Heatsink calculator
Three resistances in series
Heat travels from junction to air through three thermal resistances in series: junction to case, case to heatsink, and heatsink to air. The first two are fixed by the device and the insulator; only the third is yours to choose. Sometimes the first two alone turn out to be enough to make the problem insoluble.
| Dissipated power (W) | |
| Maximum junction temperature (°C) | |
| Ambient temperature (°C) | |
| Safety factor on Tj | |
| Rθ junction → case (°C/W) | |
| Rθ case → heatsink | |
| Airflow | |
| Result | |
| Target junction temperature | |
| Permissible total thermal resistance | |
| Needs a heatsink with Rθ no worse than | |
| With forced air this is enough | |
| Rough estimate of flat-plate heatsink area | |
📐 The formulas
Rθ(total) = (Tj − Ta) / P
Rθ(heatsink) = Rθ(total) − Rθ(junction→case) − Rθ(case→heatsink)
Tj = Ta + P · (Rjc + Rcs + Rsa)
Thermal resistance of the interface:
| Arrangement | Rθ, °C/W |
|---|---|
| No compound | 1.2 |
| With thermal compound | 0.4 |
| Mica with compound | 1.0 |
| Ceramic insulator | 2.0 |
Forced air reduces a heatsink's thermal resistance by a factor of 2 to 4.
A check point:
Tj = 125 °C, Ta = 40 °C, P = 20 W, Rjc = 1.0, Rcs = 0.5 → you need a heatsink of 2.75 °C/W or better.
Where this calculator stops:
- The flat-plate area estimate is very rough: a finned heatsink of the same base area does far better
- Thermal radiation is not accounted for — on a black anodised heatsink it adds up to 20 %
- Heat from neighbouring components, and the enclosure's overall thermal situation, are not accounted for
- A heatsink's rated Rθ is usually given for natural convection in the vertical position
🎯 In practice
- Choosing a heatsink for a linear regulator or an output transistor.
- Checking whether the heatsink you have is enough for the power involved.
- Deciding whether a fan is needed.
Practical advice:
- Thermal compound costs pennies and buys up to a degree per watt — often more than a heatsink twice the size.
- Leave margin on Tj: use 0.8 of the datasheet maximum, not the maximum itself.
- The fins must be vertical, or convection does not work and the rated Rθ is unreachable.
- A mica washer is needed for isolation, but it costs a whole degree per watt.
© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.
How to cite this calculator
UR3PKI. «Heatsink calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/heatsink (licence CC BY-NC-SA 4.0).The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.