Heat Sink Calculator

This heat sink calculator works out the maximum thermal resistance a heat sink can have while still keeping an electronic component below its safe operating temperature, which matters because overheating a transistor, MOSFET or processor shortens its life or causes it to fail outright. You enter four figures: the maximum junction temperature the device is rated for, the ambient air temperature around it, the power in watts it dissipates, and the combined junction-to-case and case-to-sink thermal resistance from the datasheet. From these the calculator works out the total allowed thermal resistance, the temperature rise budget divided by the power, then subtracts the junction-to-case and case-to-sink figure to give the maximum heat sink rating you can use, in degrees Celsius per watt. Alongside the main result it also shows the total allowed thermal resistance and the temperature rise budget, so you can see how the numbers add up. A lower degrees-per-watt figure means a more effective, usually larger or finned, heat sink; if the result is negative, no passive heat sink will do the job and you need to lower the power, allow a bigger margin, or add active cooling such as a fan. Try the worked example figures of 125, 25, 10 and 1.5 to see how the maths flows through. Treat the result as an indicative engineering guide and confirm against the datasheet before relying on it.

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Max junction temp Tj (°C)

Ambient temp Ta (°C)

Power P (W)

Rth jc + cs (°C/W)

Heat sink ≤ 8.5 °C/W

Total allowed Rth10 °C/W

Temperature rise budget100 °C

The formula

The total allowed thermal resistance is (Tj minus Ta) / P. The heat sink resistance must be at most that total minus the junction-to-case and case-to-sink resistances: Rsa = (Tj minus Ta)/P minus (Rjc + Rcs). Lower is better.

Worked example

For a device at 10 W, max junction 125 degrees, ambient 25 degrees, with 1.5 degrees per watt from junction to sink: total allowed is (125 minus 25)/10 = 10, so the heat sink must be 10 minus 1.5 = 8.5 degrees per watt or less. Enter 125, 25, 10, 1.5 to confirm.

Frequently asked questions

How do I size a heat sink?

Find the total allowed thermal resistance (temperature rise over power), then subtract the junction-to-case and case-to-sink resistances. The remainder is the maximum heat sink rating.

What does degrees per watt mean?

How many degrees the temperature rises per watt of heat. A lower figure is a more effective heat sink.

What if the required value is negative?

No passive heat sink can do it; you need lower power, a larger temperature budget, or active cooling.

Who this calculator is for

This calculator is for electronics students, hobbyists and engineers.

What this calculator assumes

You enter values in the units shown.
Ideal components are assumed.
Results are rounded for display.

Formula and sources

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