Radiator Sizing Guide: How Much Cooling Does Your GPU Need?

The One Rule That Covers 90% of Builds

Radiator sizing guides tend to overcomplicate things. There are charts with 15 variables, calculators that ask about ambient temperature and altitude, and 3-part blog series that take an hour to read. For most PC water cooling builds — including AI workstations — you need one rule:

120mm of radiator per 100W of heat, plus 120mm of overhead.

That is it. A 450W RTX 4090 needs 540mm + 120mm = 660mm minimum. Round up to the nearest common radiator size: 360mm + 360mm (720mm), or 480mm + 240mm (720mm).

This rule gives you a loop that keeps coolant temperatures under 50C at moderate fan speeds (800-1200 RPM) in a 22-25C room. If you want near-silent operation with fans at 600-800 RPM, add another 120mm. If you are in a hot climate (30C+ ambient), add another 120mm.

The rest of this article covers the details for people who want to optimize further.

Sizing for Common GPU Configurations

Single GPU Builds

GPU	TDP	Minimum Rad	Recommended (Quiet)	Maximum (Near-Silent)
RTX 3090 / 3090 Ti	350W / 450W	360mm + 120mm	360mm + 240mm	480mm + 240mm
RTX 4090	450W	360mm + 240mm	480mm + 240mm	480mm + 360mm
RTX 5090	575W	480mm + 240mm	480mm + 360mm	480mm + 480mm
RTX 4070 Ti Super	285W	360mm	360mm + 120mm	360mm + 240mm
RTX 4060 Ti	160W	240mm	360mm	360mm + 120mm

Note: These assume GPU-only cooling. If you are also water-cooling the CPU (adding 65-125W depending on the processor), add radiator capacity for the CPU's TDP using the same 120mm-per-100W rule.

Dual GPU Builds

Configuration	Combined TDP	Minimum Rad	Recommended (Quiet)
Dual RTX 3090 (NVLink)	700W	480mm + 360mm	480mm + 480mm
Dual RTX 4090	900W	480mm + 480mm	480mm + 480mm + 240mm
Dual RTX 5090	1150W	480mm + 480mm + 360mm	External radiator recommended

Dual GPU builds are where radiator sizing becomes a genuine constraint. 900W+ of heat in a standard mid-tower case pushes the limits of internal radiator mounting. Some builders use external radiators (MO-RA 420, or a DIY radiator outside the case connected with quick-disconnect fittings) to handle the thermal load.

For dual 3090 builds specifically, see our dual RTX 3090 NVLink cooling guide.

Radiator Thickness: Slim vs. Standard vs. Thick

Radiators come in different thicknesses, and thickness affects cooling capacity per unit of length.

Thickness	Typical Range	Performance	Trade-off
Slim	25-30mm	Baseline — good airflow, adequate cooling	Fits in tight cases; lower absolute capacity
Standard	35-45mm	15-25% better than slim at the same fan speed	Slightly tighter case fit; best all-around choice
Thick	55-65mm	25-40% better than slim at high fan speeds	May not fit with fans in many cases; needs high fan speed to utilize the extra depth

The Thickness Trap

Thick radiators (55mm+) have more cooling surface area, but they require higher airflow to push air through the dense fin stack. At low fan speeds (600-900 RPM), a thick radiator may perform only marginally better than a slim one — the air simply does not penetrate deep enough to reach the inner fins.

For AI workstation builds where silence is a priority (fans at 600-1000 RPM), a 30-40mm radiator with more surface area (i.e., a longer radiator) is better than a shorter, thicker radiator. A slim 480mm outperforms a thick 360mm at low fan speeds in most configurations.

FormulaMod Radiator Options by Thickness

30mm (slim):

• Barrow Dabel-30a 240mm — 30mm slim, 14 circulation channels, fits almost any case
• Barrow Dabel-30a 360mm — same slim profile, excellent for space-constrained builds
• Barrow Dabel-30a 480mm — maximum surface area in a slim package
• Bykski RC Series 30mm — available in 120/240/360/480mm lengths

40mm (standard):

• Bykski RC 240mm 40mm — sweet spot for secondary radiators
• Bykski RC 480mm 40mm — high capacity, fits most mid-towers in top or front mount
• Bykski RC Series 40mm — available in 120/240/360/480mm lengths

60mm (thick):

• Barrow Dabel-60d 360mm — 60mm thick, maximum density. Use only if you plan to run fans at 1200+ RPM.

