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Enter your CPU and GPU TDP above to get a radiator size and pump recommendation.
How to Size a Custom Water Cooling Radiator
Radiator sizing is not about picking the biggest option — it is about matching surface area to heat output at your target noise level. Every 120mm section of a radiator fan acts as an independent heat exchanger. At medium fan speed (around 1,200 RPM) with a coolant temperature 25°C above ambient, each 120mm section rejects approximately 150 watts. Run fans slowly and that drops to 90 watts; push them to 1,800 RPM and it rises to around 210 watts.
The target delta-T (ΔT) is the gap between your coolant temperature and the room temperature. A lower ΔT (15–20°C) means cooler, quieter hardware but requires more radiator surface. A higher ΔT (25–30°C) needs less surface but means the coolant runs hotter. Most builders target 20–25°C above ambient as a practical balance.
| Radiator Size | 120mm Sections | Quiet (~800 RPM) | Balanced (~1200 RPM) | Performance (~1800 RPM) |
|---|---|---|---|---|
| 240mm | 2 | ~145W | ~240W | ~340W |
| 360mm | 3 | ~215W | ~360W | ~505W |
| 480mm | 4 | ~285W | ~480W | ~670W |
| 360mm + 240mm | 5 | ~360W | ~600W | ~840W |
| 2× 360mm | 6 | ~430W | ~720W | ~1,005W |
Values at ΔT = 25°C coolant above ambient. Actual results vary by radiator FPI density and fan model.
D5 vs DDC Pump: Which Should You Choose?
Laing D5 — High Flow
- ✓High flow rate at low head pressure — ideal for large loops with two waterblocks
- ✓Quieter at low speed settings than DDC due to larger impeller diameter
- ✓Available in PWM variants for motherboard fan header control
- ✗Larger footprint — requires a separate reservoir or top-mount combo
Best for: CPU + GPU loops, any build with two or more radiators
Laing DDC — High Pressure
- ✓Higher static pressure — better for tight loops with restrictive CPU blocks and many fittings
- ✓Compact — fits in small cases and integrates into many reservoir tops
- ✗Can be louder at high speed due to smaller impeller turbulence
- ✗Lower max flow volume than D5 in low-restriction loops
Best for: CPU-only loops, SFF cases, compact builds
Custom Water Cooling vs AIO: When Is It Worth the Switch?
An all-in-one liquid cooler is the right choice for most users. It is simpler to install, covered by a manufacturer warranty, and costs a fraction of a custom loop. A 360mm AIO adequately cools any consumer CPU at stock settings and handles moderate overclocking on most platforms.
Custom loops make engineering sense when sustained thermal output exceeds what a single AIO can handle quietly — roughly 250W+ total or when a GPU waterblock is needed. An RTX 5090 at 575W under full ray tracing cannot be cooled by any standard AIO; it requires a full-cover waterblock. A workstation running simultaneous CPU rendering and GPU inference may also benefit from a loop that shares radiator surface between both components.
The total cost of a quality custom loop — good waterblocks, a D5 pump, reservoir, 360mm radiator, EKWB or Alphacool fittings, and tubing — typically starts at $400–600 USD and rises quickly with larger configurations. Plan the build fully before purchasing: custom loops are difficult and messy to modify after the case is sealed.
Planning Your First Custom Loop: Component Checklist
1. CPU Waterblock
Confirm socket compatibility (AM5 / LGA1851 / LGA1700). Jet-plate designs (EKWB Quantum, Corsair XC series) suit wide sockets. Coldplate material: copper over nickel.
2. GPU Waterblock (if included)
Must match your exact GPU PCB revision — not just the chip. Check the manufacturer's compatibility table against your GPU serial number prefix.
3. Pump + Reservoir
D5 or DDC pump; combo reservoir-tops simplify installation. Buy a pump with PWM control so you can tune speed via motherboard headers.
4. Radiator(s)
Match case radiator mounts. Measure clearance with top-mount rads carefully — RAM height and motherboard power connectors are common fitment issues.
5. Fans
Radiator-optimised fans (high static pressure). Noctua NF-A12×25, Be Quiet! Silent Wings 4, or Lian Li Uni Fan AL series are proven choices.
6. Fittings + Tubing + Coolant
Soft tubing (EPDM or silicone) is beginner-friendly. Hard tubing (PETG/acrylic) needs a heat gun and patience. Use distilled water with a biocide inhibitor — never tap water or pure glycol.
Coolant, Biocides, and Loop Maintenance
Distilled water is the base fluid for all custom loops — never tap water, which contains minerals that deposit on block surfaces and corrode metal components over months. Add a biocide inhibitor (5–10% by volume): Mayhems Pastel, EKWB CryoFuel, Thermaltake TT Premium, or Feser One are industry standards. These prevent algae growth, inhibit metal corrosion, and maintain surface tension.
Flush the loop annually. Drain completely, flush with distilled water, then refill with fresh coolant. Ignore this and you will see milky discolouration, reduced flow, and eventually a blocked coldplate. Avoid mixing copper and nickel components in the same loop — the electrochemical potential difference accelerates galvanic corrosion. Check the metal composition of every block and fitting before building.
Custom Water Cooling FAQ
What size radiator do I need for a custom water cooling loop?
Divide your total CPU + GPU TDP by the watts-per-section at your target fan speed. At balanced fan speed (~1,200 RPM) and a 25°C coolant rise, each 120mm section handles ~150W. A 500W build needs at least 3.3 sections — round up to 4 sections (a 480mm or 360mm + 120mm configuration). Always add a 10–15% safety margin.
How many 120mm sections do I need for an RTX 5090?
The RTX 5090 has a 575W TDP. At balanced fan speed targeting 25°C ΔT, that alone needs 3.8 sections — round up to 4 (480mm). Add a CPU at 170W and you need 4.9 sections — so 5 sections (1× 360mm + 1× 240mm). Plan for at least a 480mm + 240mm configuration if overclocking either component.
Is a single 360mm radiator enough for CPU and GPU?
Only for lower-TDP combinations. A 65W CPU + 200W GPU (265W total) is comfortably handled by a 360mm at balanced fan speed. A 170W CPU + 300W GPU (470W total) exceeds what a 360mm can dissipate quietly — you need a 480mm at minimum, or a 360mm + 240mm at balanced fan speed.
What flow rate do I need for a custom water cooling loop?
The thermodynamic minimum is lower than most builders expect — typically 0.2–0.4 L/min. However, practical recommendations start at 1.0 L/min to account for flow losses through fittings and to maintain good turbulence inside waterblock channels. For CPU + GPU loops, 1.5–2.0 L/min is the standard target. Flow above 2.5 L/min gives diminishing thermal returns.
Can I mix copper and nickel components in a water cooling loop?
Avoid it where possible. Copper and nickel have different electrochemical potentials, and when submerged in the same coolant circuit they can accelerate galvanic corrosion — particularly on the more reactive material. Biocide inhibitors reduce this risk but do not eliminate it. Check every block, fitting, and radiator: stay within one metal family (all copper or all nickel-plated copper).
How often should I change my custom loop coolant?
Annual coolant changes are the industry standard. Biocide inhibitors degrade over time, and dissolved metal ions accumulate in the fluid. If the coolant changes colour (milky, brown, or green tint), flush and refill immediately. Systems running dye are harder to assess — plan for a flush every 12 months regardless of appearance.
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