How hot will my CPU run with air cooling versus water cooling?
Heat moves through a cooler like current through a resistor. CPU temperature is the case air temperature plus power times thermal resistance, in °C per watt. A good air tower is about 0.2 °C/W, and a big water cooler 0.1 to 0.15. Water's extra mass also makes it heat up more slowly.
- CPU temperature under load
- 44.3 °C
- Headroom before throttling
- 50.8 °C
- Most power this cooler can hold
- 268 W
- Warm-up time constant
- 450 s
Challenge: Run 200 W or more with at least 10 °C of headroom.
More settings
Play
Put a 250 W CPU on each cooler in turn. Find the first one that doesn't throttle.
Challenge: Run 200 W or more with at least 10 °C of headroom. The box under the picture turns green when you get it.
Stuck? Pick one of the examples from the “Try an example” menu, or press “New example.”
Understand
Heat flowing through a cooler behaves like current through a resistor. That's Ohm's law for heat:
with watts for amps, degrees for volts, and thermal resistance (°C/W) for ohms. The chip, the paste, and the cooler add in series, like resistors.
A cooler also stores heat, like a capacitor. So the temperature doesn't jump. It climbs along an RC curve with time constant . Water loops hold much more heat, so they warm up over minutes, not seconds.
The bold curve is your cooler. The faint ones are the others at the same power, with their steady temperatures listed on the right.
Use
Every input has a unit menu, so you can type values in the units you already have. Results follow your units.
Show the work
- Ohm’s law for heat: temperature rise = power × thermal resistance
\Delta T = P\,R = 65 \times (0.1 + 0.15) = 16.25\ ^\circ\mathrm{C} - Steady temperature
T = 28 + 65 \times (0.1 + 0.15) = 44.25\ ^\circ\mathrm{C} - Most power before throttling
P_{\max} = \frac{95 - 28}{0.25} = 268\ \text{W} - Warm-up: an RC circuit, τ = R × heat capacity
\tau = 0.25 \times 1800 = 450\ \text{s},\quad T(t) = T_a + PR\,(1 - e^{-t/\tau})
Export
Set the CPU's power under load and pick a cooler. "More settings" changes the case air temperature and the throttle point.
- The cooler numbers are typical values. Measured reviews at your wattage are the best guide.
- Short bursts can exceed the steady limit, because the cooler's mass soaks up heat for a while.
For learning and estimation. Verify with applicable codes, standards, and a qualified professional before using in design, construction, or safety-critical work.
Cheat card
| Symbol | Meaning | Unit |
|---|---|---|
| CPU power | W | |
| thermal resistance, chip to air | °C/W | |
| heat capacity of the cooler | J/K |
- It's Ohm's law with watts for amps, degrees for volts, and °C/W for ohms.
- Case airflow sets the ambient temperature. A hot case adds degree for degree to the CPU.
- Water coolers don't make heat disappear. They move it to a bigger radiator, and their extra mass slows the warm-up.
Where it’s used
- Computer Science
PC builders and laptop designers size coolers by thermal resistance and power limits. - Electrical & Electronics
Power electronics engineers use the same °C/W math for heatsinks on transistors and LEDs. - Hobbies & Crafts
Pick a cooler for a new build, or see why a small-form-factor PC throttles.
Related exhibits
Start hereOhm’s law: how much current flows?
Push harder (volts) or squeeze tighter (ohms), and watch the current change.
Go furtherDoor closer: slam, bounce, or glide
One damping knob turns a slam into a glide. Where two dots (the poles) sit tells you which.
Go furtherThe Bartz equation: how regenerative cooling keeps a rocket throat from melting
Heating grows with chamber pressure to the 0.8 power and peaks at the throat. The fuel flows through the wall to carry it away.
Also tryHome heat loss: where does the heat go?
Heat leaks through each surface in proportion to area and temperature difference, divided by R. Windows and drafts punch far above their size.
Also tryRapid cooling: 135 °F to 70 °F in two hours
Cooling slows as food nears the fridge temperature. A logarithm says how long it takes, and why shallow pans win.
Questions people ask
Is water cooling better than air cooling?
Big water coolers usually run a few degrees cooler than big air coolers at high power, and they warm up more slowly because water holds a lot of heat. At moderate power, a good air tower performs about as well for less money.
What does °C/W mean for a cooler?
How many degrees the CPU rises for every watt of heat. At 0.2 °C/W, a 150 W CPU runs 30 °C above the case air.
Why does my CPU throttle?
If power × thermal resistance pushes it past its limit (often 95 to 105 °C), the CPU cuts its own power until the temperature holds. The most power a cooler can hold is (limit − case air) ÷ R.