Fan Configuration: Push vs. Pull vs. Push-Pull

How you mount fans on the radiator affects cooling performance significantly.

Configuration	Description	Performance	When to Use
Push	Fans blow air through the radiator	Baseline	Default choice for most builds; fans mounted on the intake side of the radiator
Pull	Fans pull air through the radiator	Similar to push (within 1-2C)	When you cannot mount fans on the intake side due to clearance
Push-Pull	Fans on both sides of the radiator	10-20% better than push alone	Thick radiators (45mm+) where single-side fans cannot fully penetrate the fin stack

Practical Advice

For slim (30mm) and standard (40mm) radiators, push-only is sufficient. The added cost and noise of push-pull fans is not justified — you get more cooling from buying a larger radiator than from doubling the fans on a smaller one.

For thick (55-60mm) radiators, push-pull makes a meaningful difference and is recommended. But remember: push-pull doubles the number of fans, doubles the noise sources, and doubles the fan cost. A 360mm thick radiator with push-pull needs 6 fans, not 3.

Fans for radiator duty: Bykski 120mm fans offer good static pressure (important for pushing air through radiator fins) and are compatible with all standard radiator sizes.

Case Compatibility Reference

The most common case types for AI workstation builds and their radiator capacity:

Case Type	Typical Radiator Support	Suitable For
Lian Li O11 Dynamic	Top: 360mm, Side: 360mm, Bottom: 360mm	Single GPU up to RTX 5090, dual GPU with planning
Lian Li O11 Dynamic XL	Top: 360mm, Side: 360mm, Bottom: 360/480mm	Any configuration including dual GPU
Corsair 5000D/7000D	Top: 360mm, Front: 360mm, Side: 360mm	Single or dual GPU
Fractal Torrent	Bottom: 360mm (front: limited)	Single GPU with limitations
Phanteks Enthoo 719	Top: 480mm, Front: 480mm, Bottom: 360mm	Any configuration, maximum radiator capacity
Mid-tower (generic)	Top: 240-360mm, Front: 240-360mm	Single GPU
ITX / SFF	Usually 240mm only	Low-TDP GPUs only; mid-range GPUs with compromise on noise

Always check clearance between the radiator + fans and other components. The most common clearance conflicts:

• Top radiator vs. tall RAM modules (especially with 40mm+ thick radiators)
• Front radiator vs. GPU length
• Bottom radiator vs. PSU shroud
• Any radiator + fans vs. motherboard power connectors or IO shields

Copper vs. Aluminum Radiators

A quick but important note: never mix copper and aluminum in the same loop. All Bykski and Barrow waterblocks are nickel-plated copper. Their Dabel and RC series radiators are copper. Mixing in an aluminum radiator from another brand creates a galvanic cell that corrodes the aluminum and contaminates the entire loop.

If you see a very cheap radiator from an unknown brand, check the material. Aluminum radiators are significantly lighter than copper ones of the same size — if a 360mm radiator weighs under 400g, it is almost certainly aluminum.

All radiators linked in this guide are copper and compatible with copper/nickel waterblocks.

How to Tell If You Need More Radiator

Already have a loop running? Here is how to check if your current radiator capacity is adequate:

• Check coolant temperature. Use a Bykski temperature/flow monitor or an inline temperature sensor. Under sustained AI load in a 22-25C room, coolant should stabilize under 50C. If it exceeds 50C, you need more radiator or higher fan speed.
• Check delta-T. Delta-T is the difference between coolant temperature and ambient room temperature. Under 15C delta-T at moderate fan speed = excellent. 15-20C = good. Above 20C = you are radiator-limited.
• Listen to your fans. If fans need to run above 1200 RPM to keep coolant under 50C, adding radiator surface would allow lower fan speeds at the same thermal performance.

Radiator Order in the Loop: Does It Matter?

Short answer: barely.

