Online Tool Store
Heat Sink Calculator
Work out the heat sink thermal resistance a component needs from its power dissipation, maximum junction temperature, and ambient conditions.
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Math & Science
Heat Sink Calculator
Frontend preview — no upload or external service.
Required sink
12 W with a 125 °C junction limit at 45 °C ambient allows 6.67 °C/W total; after junction-to-case and interface resistance, the sink must be about 4.7 °C/W or better.
How the Heat Sink Calculator works
- Enter the power the device actually dissipates, not its rated maximum.
- Set the junction temperature limit below the datasheet maximum — designing to the absolute limit leaves no margin.
- Subtract junction-to-case and interface resistances from the total to get the sink figure, then choose a sink rated below it.
The method
Thermal resistances in series add, and the total allowable resistance is the temperature rise divided by the power dissipated.
R_total = (T_junction - T_ambient) / P; R_sink = R_total - R_jc - R_cs
(125 - 45) / 12 = 6.67 °C/W total. With 1.2 °C/W junction-to-case and 0.8 °C/W through the interface, the sink must be 4.67 °C/W or lower.
FAQ
Does thermal paste matter much?
At these numbers, yes. An interface resistance of a degree per watt is a significant share of the budget, and a dry or uneven mounting can double it.
Should I design to the maximum junction temperature?
No. Semiconductor life falls sharply with temperature, so derate — designing 25 to 30 degrees below the maximum is common practice.
Is natural convection enough?
It depends on the sink's rating in still air, which is what manufacturers quote. Forced air changes the figure dramatically, so check which condition a rating refers to.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Full resistance chain | ✓ | Manual | Yes |
| Includes interface resistance | ✓ | ✗ | Sometimes |
| No simulation software | ✓ | ✓ | ✗ |
| Airflow modelling | ✗ | ✗ | ✓ |
Heat Sink Calculator adds the whole resistance chain, including the interface — the layer most often left out of a thermal budget.