CPU cooler TDP headroom check: will your cooler keep up with the overclock?

An overclock adds heat faster than it adds speed. This check estimates how much heat, from figures you already have, and sets the estimate against your cooler's watt rating. You get a verdict and four numbers: estimated heat, the scale against stock, the cooler margin in watts and the ratio of rating to estimate. It is a planning tool for deciding whether the cooler needs an upgrade before you raise the clock. It is not a thermometer.

Filling it in

1. CPU TDP at stock (W): the thermal design power Intel or AMD publishes for the chip, such as 80 W for a Xeon E5450 or 95 W for a Core 2 Quad Q9650.
2. Stock clock (GHz): the rated clock.
3. Stock core voltage (V): read it at stock in the BIOS hardware monitor or a monitoring tool. A reading under load is the better one, since most boards let the voltage sag a little under load.
4. Target clock (GHz): the clock you are aiming for. The FSB overclock calculator gives it for an FSB overclock.
5. Target core voltage (V): what the chip needs at that clock. Until you have tried it, this is a guess; rerun the check with the real figure later.
6. Cooler rating (W): the cooler maker's TDP figure, if it publishes one.

Every field needs a positive number, otherwise the tool asks you to fill them all.

The method in plain words

The heat a CPU makes by switching its transistors scales roughly with frequency times voltage squared. Clock counts once, voltage counts twice. The check takes the stock TDP as the starting heat and scales it:

scale          = (target clock / stock clock) x (target voltage / stock voltage)^2
estimated heat = stock TDP x scale
cooler margin  = cooler rating - estimated heat
ratio          = cooler rating / estimated heat

The ratio picks the verdict:

Worked example: the default settings

The check opens with an 80 W chip at 3.0 GHz and 1.20 V, a target of 3.6 GHz at 1.30 V, and a cooler rated at 150 W. Think of an E5450 class Xeon taken from a 333 MHz to a 400 MHz bus at its fixed multiplier of 9.

Frequency factor   3.6 / 3.0          = 1.200
Voltage factor     (1.30 / 1.20)^2    = 1.174
Scale              1.200 x 1.174      = 1.408    shown as 1.41x
Estimated heat     80 W x 1.408       = 112.7 W  shown as 113 W
Cooler margin      150 W - 112.7 W    = 37.3 W   shown as +37 W
Rating / estimate  150 / 112.7        = 1.33

A ratio of 1.33 sits between 1.2 and 1.5, so the verdict reads "Workable headroom". The heat figures keep full precision and round only for display, which is why 80 x 1.41 (112.8) and the shown 113 W agree. The verdict follows the ratio exactly as shown, to two decimals, so a ratio shown as 1.5 always reads Plenty. Note what happened: a 20 percent clock gain brought about 41 percent more heat.

Two what-ifs show how the bands move. Keep the 150 W cooler and settle for 3.4 GHz at 1.25 V: the scale drops to 1.23, the estimate to 98 W and the ratio to 1.52, which is "Plenty of headroom". Or keep 3.6 GHz at 1.30 V and put a 130 W cooler on it: the ratio falls to 1.15, and the verdict turns to "Tight: expect high temperatures". On a cooler rated 95 W, the ratio is 0.84 and the margin reads -18 W.

Limits

• TDP is a design figure, not measured power. It is the heat a cooler should be built to remove at sustained load, and the estimate inherits whatever gap there is between that figure and what your chip really draws. Sizing a CPU cooler by TDP explains TDP, Intel's PL1 and PL2 and AMD's PPT.
• The formula covers switching power only. Leakage current grows with voltage and temperature, so a hot, high-voltage chip makes more heat than the estimate. Treat the result as a floor.
• Cooler ratings are not standardized. Each maker picks its own heat source, fan speed, air temperature and temperature limit, so compare ratings only within one maker's range and prefer its per-CPU compatibility list.
• Case airflow decides what air the cooler works with. A tower cooler breathing warm exhaust from a graphics card falls short of its rating.
• Ambient temperature adds almost directly: a few degrees warmer in the room is a few degrees warmer at the chip, so a margin that is fine in winter can be tight in August.
• The paste joint costs degrees at every load if it is old or patchy, and repasting a CPU is the simplest fix there is.
• The board's voltage regulators heat up with the overclock too, and the check does not model them.

For the clock and voltage side of the plan, overclocking a Xeon on an LGA 775 board covers straps, voltages and stability testing.

Questions

My cooler has no watt rating. What should I enter?

Many makers have dropped watt figures in favor of per-CPU compatibility lists. Use the TDP of the hungriest chip the maker lists for your socket as a stand-in, and read the verdict with extra caution, since a list entry says the cooler copes, not by how much.

Why does voltage count twice?

Each switching transistor charges and discharges a small capacitance, and the energy per switch rises with the square of the voltage. In the default example, 1.30 V is 8.3 percent more than 1.20 V, yet it adds 17.4 percent more heat on its own.

Can I use the check for a modern chip with turbo?

Partly. The model assumes a fixed clock and voltage, which suits Core 2 and Xeon chips without turbo. Modern chips change both all the time and run up to power limits, so the sustained limit (Intel's Maximum Turbo Power, AMD's PPT) says more than any scaling estimate. Enter that limit as the TDP, keep target clock and voltage equal to stock, and the check compares it with your cooler directly.

Is Workable headroom enough?

For most builds with sensible airflow, yes. Near the bottom of the band (1.2 to 1.3), plan for summer temperatures and a busy fan. Dropping the target by 100 or 200 MHz, and the voltage with it, often moves the estimate into Plenty.