The temperature difference between "GPU → radiator → pump" and "GPU → pump → radiator" is typically 1-2C. Coolant temperature equalizes quickly across the loop because the water volume moves through all components many times per minute.

What matters more: place the radiator(s) where case airflow works best. If your case has a mesh top panel, the top radiator mount will exhaust heat efficiently. If the top is solid, use front-mount intake (pull cool outside air through the radiator).

For builds with two radiators: use one as intake (pull fresh air in through the radiator) and one as exhaust (push hot air out through the radiator). This creates positive airflow through the case and keeps all components — not just the water-cooled ones — at reasonable temperatures.

Quick Decision Matrix

Your GPU	Your Priority	Buy This
RTX 4060 Ti / 4070	Any	Single 240mm or 360mm radiator
RTX 3090 / 4070 Ti Super	Quiet operation	360mm + 240mm
RTX 4090	Quiet operation	480mm + 240mm (minimum 360mm + 240mm)
RTX 4090	Near-silent	480mm + 360mm
RTX 5090	Quiet operation	480mm + 360mm
Dual 3090	Quiet operation	480mm + 360mm (minimum)
Dual 4090	Quiet operation	480mm + 480mm

Common Radiator Sizing Mistakes

Understanding what goes wrong helps more than theoretical perfection. Here are the radiator sizing errors that cause the most grief in AI workstation builds.

Sizing for the Wrong TDP Number

GPU manufacturers list TDP (Thermal Design Power) as a single number, but the actual power draw varies with workload. An RTX 4090 is listed at 450W, but under sustained AI inference it may pull 380-430W depending on the model and power limit setting. Gaming pulls 300-380W on average with spikes to 450W.

For radiator sizing, use the sustained power draw under your actual workload, not the peak TDP. If you run Ollama inference, monitor power draw with nvidia-smi for 10 minutes and use that number. You will often find you need slightly less radiator than the TDP number suggests — but size for the higher number anyway, because the extra capacity means quieter fans.

Forgetting CPU Heat Load

If you plan to add the CPU to your loop later (a common upgrade path), size your radiators for the combined heat load from day one. A typical AI workstation CPU (AMD Ryzen 9 or Intel i9) adds 125-170W of heat. If you sized your radiators for a 450W GPU only and later add a 125W CPU, you are suddenly 125W short — meaning either louder fans or higher coolant temperatures.

Ignoring Ambient Temperature

The 120mm-per-100W rule assumes a 22-25C ambient room temperature. Radiators reject heat by transferring it from coolant to air. The closer your coolant temperature is to the air temperature, the less efficient the transfer becomes. In a warm room (28-32C), you need roughly 20-30% more radiator surface to achieve the same coolant temperature. This matters for AI rigs in garages, server closets, or un-air-conditioned rooms during summer.

Buying Slim When You Have Space for Standard

Slim (30mm) radiators are great for tight cases, but if your case can accommodate a 40mm radiator in the same spot, the thicker radiator provides 15-25% more cooling capacity with the same fans. Do not choose slim just because it is cheaper — the price difference between a 30mm and 40mm Bykski radiator is $5-10, but the cooling difference is significant.

Maintenance Considerations for Radiators

Radiators require minimal maintenance compared to other loop components, but they are not zero-maintenance.

• Dust accumulation on fins: Over 6-12 months, dust builds up between the radiator fins and fan blades, reducing airflow by 10-20%. Clean radiator fins with compressed air every 6 months. Remove fans for thorough cleaning if dust is heavy.
• Internal flushing: New copper radiators may contain flux residue, solder particles, or manufacturing oils. Before first use, flush with distilled water until the water runs clear. For established loops, flushing during coolant changes (every 6 months for AI rigs) keeps internal channels clear.
• Corrosion check: During coolant changes, inspect the coolant for discoloration. Green tint indicates copper corrosion (often from mixed metals). Brown particles indicate severe corrosion. See our maintenance guide for the complete inspection procedure.

Browse all radiator options in the AI Workstation Cooling collection. For a complete build walkthrough with specific radiator recommendations, read our RTX 4090 water cooling guide or the cost breakdown by tier.