if you also cool the CPU. Aim for 600mm of total fan length split across two radiators, such as a 360mm up top and a 240mm in front, or a 420mm in front and a 240mm on top. Splitting surface improves coolant flow and lets you run all fans at 900 to 1200 RPM while keeping the water delta under 10C. A common balanced setup is a 360mm x 30mm (top exhaust) plus a 240mm x 30mm (front intake) for a combined 600mm. If your case supports 140mm fans, a 420mm x 30mm plus a 280mm x 30mm provides even more surface with fewer, larger, quieter fans.
Small Form Factor and ITX (240mm to 280mm Limit)
In small form factor cases that max out at 240mm or 280mm, a 300W overclock is possible but not ideal for silence or sustained boost clocks. A single 280mm x 30mm with good fans can dissipate the load, but you will see coolant temperatures 14C to 18C above ambient under extended load, which pushes GPU core temperatures higher and can reduce overclock stability. If you must build in this size, use a 280mm x 45mm if it fits, undervolt slightly to 270W to 285W for daily use and only run 300W for benchmarks, and consider an external radiator or side-mounted 240mm expansion if your case supports it. Pump placement is critical here; a combined pump and reservoir that fits in 60mm of clearance helps keep tubing runs short.
Open Bench, Wall-Mounted, or External Radiator Setup
For test benches or users who want the lowest possible noise, external surface removes case limits. A single external 480mm x 45mm or dual 360mm external unit will cool a 300W GPU with fans at 700 to 900 RPM and keep the coolant delta under 8C. External mounts also let you use thick 60mm radiators without worrying about RAM clearance. This approach is popular as of early 2026 among overclockers who want to keep the main case compact while moving heat outside the chassis.
Common Sizing Mistakes
The most common error is buying too little surface and trying to compensate with fan speed. Going from 240mm to 360mm of surface reduces required fan speed by roughly 300 to 500 RPM for the same water temperature, which is a much larger noise reduction than any fan upgrade alone. Second, mixing aluminum and copper without a proper inhibitor causes corrosion. Stick to all copper and brass radiators if your block is copper or nickel-plated copper, which most GPU blocks are. Third, choosing an extra-thick radiator for a restricted case. A 60mm thick radiator behind a solid front panel or dense dust filter often performs worse than a 30mm radiator with good airflow because the extra thickness needs higher static pressure to be effective. Fourth, ignoring total heat load. A 300W GPU plus a 150W CPU is a 450W loop, not a 300W loop. Size for total wattage plus 20 percent headroom for transients during overclocking. Finally, forgetting fitting clearance. Measure port offset before you assume standard straight fittings will fit.
FAQ
Is a single 360mm radiator enough for a 300W overclocked GPU?
Yes, if the GPU is the only heat source on the loop and you are comfortable with moderate fan speed. A 360mm x 30mm copper radiator with three 120mm fans at 1200 to 1500 RPM will keep coolant about 10C to 12C above room temperature with a 300W load in a case with decent airflow. For quiet operation under 1000 RPM or for long gaming sessions in a warm room above 26C, a 360mm plus an additional 120mm or 240mm is a better target. If you add a CPU to the same loop, a single 360mm is undersized.
Should I get a thicker radiator or a longer radiator?
Longer is usually better for noise and case compatibility. Adding 120mm of length adds about 33 percent more surface area with no increase in airflow resistance per fan, while adding 15mm of thickness adds about 15 to 25 percent more cooling but requires higher fan speed to push air through the deeper fin stack. For most builds, a 360mm x 30mm will beat a 240mm x 60mm at the same fan speed and fit in more cases. Choose thickness only after you have maximized length that your case supports.
Do I need push-pull fans for a 300W GPU?
No, push-pull is not required at this wattage. A single set of fans in push or pull is sufficient for 30mm to 45mm radiators. Push-pull can lower coolant temperature by 1C to 3C or let you drop fan speed by 100 to 200 RPM, but it doubles fan cost, adds 25mm to stack height, and increases weight to 5 lb to 7 lb / 2.3 kg to 3.2 kg per radiator. It is more useful on thick 60mm radiators or when you are limited to a single 240mm to 280mm mount and need to extract maximum performance from that small surface.
What coolant temperature should I target and how much surface keeps me there?
Keep coolant under 45C for component longevity, with 35C to 40C ideal for overclocking stability. Coolant temperature is ambient plus delta. With 22C ambient and 480mm of total surface (for example, 360mm + 120mm) you can expect roughly 7C to 10C delta at 300W with fans at 1000 to 1200 RPM, for a 29C to 32C coolant temperature. With 360mm only, expect 10C to 14C delta. With 600mm (360mm + 240mm) you can hold 6C to 9C delta even with a CPU added. If your room is 28C to 30C in summer, add 120mm to 240mm to your plan to maintain the same coolant target without raising fan speed.
